DRE Motorsports Technical Data

JR Racecar Titan Tuning Spec's

Here are some of the typical tuning parameters of a JR Racecar Titan Motor.

Recommended Engine Break-in Procedure:
For breaking-in your Titan engine, we recommend the following procedure. Fill the engine with 10 ounces of a non-synthetic oil. The recommended weights are 0-30, 5-30 or 10-30 weight oil. When starting or running your engine, please follow the IHRA or NHRA warm-up procedures found the their respective rulebooks. This is for safety reasons only. Start and run your engine until a good temperature is obtained in the engine. Then shut the engine off and let it cool leaving the oil in the engine. After the engine has cooled, start the engine and repeat the steps. After this process has been repeated, change the oil. Again, use a non-synthetic oil. At this time you are ready to race. After 3 to 5 passes down the track you will need to change to oil again. At this point, the engine should be broke-in. You can now switch to your good lightweight synthetic racing oil. It is recommended that you change the oil in your engine every 3 to 5 passes. If you have any questions, please feel free to contact the JR Raacecar technical support staff.

Tuning Guide:
The following are tuning recommendations any may vary depending on your race car and racing program.
Oil: Run 10-ounces of oil in your Titan Racing engine. Run synthetic racing oil only after the above engine break-in procedure has been completed.
Target EGT and CHT:
Idle EGT is approximately 750 to 800, CHT 150 to 160.
Max recall EGT after a run is approximately 1200 to 1250, CHT 280 to 300.
Idle RPM:
Approximately 4000-4200 RPM with your clutch engagement at approximately 4600-4800 RPM.
86 tooth rear sprocket and a 15 tooth front sprocket for a gear ratio of 5.73. This gear ratio is for 18x8x8 rear tires. Please remember that the gearing is a estimate and that changes may be necessary due to package weight and elevation. Please give us a call for further gearing assistance.
Max RPM:
Max recall RPM should be between 7200 to 7400 RPM. Anything outside of this range is not recommended.
Torque Specs:
Head Stud (nuts) 150 inch-pounds (or 12.5 foot-pounds).
Rod Bolts 150 inch-pounds (or 12.5 foot-pounds).
Spark Plug gap should be approximately 0.025”
Coil to flywheel air gap should be approximately 0.025”
Ignition Timing:
The ignition timing recommended range is 0.450” BTDC to 0.475” BTDC. This depends on your application.
The Titan racing engine bolt in the set of 4 holes closest to the side cover on the clutch side.
Watch your chain tension. Set the chain tension with the driver in the race car on the ground.

MyChron 4 660 Tuning Spec's

Here are some of links to MyChron 4 660 tachometer. AIM MyChron4 Webite link.

CVT - Constant Variable Transmission, it's your Salisbury style clutch drive system - which ultimately provides an infinite number of "gears" if compared to a manual or automatic transmission. So the CVT is telling you the effective "gear" you are in.

Drive - expresses the compliment of CVT as a percentage to drive ratio. For example, when CVT shows 1.0, Drive will show 100%, and when CVT shows .70, Drive will show 130% - that is 30% overdrive.

Linear - Linear Acceleration expressed in g's. It's computed by the rate of acceleration of the speed, which is effectively the jackshaft. It's uses are limited, as any sort of slip, or slip-then-grab, will show an exaggerated amount of linear g-forces - NHRA forbids using an actual g-sensor on jr dragsters. However, this does give a way to sort of quantify the slip amount.

The No-Lift ET prediction is designed to detect a lift or a tap of brakes before the finish line - when the driver "dumps" the car. It should accurately detect a lift more than 98% of the time. The idea behind the feature is that as the day progresses, weather changes, track conditions change, etc. - you set your dial, and then your driver dumps the car to avoid a break-out - but now you're left wondering how close your dial was... This is where the No-Lift ET is treasured. In cases like this, the NLET will accurately tell you what the car would have ran had the driver not lifted, giving you the knowledge so you can determine you next dial. I've also seen this used as a training aid to teach drivers what a lift or brake tap does to the ET. They'd make a pass, dump the car, and then let the software show them how much ET that dropped (added).

Mikuni 42mm Tuning Info

the air jet works with main jet ... the carb draws a mixture of fuel and air into the inlet of the throat ..... so a smaller air jet will richen it up just as a larger main jet will do ...... a bigger air jet allows more air into the mixture thus leans it out , just like a smaller main ....... most 42's have the air bleed plugged off so tuning is done by installing a larger or smaller fuel jet

pilot jet is the idle mixture's fuel jet ..... the screw on the side of the carb is a adjustable air bleed for the idle circuit

the 1.6 to 2.0 air bleed will lean it out big time ...... think of this , the main fuel circuit from the carb throat has one feed into the barrel .... that feed draws fuel from the main jet and air from the air bleed at the same time , restricting the air from the bleed will richen the mixture ..... you can figure it out from here ......different builders convert them to alcohol different ways .... if it was mine I would plug the air bleed so it doesn't feed air into the main circuit ... then just tune with the main jet... Check the pilot jet carb body opening to make sure it has been drilled larger than stock for at 42. If not, changing to a larger pilot will not make any changes. Some Purepower 42 carbs were not drilled.

Additional PP Notes

that is exactly right the same goes for me but they also didnt drill the main jet orifice where the main goes the right size either its only about .110 so any jet above that size will not do anything it needs to be drilled out to about .130 thst is also the anwser to your surge problems if you are having them but once you get everything drilled a .55 idle jet is plenty you shouldnt have to go higher than that

How Many Inches Did I Win By?

How to Figure Out Margin of Victory in Feet and Inches

By Bill Pratt

The other day on nitromater.com, we were discussing how to figure out the distance for a particular margin of victory. Since IHRA Richmond was rained out Sunday, I guess I’ll post my response to that discussion here as a makeshift story of the day.

Here’s the step by step method to figure out the margin of victory in feet and inches.

In the nitromater discussion, there was a Super Stock race that had an amazing .0003 second margin of victory. The newest versions of the Compulink timing system actually read out to four decimal places.

To figure out the approximate distance in a margin of victory, you need to start with two things: the margin of victory in seconds and the LOSER’s trap speed. The winner's speed doesn't matter. You need to know the loser's trap speed because what we are really measuring is how many feet the LOSER has to travel for those .0003 seconds after the winner hits the finish line.

For the purposes of this demo, let's assume the loser was running exactly 120 mph.

You need to convert this to Feet Per Second. The way to do this is to take the MPH and multiply it by 1.4666666 (infinity). Using 1.466 is probably close enough for this purpose.

So, 120 mph equals 175.999 feet per second. Let's just say it's 176 Feet Per second.

Now you simply take the true margin of victory (in this case, .0003 seconds) and multiply it by the Feet Per Second. At 176 FPS, this turns out to be .0528 feet.

If you get something less than one foot, which we did here, you need to convert to inches, so take .0528 and multiply it by 12.

In this case, the margin of victory is less than an inch! In fact, it's .633 inches! For those of you who are better with fractions, take .633 times 16 (to convert to 16ths of an inch). You get 10/16s, or 5/8s.

SO... the front wheels of the losing car in this Super Stock match were FIVE EIGHTHs of an inch behind the front wheels of the winning car!

This formula works for any type of drag racing vehicle. You just need to get the true Margin of Victory by adding each racer's ET and RT and then subtracting the winning time from the losing time to get the MOV in seconds... It's a little more complicated for Dial Your Own bracket racing because you have to convert the ETs relative to each racer's dial in. We can talk about that later if you want.

Another thing, if you want to talk about car lengths of victory, just compare the result you get in feet and inches to the following rough measurements: Top Fuelers: 25 feet, Funny Cars: 18 feet, Pro Stock and other street cars: 15 feet. I have no idea how long a bike is <g>.

Bill Pratt

Tips for a struggling "Bottom Bulb" Racer

Written by Sal and Peter Biondo

Would like to share with you a little something that may help a "bottom bulb" racer with his / her reaction times. There are times when we feel "lost" on the bottom bulb. Those are the worst days for any racer, you have a lack of confidence, and it may take you weeks to get back "on track". But, we tried something with a few racers who could not get their "bottom bulb" reaction times consistent. We have also used this method to prove to racers that they may or may not be using the correct rollout in the practice tree.
The main idea is to leave as soon as you see the bottom bulb, react to the "flash", that is what makes you consistent. But due to the "distraction factor" of the top two ambers we can sometimes be inconsistent. So, what we suggest to do is the following : take the top two ambers out in each lane, you can do this with the table version "Final Round 2, or 3" by unscrewing the top 2 lenses, and pulling out the bulbs. The bulbs can not be pulled out of the versions with the LED bulbs, such as the Final Round 4 (FR4), but the FR4V2 model has an option to shut the bulbs off) Or, you can do this with the Full Size Tree by simply unscrewing the bulbs. This makes it easier to put your focus solely on the botton amber. (Of course you should make sure there is no handicap set in the unit, because we want your bottom amber to come on before, or simultaneously with the bottom amber in the other lane).

React to that amber as fast as you can, (for those who are not used to this, it may take a few shots). This may take some more concentration now, since we don't have the top two ambers giving us the warning of the third coming on. But, those two top two ambers are there for no other reason but to distract you!!

With this method we can now figure out the rollout to use in the practice tree by adjusting the rollout number until you are in the 5 - teens, or 5 - twenties. After you have established an average competitive reaction time, you are ready for the final step:

Put all the bulbs back in, leaving the rollout number the same, and "hit" the tree just as you did before. Try to forget that the top two ambers are there. If your reaction times are about the same and as consistent as they were before, you are learning to master the full tree, which is the hardest "tree" to conquer. (it requires the most discipline). If you master that, set the tree up, so that you give the other lane a 1/2 second or full second handicap, because that will create more distraction, and if you can master that, then you are ready for anything.

Guide to Weather Stations and Predicting
Written by Peter Biondo

I put together a weather station guide to help you get the most out of your weather station. Regardless of which weather station you purchased from us, these tips will help get you going and hopefully lead to more round wins!
Taking Weather Readings;
Trailer Based Weather Stations- The first and most important step in predicting your ET or throttle stop settings is taking weather readings properly. If you are using a trailer mounted weather station such as the Altacom 2 or Performaire Weather Center there is not much you have to worry about as the fan aspirated sensors utilized by these 2 systems will accurately and automatically take weather readings for you all day long. These stationary weather stations use high end sensors and incorporate a continuous fan blowing through the sensors which makes it the most fail safe way to get accurate readings with minimal effort. Add a pager to the mix and you have combined the ultimate in accuracy and convenience.

Hand Held Weather Stations- With a hand held / portable style weather station, it is much more important to take care in where and how you take your weather readings. To obtain the most accurate readings you should follow these guidelines:
- Allow 10 seconds after turned on for barometer to settle
- Take readings in the open air, out of direct sunlight, and away from hot vehicles. (if there is no shade around- use your body shade)
- Take readings in the same place every time. (whether in the lanes or the pits)
- Use common sense when taking weather (don't leave it in your hot racecar for all day and hold it out the window to take the weather, it may take up to 10 minutes for the heat to dissipate from the unit.)
- A hand held weather station that has a built in fan to blow over the sensors will be more forgiving in where and how you take your readings.
The bottom line with hand held weather stations is that you can achieve the same accurate results as you would with a trailer based weather station, but it does take a little bit more effort on the user’s part.

Weather and Predictions
After gathering accurate weather readings from your weather station, the next step is making predictions to your vehicles performance. The most common question I am asked is “should I do manual predictions or should I use a program to do the predictions for me?” My best answer to that is to thoroughly learn how to do the predictions on your own regardless of whether you decide to use a program to predict. Like anything else in life, if you really want to master something you should not only look at results but you should look into the “why” and “how” you get to that result. Later on, I will talk about some prediction programs, but before that I want to share some of the “why’s” and “how’s” on weather and how it effects your vehicles performance so that you will not only get a good understanding of this but you will also be able to effectively predict your vehicles performance on your own.

Predicting on your own- As we would all love to be able to push a “magic button” and automatically be able to predict to the thousandth of a second, we also have to realize that there is a lot more to it than that. With all the variables out there on each pass, it is important to look at these variables and see how they effect your vehicles performance and by how much. This all starts with taking efficient notes- logging all of the weather variables in your logbook. Take the logbook home with you and study it. Pretty soon you will see a pattern developing and you will learn how much of an effect each weather variable has on your vehicle. The main weather variables you want to look at are

1) Density Altitude.
2) Barometer.
3) Humidity.
4) Water grains.
You can also look at temperature but the reason I didn’t put that in here is because temp is heavily taken into consideration in the density altitude number, therefore there is little reason to look at both temp and density altitude. Gasoline burning vehicles tend to be mostly effected by the density altitude change and less effected by moisture (humidity and water grains) where as if you run an alcohol burning vehicle, you would want to keep a very close eye on humidity and water grains and put more weight on any moisture change, and put less weight on the density altitude change. After studying your logbook, you will soon learn how much of a weight to put on each weather variable.

Here are some tips and generalizations I have learned with my vehicles;

Gas vehicle (¼ mile)- a change of 150 feet in density altitude will change your vehicle .01. A change of 18-20% of humidity will change your vehicle .01. A .10 (ex 29.90 to 29.80) change in barometer will change you vehicle .01.

Alcohol vehicle (¼ mile)- a change of 300 feet of density altitude will change your vehicle .01. A change of 10-13% in humidity will change your vehicle .01. A .10 (ex 29.90 to 29.80) change in barometer will change you vehicle .01.

Water Grains- If you see water grains go up at the same time humidity goes up, you are generally going to see a significantly more slow down than if humidity goes up and water grains stays the same. The same is true when humidity and water grains go down.

Track Conditions- Track temperatures and track prep will also affect your vehicles performance. The ideal track temperature is in the 70 to 90 degree range. Here the rubber on the track is the tightest. The further the track temperature gets from this ideal temperature, the more negative (slowdown) effect on your ET. Too cold of a track and there may not be enough adhesion. Too hot of a track and the surface tends to be greasy and is susceptible to bald spots. As with any other variable, pay attention and share information with your buddies. Have 500 cars been down each lane since the track was prepped last? Is the sun beating down on it on a hot day? How far the track temp from the “ideal temp” is and which direction is it heading? What were the characteristics of this track the last time I ran on it at this time of the day? On very hot surfaces it has also been thought (but not proven) that the actual heat from the track surface will slow down your vehicles performance (the entire length of the ¼ mile), not because of traction, but because the heat off the track actually heats up the temperature of the air a few feet above the track, which is the air the carburetor “sees”.

Wind- Wind is often an underplayed factor especially when predicting ¼ mile performance. It is also the hardest variable to pinpoint because it’s always changing and swirling. The best thing to do is pick a reference point for how and where you will determine the wind at each track and come up with an average wind reading over a period of 20 seconds. It could be a flag or your hand held windmeter at the 1000’ mark, your stationary wind meter on top of your trailer, burnout smoke, or a combination of these. It is not only important to pay attention to the level of the track, but to the obstacles surrounding the track. A generalized chart (average) for wind would be a 4 mph tail wind = .01, a 7 mph tail wind = .02 and a 10 mph tail wind = .04. The same applies for headwinds except the value for headwinds will tend to be slightly higher because you are going “against the grain” so to speak.

An advanced tip (looking deeper into it)- Whether you are talking about density altitude, barometer, humidity, track temperature, or wind, the further you get from the ideal point, the more of an affect that same change will have on your vehicles performance. This is assuming you don’t make any “set-up” changes (gear, converter etc) to the car to compensate for significantly worse or significantly better conditions. For instance, let’s say my stocker runs a 10.45 at 1000 feet of DA and 40% of humidity. If the DA goes to 1600 feet and the humidity goes to 55%, I would then run an ET of 10.50. But if the next day we have totally different conditions and the DA goes to 4000 and the humidity goes to 85%, I would probably slow up more than what I would have figured using my original formula. My original formula would tell me I should dial a 10.68 but because the conditions veered “so far off center”, it is likely the ET would be slower. The same concept applies when all the weather variables move together. Same concept goes for track temperature and wind. For instance, if the temp, humidity, and vapor pressure all move significantly down and the barometer moves significantly up it is more likely your car will speed up more than you would anticipate using your original formula.

Prediction Programs
Some weather stations are equipped with built in prediction programs where you can enter your vehicles runs and the weather that corresponds with the runs. The prediction programs then use its own formula’s to figure out how the weather change will affect your vehicles performance. When using these prediction programs it is important to remember the following:

- All runs in a database should correlate or make sense with each other. The runs that do not should not be entered or should be deleted. An example a database with 2 runs that don’t correlate with each other would be; the density altitude of RUN A is 1000 with an ET of 10.19 and the density altitude of RUN B is 1500 with an ET of 10.17. Run B had worse/ “slower” weather but netted a quicker ET. Possibly run A is not a good run and should not be entered in the database. Possibly run A had a significant head wind, tire spin, or a broken valve spring? It is not only important you keep this run out of your database (as it will confuse the program and lead to inaccurate future predictions) but it’s also important that you figure out why this run was slow for future reference.

- A database should consist of runs that have a large range of density altitude. Having different density altitudes in your database (ex.200 feet - 2100 feet) shows the computer how your car responds to a big weather change. A throttle stop database should consist of runs with a large spread of throttle stop data as well. We cannot emphasize enough the importance of quality runs in your database. A database with four or five quality runs will predict better than a database with 15 runs where 5 of them are "bad". Predictions from one track to the next are usually accurate, However- due to variance in rollout, wind direction, track surface, and traction form track to track- predictions may be off. If after your first run at a new track, your prediction is way off, you should start a new database with that track.

If you are looking for a stand alone prediction program for your computer or laptop, I would highly recommend Crew Chief Pro. This computer program is very comprehensive and detailed, more so than the more generic programs that come with weather stations. If you purchased a weather station from us we will give you 10% off the Crew Chief Program. Ask for Peter Biondo and mention this write-up.

When it comes to jetting, the common question always is- “Is my car jetted properly?” If you are trying to predict your vehicles performance to the .001 of a second, it is important your jetting be right so that your vehicle responds to weather changes predictably and consistently. There are many ways to see if you are jetted properly. From more traditional ways like reading spark plugs to using modern technology like onboard computers, EGT gauges, and O2 sensors. A more common sense approach is to monitor how much your vehicles performance changes when the air changes. In other words, is your car reacting to weather changes like it is supposed to? If you make a run in the morning where the DA is 1000 feet and run a 10.10 and then make another run in the afternoon where the DA is 1800 feet and your car falls off to a 10.21, chances are you are jetted too rich. If you actually pick up in the afternoon when the air gets worse, chances are you are too lean. As a rule of thumb, talk to a few of your friends at the races that have reliable cars and compare notes. See if your vehicle is corresponding to what the weather and what the other reliable cars are doing. Another question is “How often should I change my jetting?” If you are trying to get the most out of every run (comp eliminator, pro stock etc), you would have to not only change your jetting every day, but you would have to change them from the morning air to the afternoon air. However in classes where you are trying to predict performance it is more beneficial to leave your jetting as a constant. I would say a gasoline burning vehicle should have 3 different jet settings over a period of a race season, one setting that works well for killer weather conditions, one for mediocre conditions, and one for poor weather conditions.

Summing it up
Hopefully this will help you get the most out of your weather station. As you will see and hear, there are a lot of ways to use weather stations in predicting vehicle performance. No matter the case, there are two things you always want to keep in mind. Make sure you have a weather station that is accurately gathering data for you and second, always use common sense, pay attention, and take notes on how the weather affects your vehicle.

Tips on Using a Throttle Stop

(Written by Peter Biondo) updated 10/17/08
Through years of on track experience with throttle stop racing I have learned a few things about throttle stops that can serve as a guideline to help in your throttle stop racing.

Finding how much to mechanically shut your throttle down is crucial. You want to find a setting that will work well and be consistent. I have found 3 blade angles that work well (find the settings below). The most accurate way to adjust your "blade angle" is by RPM- (the rpm your engine drops to while the throttle stop is engaged). Once you have the right throttle stop RPM, you are done with the mechanical part of it, and all ET adjustments should be done with a timer.

As mentioned above, I have found 3 blade angles that work well:
- A "throttle stop rpm" of 3900- this will work well if your car runs 1 second under the index.
- A "throttle stop rpm" of 4300- this will work well if your car runs .3 to .9 under the index.
- A "throttle stop rpm" of 4800- this will work well if your car runs less than .3 under the index.

*** If shifting on time, please refer to that section below as the suggested t/s rpm is different.

Before figuring out your ratio you first must enter a number in timer 1 of your throttle stop timer. This number indicates when the throttle stop will come on after launch. Most people prefer to have this number set early for high mph. I recommend having the throttle stop come on between .1 and .3. Once you set this, you will never adjust it again. To adjust your ET you will change timer 2.

Whether you are using a weather station to predict a throttle stop or not, I highly recommend you learning your throttle stop ratio. The Throttle Stop Ratio is the effect the throttle stop time has on your ET. Here's an example- if you add 2 tenths (.2) to your throttle stop timer and it changes your ET by 1 tenth (.1), then you have a 2 to 1 ratio. To learn your ratio do the following:

Make one run with a small amount of time (duration) in the throttle stop timer (.5). Make a second run with a large amount of time (2.5). Let's say run # 1 was an 8.40 and run # 2 was an 9.40. You can figure out your throttle stop ratio by dividing the change in the throttle stop time by the change in ET. The change in throttle stop time divided by the Change in E.T = T/S Ratio. OR (2.00 divided by 1.00 = 2).

This is called a 2 to 1 ratio. Learning your ratio will allow you to correct for changing track and air conditions. Your ratio depends on your "throttle stop rpm". For most applications a 3900 T/S rpm results in a 2 to 1 ratio, a 4300 T/S rpm results in a 3 to 1 ratio, and a 4800 T/S rpm results in a 5 to 1 ratio. These ratios are based on cars equipped with converters that stall in the 5600-6400 area. Extremely loose or tight converters will result in different ratios.

An "In-linkage" throttle control is sensitive to the entire throttle linkage system. It is very important to have an absolutely solid and rigid pedal stop. Without this you can stretch your linkage causing inconsistency. Your cable attaching bracket must also be rigid. Any flexing or binding will ruin the consistency.

Is it beneficial to shift on time (have a timer shift the car during the stop duration) while on the stop? The answer really depends on how fast your car runs. Example: If your car runs well under the index (over 1 second under the index), you can gain consistency by shifting on a time. There are 2 major benefits for shifting on time.

The car will come off the stop in high gear, lessening the chances of spinning the tires at that point.
The rpm’s on the stop will be much more stable when in high gear. In other words, your stop rpm’s will climb at a much slower rate when in high gear compared to low gear. This will result in more consistency and a more predictable throttle stop ratio.
*** Cars running less than 1 second under the index will most likely not benefit from shifting by time.
*** When shifting on time, it is good have it shift a few tenths (.3 to .9) after the stop comes on.
*** When shifting on time you should raise your throttle stop rpm 300 higher than the suggested rpm mentioned in the above #1 example. (Example: cars running one second or more under the index should have a throttle stop rpm of 4200 as opposed to the 3900 suggested rpm described above.)

5/ SPEED CONTROLS- Necessary or not?
Speed controls are a way to slow down how fast a (CO2 powered) throttle stop either comes on or comes off. This can be especially beneficial in higher powered cars and on greasy, hot tracks. If you have a high horsepower car, and the car comes off the stop in first gear (shifting on rpm), it is a good idea to slow down the throttle stop opening speed to 50% to 60% speed. The Co2 powered stops that we sell have the capability to slow down and regulate this speed. This will keep your vehicle hooked up when the stop comes off and it will also be a smoother transition from part to full throttle. On the other hand, it you are coming off the throttle stop in high gear (shifting on time), it is not necessary to slow the throttle opening speed down as the transition from part to full throttle is smoother in high gear. There are also arguments about whether or not the throttle stop closing speed is more consistent if left at full speed or a slowed down. I have found that having the throttle slowed down slightly (80% of full speed) or at full speed seems to work well for most vehicles.


Using a Practice Tree Effectively and Setting Rollout
Dollar for dollar, the practice tree is the best investment a racer can make towards his racing operation. A minimal investment in money and time can help you achieve consistently good reaction times which is arguably the single most important part of a race. It will also help you build your confidence level and that in itself will lead to more round wins.

Setting Your Rollout

Let’s start by answering the question “What is rollout”? Simply put, rollout on the track is the distance it takes from when your car is staged to when your car breaks the starting line beams and starts the clocks. Breaking this down into time, it takes most cars between .22 to .35 from the time the car begins its launch, to the time it breaks the starting line beams. When using a practice tree, you are not using moving vehicles and racetracks; you therefore have to break the rollout down into time.

A very common question is, “What number do I use for rollout in my practice tree?” To simplify this, I broke this down into categories based on your tree type below. Ultimately, the most important part of this is you achieving consistently good reaction times on the practice tree and then hopping in your car at the races, hitting the SAME spot and hitting the same consistently good reaction times.

Rollout for Pro Tree and Top Bulb Racers:
I will break this down into 3 ‘tree types’. Top bulb racing, pro tree racing, and bottom bulb racing. The first 2 types allow racers to use a delay box and therefore are much easier to find your spot and to relate the practicing to you and your car. To do this set the delay box on the practice tree to the same delay box numbers you would use in your car. Now all you have to do is adjust the rollout until you get the reaction times you are looking for. It’s the easiest and most accurate way of matching your car’s rollout to the rollout you need in your practice tree. For instance, in my bracket dragster (top bulb/ .500 racing) I use 1.150 in my delay box at the track. I go to my Final Round 4 tree and set my delay box to the same 1.150. After a handful of practice runs I realize that I need to have .22 in the rollout to achieve .00 and .01 reaction times. If I use the same dragster to run super comp (pro tree/ .400 racing), I go about it the same way- set the delay for .050 (which is the delay I typically use for the four tenths pro tree), and after a handful of practice tree runs I see that I again need the same .22 in the rollout to achieve the .00 and .01 reaction times. The rollout for my dragster (the time it takes for my dragster to go from “staged” to “breaking the starting line beams” is .22, or 22 hundredths of a second). As a general guideline the “rollout” for a typical seven second dragster will usually be within the range of .22 to .28. A slower leaving 10 second door car will usually take longer to break the beams and typically have a rollout within the range of .28 to .35.

Rollout & Pro Category Type Cars (Top Fuel to Top Alcohol) is tougher to figure out because of the many different and ever changing mechanical aspects of car that effect the vehicle reaction time. The most effective way to use a practice tree for a driver of one of these classes is to set the rollout for .27 for a pro stock car or bike and for .32 for Top Fuel, Funny cars and .29 for Top Alcohol cars. Set the tree type to 4 tenths pro tree and try to get your reaction times as quick and consistent as you can.

Rollout & Bottom Bulb Racing:
Now that we explained rollout and how to set the rollout on your practice tree for ‘top bulb’ and ‘pro tree type’ racing, lets now talk about setting the rollout for bottom bulb racers. Bottom bulb racers do not use delay boxes and therefore any adjustments they need to make to their reaction times need to be made by adjusting your vehicle reaction time. There are many ways to adjust vehicle reaction, among the easiest ways to do this are changing launch rpm, front (and rear) tire pressure, and front tire size. In transbrake equipped cars you can do also do this by adjusting the travel of your transbrake button. Getting back to the practice tree, the following is the best route to take to help you figure out your rollout on a practice tree for a bottom bulb racer. The first thing you should do is set the tree to 5 tenths PRO TREE. Although bottom bulb racing is done on a 5 tenths FULL TREE, by taking the time to take a few hits at the 5 tenths PRO TREE, you will quickly and accurately figure out YOUR correct rollout because the PRO TREE gives you the easiest opportunity to react off the flash and correctly react to the light rather than anticipate the light. On an LED equipped tree such as the Final Round 4, most racers will see that they need a rollout within the range of .30 to .37. The fastest reacting person may need a .37 whereas a slower reacting person may need a rollout number closer to .30. Once you have taken a few shots at the 5 tenths PRO TREE and have set your rollout accurately to where you are hitting consistently good reaction times, you can then switch the tree over to the 5 tenths FULL TREE and feel confident that the rollout setting you have is correct. This is very important for training because now if you have a late reaction time or a red light you know it is you that reacted early or late and you will have no questions on whether the rollout number is correct.

A very important and common question then would be “Now that I have practiced and can hit consistently good reaction times on a practice tree, how do I relate this information to ME, in MY CAR, at the races. The answer is simple; hit that same spot on at the track that you have ”learned” on the practice tree. Now if do this and your reaction times at the track are on target (.01 and .02) reaction times, you are all set. Your car conforms to your “spot” on the tree and your job now is to consistently hit that spot that you have trained yourself over and over again. BUT, if you confidently hit your spot on the track and you are coming a few hundreths red or late, you then have to make changes to your vehicle reaction time. Adjust your leave rpm, tire size, tire pressure, or transbrake button travel to get where you want to be. A common mistake bottom bulb racers make is that they get in their car, hit the correct spot on the tree, and when it comes up red or late they try to adjust themselves to leave early or late. They end up out in left field. There is only one spot on the tree a bottom bulb racer should leave on to achieve the highest level of success on the bottom bulb. That spot is leaving as soon as they see the bottom bulb or “leaving on the flash”. This is the spot you have learned on the practice tree and the most consistent way in the long run.

The most effective way to use a practice tree

PHASE 1: The Big Picture
When you first get a practice tree, you should spend a considerable amount of time with it learning the “big picture”. Here I mean you should try to relate your car and your racing style (full tree/ pro tree/ bottom bulb/ top bulb) to it. Earlier, I covered setting rollout and different tree styles. Once you get your spot and figure out the roll out you are going to be using, spend a considerable amount of time training. Here you want to train your mind to ‘consistently react’ by repetition.

PHASE 2: Short Interval Practicing
After you get to a point where you are comfortable with ‘your spot’ you should go to the next phase of practicing which I will call ‘segment practicing’. The goal here is to train in segments or short intervals (I like to hit the tree 10x and then walk away). By keeping your sessions short, you have a greater chance of giving your brain 100% focus on the tree. The longer the sessions, the more like a game it will seem like and the more of a chance you give your mind to wander. The more your mind wanders, the more inconsistent your reaction times will be and the more inconsistent your reaction times are the less confident you will be in your abilities. Needless to say, an important goal here is to build your confidence, not to diminish it. So, don’t treat it like a game. Hit it 10x, document your results and look for consistency and improvement.

PHASE 3: Create Real Race Situations
The last phase of practicing is where I really want you to put yourself to the test. Here I want you to take a piece of paper and write on it ‘time trial 1’, ‘time trial 2’, ‘eliminations round 1’, ‘eliminations round 2’, all the way to the final round. The important thing here is to make one run and leave for a while then come back and make another run. Spacing the ‘rounds’ apart exactly like you do at the races will 1)- force you to out every ounce of focus you have into that round and 2)- create a real race feel. I even go as far as writing down who my opponent is (you can make up opponents based on who you typically race against) in that particular round and what reaction time I need to ‘win’ the round. This puts some pressure on you where if you lose, you are done and have to start all over another day. When you win the race you will realize that you can ‘do this’ and your confidence level will skyrocket. Bringing this confidence and ‘know how’ to the races will undoubtedly lead to more race wins.

Another effective way to simulate real race situations is to sit in by sitting in your own car and practicing in front of a full size practice tree. While doing this, don't be afraid to throw your helmet and other safety equipment on. You can hook up the button in your car, (if you purchased a full tree package from us, we included the proper wire to connect the practice tree to the button in your car). When doing this, you should have the power off in your car and also disconnect your transbrake wires from your button. This is the best simulation you can get, if you can consistently achieve good reaction times here, you can confidently go to the races knowing you can do this and do it well.

A Quick Tip to improve your "Finish Line Driving"

(This section written by Sal Biondo)
I never claimed to be the best finish line racer out there...But, on the same note I've seen people out there who are worse than me. But, for some reason or another, I've had dozens of people come up to me and ask me how they could become a better "Finish Line" racer.
Well, the answer is simple, I think. Race as much as you can, and with the experience you gain, in time, your "Finish Line" driving will improve. I'm only kidding, I would not make you come to this section and just tell you to race more often.

I really did think of a method to improve a racer's "Finish Line" driving. For the most part, it is best applicable for "Super" racers, who make time trials alongside racecars that run almost the same E.T. as each other. What I suggest is this: when making your run, always check out your opponent as you are headed down track. (Of course, don't do this if you are fighting the car, or there are some severe side winds present! I'm assuming that everyone who enters this section is an experienced drag racer, and I don't need to give out driving lessons!) When you approach the finish line.... that is where I suggest you pay the most attention. What you should do is take a look over, and see where you are in comparison to your competition. Keep a mental note of that distance, and try to decide who got to the finish line first, and by how much. Decide on a number before you pick up your time slip, and see how close you can get. If you do that all the time, you may get a better idea of what's going on at the finish line.

I can't tell you how many runs I see from the finish line, even during time trials, and I see racers looking straight ahead. Maybe sometimes you need to keep your eyes on the guages, or other times you can see well out of your "peripheral vision", but when its real "tight" during a time run, I'll always use that time to try and learn something.

This method of guessing who got to the "stripe" first, and by how much should be done during eliminations also. I know it's probably the furthest thing from your mind when you are at a big race, but think of the long term benefit it could have. For dial-in racers, eliminations are probably the only time you can practice this method. I know in all my past experience of making time runs in brackets and Super Stock, I've rarely made a run with someone who ran within a couple of hundredths of me.

Understanding 4 Link Suspensions

This website has grown far beyond my original intentions of helping the shadetree mechanic in understanding a drag racing suspension in simple terms and diagrams. Initially I wrote this page as a detailed explaination of a Drag Car Suspension using explainations that the do-it-your-selfer can easily understand and do in the driveway with average tools, pencil and paper, etc. Things have changed over the years since this site was published. By the photos on this site you can see the variety of racers that have benifited from the info in this page and for this reason I am updating this site with some explainations that will be different than originally written. There is also information that I still have not gone into due to the complexity of writing it down or creating the diagrams and my goal for this site is to keep this information as simple as possible.
I commonly read on bullitin boards that this site is Only for stock suspension cars or Only for tube chassis cars but in fact the info below is for both types of vehicles. Whether the car is a stock suspension car running 13's down to 7's or a full tube chassis car running 13's down to 7's this info can be used successfully. A stock suspension car that uses 4 trailing arms is a true 4-link and can be treated, plotted, etc, as a true 4-link car so the savy racer looking for an edge can benifit from this information.
There are sections below that do separate the different ideas for a Stock Suspension car vs a True 4-link car and the reasoning will be explained below.
Here is how I learned to think about launching a car. Imagine pushing on a refrigerator on a tile floor. Your feet are representing the tires on your car. The Center of Gravity (CG) of the refrigerator is imagined to be in the very MIDDLE on one of the shelves. If you were to push on the refrigerator somewhere above the CG then you will probably push it over (the refrigerator won't move) and you won't feel much pressure on your feet. Now, if you push on the refrigerator 2" from the ground, you will slip on the ground (not getting traction) and the refrigerator won't move. If you push on one side of it then it will turn and not go straight.
But somewhere in between the CG and the ground will be the "sweet spot" where you will get the most traction and the refrigerator will move the easiest. When changing the suspension, what you are looking for is the Least amount of power to move the car the quickest. This is Efficiency.

Center of Gravity (CG):
Imagine the weight of your car concentrated in an area the size of your fist and located, for example, on top of your shifter handle. The actual position will need to be calculated but this is just for a visual aid.

Instant Center (IC):
Ladder Bar- The IC is the front ladder bar mounting hole.

Four-Link- The upper and lower bars are angled toward each other. The IC is the imaginary point of intersection if you were to draw a line along the length of the lower and upper control arms forward.

Percentage of Rise (PR):
Percentage of Rise (PR) is best used for ladder bar cars but a few do use it for 4-link cars. Imagine a line drawn down to the ground from the Center of Gravity (CG) like your shifter handle in the example above. Now draw a line forward from the contact patch of the rear tire through the line from the CG (the line should be drawn so it is below the CG). This intersection is the PR.

Getting back to the CG. Let's say the CG is 26" above the ground, for example, sitting on top of your shifter handle. This 26" is represented as 100% and 13" (halfway up from the floor) is represented as 50%.

When you found the PR (when you drew the imaginary line from the contact patch through the virtical line from the CG) it intersected it at, let's say 18" from the ground. Then the suspension is said to have a PR of 69% (18" / 26" = 69 or 69%). Generally, cars equipped with automatic transmissions need a PR greater than 50%. Cars equipped with manual transmissions need a PR less than 50%.

The HIGHER the PR the HARDER the suspension Hits the tires. The LOWER the PR the SOFTER the suspension Hits the tires.
Manual transmission cars already hit the tires plenty hard when the clutch is dropped so need very little PR compared to an automatic equipped car. By comparison, automatic cars (no T-brake) don't hit the tires hard at all.


Finding the center of gravity height can be done in several ways, none of which are accomplished very easily and without some work. Presented here is the easiest method. The center of gravity height is calculated by weighing the car when level and then raising the car at least 10 inches at the rear and weighing the front again. Enter the data into the program below to calculate your center of gravity height.

Before you begin:


Be sure that all fluids are full

Replace each shock absorber with a solid link to eliminate suspension travel

Make sure the tires are inflated to the maximum pressure as specified by the manufacturer to eliminate any sidewall flex

Note: If these steps are not taken, the calculations will be inaccurate

Center of Gravity Height Formula

Definition of Variables

* CGH - Center of Gravity Height
* WB - Wheelbase (inches)
* TW - Total weight
* FW1 - Front weight LEVEL
* FW2 - Front weight RAISED
* FWc - FW2 - FW1 (change in weights)
* HT - Height raised (inches)
* Adj - Adjacent side (see below)
* Tan q - Tangent of angle (see below)
* CLF - Left Front tire circumference
* CRF - Right Front tire circumference
* C - (CLF + CRF) / 2 (average circumference)
* r - Axle Height

Related Formulas



We will list some of the common problems you may encounter with the Briggs motor on Methanol and the most typical solutions. Some of these can be very frustrating at times but if you check everything completely you'll find the problem.

NO COMPRESSION - This problem exhibits itself when you try and turn the motor over by the cranker and cannot feel it build resistance as it comes up to TDC.

. Blown Head gasket - Replace gasket and double check the head bolt torque after running the motor once. The area around the bolt on the exhaust side of the cylinder is where 90% of all failures occur.

. Stuck Exhaust valve in the open position -This is most often from a close fit of a new exhaust valve guide. At the track you can see the valve stuck open by removing the plug. Try and push the valve down with a screwdriver and then shoot some penetrating oil on the stem. Once the motor cools down you have a shot at making it run and wear in the needed clearance. The best way is to go back and hone the guide for proper fit(.002-.0025). A loose guide will also lose compression.

. Bent valve - On rare occasion either valve can get bent by hitting the cylinder head and thus loose seal. Simply replace the valve if it will not seal after attempting to re-lap it.

. Bad Seat - this one will drive you crazy!. If the seat is loose it will loose seal as the motor heats up and you motor will exhibit many symptoms such as skipping or no real power. Sometimes the engine will cut off after it gets up to temp with a loose seat. Drive the seat back down in it's pocket first. If this does not fix the problem replace the seat with a new one or use a high temperature seat locking sealant that you can find at automotive stores.

SKIPPING - With a methanol motor where the motor skips is of importance in isolating the root cause and fix.

. Skipping at the end of the straight or max RPMs. - lower your jet size. Normally shows up when the weather changes from winter to summer and the air gets thinner(worse). Go down at least two jet sizes(.002). A bad coil can also cause the problem(especially when hot) or a fouled plug. Change the plug first then start looking at the other solutions.

. Skipping in the turns - This is most often a problem with the fuel being thrown out of the small pick tube's pocket in the tank. Most newer tanks from Briggs have rather large slots built into the small pickup tube area to let excess fuel flow back into the main tank portion. These large slots can contribute to the fuel being tossed out in high G turns. The solution is to find Briggs part # 555520 which is a small spring like piece of steel that goes into the small pickup area and decreases the slot size.

. Low/medium speed skipping - Timing is off. Check you timing again to insure the flywheel has not slipped. If it did then re-lap the flywheel to the crankshaft and torque to 70 Ft. .Lbs.


Tuning with Temperature

Lately a lot of questions have come in regarding temperature - more specifically cylinder head teamp, CHT. Maybe it's the few over 90 degrees race days followed by 50 degrees nights that we see this time of year, but there's definitely some aggravation about tuning the Briggs. Tuning with temp will be the focus of this tech article, but even if you're not experiencing tuning or temp problems I hope that there will be something here of interest for everyone.

Since an aircooled engine is very sensitive to temperature (atmospheric) changes and relies on outside air to cool itself, we need to understand better how to use temperature to our advantage - i.e. make more power without hurting parts. This can easily be accomplished by at-track tuning - I say "easily" because it really is.

There are several methods of increasing or decreasing CHT. A quick list would include fuel jetting (fuel-air ratio), spark plug heatrange, exhaust pipe diameter and length, flywheel screens, RPM, and ignition timing. There are more complicated ways, but these are all very simple and can be done at the track and between races if necessary.

My approach to tuning with temp varies from some other engine builders undoubtedly, but the fundamentals are sound. Every engine builder will agree that to get the most umpff out of your engine - a basic understanding and a willingness to make tuning adjustments (the right ones), is necessary to make your engine a front runner instead of a backmarker. I prefer to run as much fuel through the engine as I can. But you need to be able to burn the fuel completely to get the most power from your engine. Burning efficiently as much as quantity.

First a few really basics - Leaner is...turning your needle screw in, clockwise, or going down in jet size -i.e. from a 52 to 50 jet - These jets are tuned and replaced much like those in a Holley carb. Although unlike a Holley, these numbers represent decimal sizes .052 and .050 respectively - This is fifty two thousandths of an inch in diameter - so you see that your Briggs is very sesitive to fuel tuning adjustment. Richer is....turning the needle out, counterclockwise, or going up in jet size, i.e. from a 52 to 54. Richening, or fattening as it is sometimes called, allows more fuel into the engine which in turn cools the engine. This is very important to understand correctly and many karters, even some who have been racing for years, still don't get it. Richening an engine will cool it, leaning an engine will increase temps - this is most easily seen in exhaust temps, or EGT, but since most everyone uses CHT, we will concentrate on cylinder head temp. in this article.

When air density is greater, your engine requires more fuel to maintain the same engine temp. While barometric pressure, humidity, and air temperature all factor into air density, air does not normally change significatntly during the course of any one heat. However, quite often we qualify or hotlap under sunny conditions and warm temps. While an approaching storm front can quickly change barometric pressures - (Remember the weatherman talking about warm fronts, cold front, high pressure, and lows-- see the weather channel), even a dark cloud passing the sun can change conditions enough to merit making changes to your engine. Okay, so we're getting pretty involved in weather stuff now and i"ve been accused of being an amateur meteorologist before, so let's move onward. - if you've got an air density gauge, you probably already know how to use it and this stuff is old hat - If you don't have an air density gauge - consider buying one - two sources are Kinsler Fuel Injection, 1834 Thunderbird, Troy, MI 48084 (248) 362-1145; and Speedway Motors, 300 Speedway Circle, P.O. Box 81906, Lincoln NE 68501-9896, (402) 424-4411. Give them a call.

Tach's & Data Recorders

Tuning Using Exhaust Gas Temperature
This article is reproduced from Fox Valley Kart .
NOTE: This article was designed for go-cart use, but all the same principles apply to Junior Dragsters.

By Kurt Huber and John Copeland

More and more serious racers are taking an alternative approach to judging the condition and performance of their engines. By using exhaust gas temperature they have added a powerful diagnostic and tuning tool to their arsenal of racetrack weapons. Exhaust Gas Temperature (EGT) measurement has been a fact of life in other forms of motorsports for years, yet it's use in karting has been relatively limited until recently. Here's the skinny.

If combustion was a perfect process, the exhaust gas from an engine would contain only nitrogen, water vapor, and carbon dioxide. But in the real world it also contains carbon monoxide, hydrogen, unburned fuel, other hydrocarbons, plus traces of aldehydes, alcohols, ketones, phenols, acids, nitrogen oxides, carbon, and lots of other stuff. And that's assuming that we're starting with conventional, legal fuel. There has been lots of things written about illegal additives and how dangerous they can be. Please think before you or anybody you know add anything extra to your fuel. Think about the price you or your friends or family might pay, health-wise, just because someone is looking for an edge. But this article is not about what fuel is composed of, but rather about the temperature of the combustion products and how their measurement can be an even bigger edge.

In the temperature measurement industry their are two basic types of measurement devices. The first is called resistance temperature detection or RTD. This type of device is basically a very fine wire encased in a container, or bulb. As the temperature of the bulb changes, the electrical resistance of the wire changes. By passing a small current through this wire and measuring the resistance, the temperature can be determined. This is the method used by your Digatron cylinder head temperature gauge. And as any of you knows who've used a CHT GAUGE for any length of time, these CHT sensors, while accurate, are relatively delicate. Rough service (like on a kart) is not generally recommended. They also have a temperature limit that makes them unsuitable for use measuring EGT.

The other major means of temperature measurement is the thermocouple. The thermocouple is a unique device. There are several different types of thermocouples, using different materials for different temperature ranges, but they all operate by the same basic means. A thermocouple consists of two wires, of different materials, welded or fused together. For the temperature range we are most interested in, the type K thermocouple is most suitable with a maximum temperature of 1900 degrees Fahrenheit. In a type K device one wire is an alloy called CHROMEL®*, and the other an alloy called ALUMEL®*. A small portion of each wire is exposed and the two are welded or fused together. That assembly is encased in an electrically insulated sheath and the other ends of the wires are connected to a very sensitive voltmeter. Now here's where the thermocouple differs from the RTD. When the fused end of the thermocouple wire is heated, it generates it's own current. It's only a matter of millivolts (that's one one-thousandth of a volt), but the voltage generated is an accurate indicator of the temperature of the end of the thermocouple. A real bonus for motorsports is that these thermocouples are remarkably sturdy and reliable. With no delicate parts to break, unless you exceed their maximum temperature, they're pretty hard to damage. In fact, every gas- fired furnace and water heater uses one to tell the gas valve that the pilot flame is lit.

The thermocouple probe is carefully fitted into the exhaust system, relatively close to the engine. For maximum accuracy you want the tip of the thermocouple to be centered in the exhaust gas stream as it comes out of the engine. But there is considerable debate about how close to the engine it needs to be. Digatron's information advises mounting the probe between 3 and 4 inches from the piston face. But many snowmobile racers routinely set their EGT pickups as much as 8 inches from the exhaust port. In fact, it really doesn't matter exactly where the probe is mounted, although the closer to the exhaust port, the less the ambient air temperature will cool the header and affect the readings. One word of caution, however. Comparing EGT readings between engines or karts whose EGT probes are not mounted exactly the same distance from the piston will get you in trouble. If you use EGT, mount the pickups in all your headers at the same length. Otherwise you might just misinterpret the readings.

On the subject of mounting the EGT probe, there is some concern among 4 cycle racers about disrupting the gas flow in the relatively small diameter headers that are most common on 4 stroke engines. Introducing the probe, with a diameter of about .125 inches, about 1/2 inch into a 1 inch diameter header will consume about .0625 sq. inches of header cross section. That's about 8% of the total area. To test just what effect this might have on the absolute flow, we checked it on the flow bench. Our testing revealed that fitting a Digatron EGT probe into a .990 diameter header reduced the flow by __%. This is approximately the same flow as using a .960 header. You may want to factor that into your pipe selection if you're going to use EGT on your 4 cycle. You'll also want to factor in the value of knowing what your air/fuel ratio is doing versus whatever minor loss is exhaust flow there might be.

There are lots of myths and questions surrounding EGT and it's use. Some folks figure that, if you have a Cylinder Head Temperature gauge (CHT), you already have all the information you need, and that EGT is redundant. While you can certainly get by on just head temp, CHT and EGT each tell you slightly different things, and using them together tells you some things that neither one could tell you alone. EGT has some advantages because of it's basic construction and it's mounting location. A thermocouple responds very quickly. Because the CHT sensor has to respond to the temperature on the outside of the head, it cannot respond to changes in combustion temperature as fast as the EGT probe that is directly in the exhaust gas stream. Secondly, the EGT probe is not exposed to the outside air, it is not affected by changes in outside temperature. By comparison, since the CHT is measuring the temperature of the cylinder head casting itself, and since the cylinder head is one of the engine's primary means of shedding heat to the air, the cooler the air, the cooler the CHT reading and vice versa. For quick, consistent temp readings, EGT is definitely worth a look.

But what exactly are we trying to determine with these temp sensors, anyway? EGT and CHT are simply ways of trying to judge the relative fuel/air ratio. We all know how critical it is to have the carb mixture correct, whether by changing the jet in a 4 cycle, or by adjusting the carb needles on a 2 cycle. And it's generally agreed that the leaner the mixture, the hotter the engine will run. But what is really happening inside there? Does hotter always mean better, or just sometimes?

Well, the truth is, it's mainly a matter of air. Many of you have had the experience of hitting the set-up just right in practice and then waiting excitedly for the race, certain you're going to blow 'em all away this time. But when the time comes for your race to start, suddenly you've lost that wonderful top-end RPM you had in practice, or the clutch just won't pull like it did in practice, or some other problem pops up to spoil your day. You havn't changed a thing, but the air may have changed things for you! As the air temperature goes up, or the humidity goes down, or a storm front blows in, the density of the air changes, and that changes the fuel/air ratio that your carb delivers. If you don't recognize what's happening and adjust accordingly, your going to suffer.

So how can you stay on top of the effect that changing air conditions is having without bringing your own weatherman with you to the track? With a EGT gauge you can take alot of the guesswork out of carb tuning. 'Remember we said that it was generally agreed that a leaner fuel/air ratio was always hotter. And when we asked if hotter was always better? Well, you guessed it, neither one is true. If you get the fuel/air ratio too lean, the combustion temperature will actually go down! Let's look at another example of this, one that you can actually see with the naked eye. An Oxy- Acetylene torch will burn with a wide variety of fuel/air ratios. Generally when you light the torch the mixture will have too much fuel (acetylene) for the amount of oxygen that's flowing. The flame will be yellow and produce alot of smoke, and not be very hot, relatively speaking. But as you turn up the oxygen valve, the yellow flame and smoke disappear, the flame turns bright blue, and the flame temperature goes up dramatically. So leaner here is definitely hotter. But as you continue to turn up the oxygen, the flame begins to shrink, and the flame temperature actually goes down, even though it's leaner! Eventually, if you keep turning up the oxygen, the flame will just go out! Believe it or not, the same thing happens inside your engine.

"Wait a minute," you say. "I know that when I lean the engine out it just keeps getting hotter until it sticks!" If all you have to go by is CHT you're absolutely right. When your engine gets too lean, the skyrocketing temperature you see on the CHT is probably not really an indication of hotter combustion. Most likely it's a warning sign of DETONATION. Detonation is the collision of two flame fronts inside the combustion chamber, where there should be just one, and it's the single biggest cause of heat related engine failures. Savvy drivers can often sense that an engine is slowing down and richen up the mixture to control the detonation. But you don't need decades of experience to spot detonation before it puts you on the trailer for the day. Just like with the Oxy-Acetylene torch, when the mixture gets too lean, the flame temperature goes down! Detonation floods the combustion chamber with heat, so the CHT goes up, but with CHT and EGT readings, if you see CHT rising and EGT going down, it's a sure sign of detonation.

A quick adjustment will restore the power and save that expensive rebuild. Even with just EGT, it's alot easier to get the most out of your engine without burning it down. EGT should climb as the RPMs come up on the straight, then drop when you lift for the corner. If it drops when you're pulling off a hard corner, or under acceleration, you're on the detonation expressway back to the shop for a rebuild. Detonation is a fascinating subject, one that is too complicated to be handled adequately here. We'll save that for another article. But trust that it is something to avoid, and the best way to avoid it is to watch the exhaust gas temperature.

So to summarize, we know we want to run the fuel/air ratio as close to ideal as possible. And we know that the ideal fuel/air ration should produce the hottest combustion flame. While the cylinder head temperature gives us some indication of the combustion temperature, it can be misleading because of air temperature or other weather conditions. Because of the mass of the cylinder head, CHT can take a few seconds to register a change in internal temperature. And CHT alone is not the best indicator of detonation. Exhaust gas temperature does all these things better that CHT; better, faster, and more reliably. So what's holding you back? If someone came up with a clutch that was better, faster, and more reliable, you'd be after it in a second. Why is this any different? Remember, the more you know, the faster you go.

*CHROMEL® and ALUMEL® are registered trademarks of Hoskins Manufacturing Company.

Chassis Dynamics


Blueprinting a Briggs Stocker

By Joey Padgett of Checkered Flag Fuels
World Karting Magazine, November, 1994

Nobody runs a box stock Briggs 5hp engine in WKA races. Everybody has "blueprinted" their engine, increasing the performance from the stock 5hp configuration to a considerably higher output. All these modifications and the increased performance hopefully falls inside the rules governing the Briggs Stock 5hp classes racing in WKA.

When it all began, Briggs racers bought a bone-stock 5hp engine, raced it for a while, then took it apart and rubbed on it, or had a buddy "who is real sharp with Briggs" rub on it till it ran faster and faster each race. The Briggs Box Stock engine rules, currently used by the WKA, mirror what evolved over time as modifications by some of the best "blueprinters" in WKA racing.

Today, a top Briggs engine shop will buy new engines, season them in their own way and then begin the blueprinting process. Usually, the engine is torn down to the bare block, all machine work done after carefully measuring the stock engine. A blue printed carb and modified cam are added, the engine is run in on the dyno. The cost for all this ranges from $500 to $700 depending on the engine builder and the end result.

Joey Padgett and his crew at Checkered Flag Racing Fuels have always wondered how each stop of their engine blueprinting process affected the overall performance of the Briggs stock 5hp engine. Well, to satisfy their curiosity, and to educate and inform WKA members, the WKA donated a Box Stock 5hp to Padgett and crew to blueprint and dyno in a step-by-step process, carefully measuring the effect of each change along the way.

The Checkered Flag crew for this test consisted of Joey Padgett, chief engine builder, and Brandon Creedle, chief dyno operator and grunt labor. Checkered Flag has a fully equipped machine shop and their shop's talents at building Briggs 5hp engines are nationally recognized with several WKA National wins. They use a Stuska water brake dyno and have worked very hard at getting accurate and repeatabe results from their dyno.

1. Engine Break-In
The first step in preparing a race engine from the Stock 5hp Briggs out of the box (hence, box stock), was to run the engine in on the dyno for three hours at 5800 rpm. This was done, monitoring the temperature and putting the engine under a small load on the dyno. The goal was to season the block and get all the moving parts familiar with one another.

A dyno pull was made after engine break-in on the stock but now seasoned engine. A reading was made at 3600 rpm., since Briggs uses that rpm figure for establishing their 5hp rating. The engine made 5.09 horsepower at 3600 rpm, just a tad more than Briggs claims for this engine. Futher runs were made to establish the power curve of the engine in the rpm range where we expect to run the engine on the track.

RPM Torque HP*
3600 - 5.09
4750 .880 4.48
5000 .750 4.02
5250 .670 3.77
5500 .560 3.30
5750 .430 2.65
6000 .330 2.12
Average .603 3.39
*All dyno runs in this article are corrected to standard temperature, humidity and atmospheric pressure to reduce any weather related influences. Torque figures are gauge readings, and not actual corrected torque.

The step-by-step modifications to e performed on this stock engine were the same as done to each blueprint Checkered Flag engine. The sequence of the modifications was selected to resemble what a typical racer might try if he were blueprinting the engine himself. Also, the sequence was selected to minimize the amount of teardowns and was accomplished with tearing the motor down only twice.

Exhaust Change
The stock Briggs muffler was removed and a .930 triple stage header was installed on the stock Briggs 5hp test engine. Again, the engine was running on pump gas with 21 ounces of 30W petroleum oil in the sump as per Briggs standard recommendations.

RPM Torque HP Change %Change
3600 - 5.52 0.430 8.44
4750 1.145 5.79 1.310 29.94
5000 1.060 5.65 1.630 40.55
5250 0.980 5.48 1.710 45.36
5500 0.905 5.30 2.000 60.60
5750 0.720 4.41 1.760 66.41
6000 0.640 4.09 1.970 92.92
Average 0.908 5.12 1.730 59.41
Obviously, the stock Briggs muffler was very restrictive to the exhaust flow. The huge increase in horsepower from 5000 to 6000 rpm reflects this.

Change to Menthanol Fuel

RPM Torque HP Change %Change
3600 - 6.35 0.830 15.04
4750 1.240 6.27 0.480 8.29
5000 1.120 5.96 0.310 5.49
5250 1.040 5.81 0.330 6.02
5500 0.950 5.56 0.260 4.90
5750 0.740 4.53 0.120 2.72
6000 0.650 4.15 0.006 1.47
Average 0.957 5.38 0.260 5.08
The change to menthanol fuel produced the biggest gain on the bottom end of the power curve and diminished as the RPM increased. The temperature indicated that a smaller jet may have produced more power with the rest of the motor in the stock condition.

Blueprinted Carb

The stock carb we had been using for the test up to this point was flowed and blueprinted and flowed again. Before the blueprinting, the carb flowed 21.03 cfm, after the blueprint job, it flowed 24.36 cfm, an increase of 16%.

RPM Torque HP Change %Change
3600 - 6.57 0.220 3.46
4750 1.320 6.66 0.390 6.22
5000 1.220 6.48 0.520 8.72
5250 1.080 6.02 0.210 3.61
5500 0.980 5.73 0.170 3.06
5750 0.830 5.07 0.540 11.92
6000 0.660 4.21 0.060 1.45
Average 1.015 5.70 0.320 5.95

The blueprint carb was a good change for the bottom of the power curve, but the .052" jet may have put a hole in the fuel curve as indicated by the percentage change around 5500, 5750 and at the top rpm at 6000. Further modifying this engine should take advantage of the extra rich fuel curve on the top end of this motor.

Deck Block and Mill Head

Now the machine work begins. Taping off all the openings, the engine was left assembled except for removing the head. The head is milled to the specs listed in the WKA tech manual. The block was decked so that the piston protruded out of the block .0035". The engine was cleaned off and the head reinstalled with a stock gasket and the bolts torqued.

RPM Torque HP Change %Change
3600 - 6.69 0.120 1.83
4750 1.140 7.16 0.500 7.51
5000 1.320 7.06 0.580 8.95
5250 1.220 6.85 0.830 15.45
5500 1.080 6.35 0.620 10.82
5750 0.950 5.84 0.770 15.19
6000 0.820 5.26 1.050 24.94
Average 1.130 6.42 0.720 12.63

The engine liked more compression and the re-chambering of the combustion chamber. A 12% average increase is great for just doing some minor machine work. Importantly, this increased the engine's ability to work better at the higher rpms and produced the most significant increases above 5000 rpm.

Porting and Valve Job

The ports were de-burred and enlarged, using information from the flow bench as previously developed by Checkered Flag. It's very easy to get the exhaust port too big, so just a general deburring and smoothing of the port was done here. Most of the port work was concentrated on the intake port. RPM Torque HP Change %Change
3600 - 7.02 0.330 4.93
4750 1.520 7.73 0.570 7.96
5000 1.420 7.60 0.540 7.65
5250 1.340 7.53 0.680 9.93
5500 1.190 7.01 0.660 10.39
5750 1.100 6.77 0.930 15.92
6000 0.920 5.91 0.650 12.36
Average 1.200 7.09 0.670 10.44
Joey felt he should have just de-burred the ports and made a dyno run so we could have compared exactly what a good port job would mean as compared to a karter just de-burring his ports with a hand grinder at home. Again, this porting work is just moving the power band up the rpm scale and allowing the earlier changes on the carb and fuel to take full effect.

Honing Cylinder and Clearance Rings

The engine was disassembled and the block was honed out to a 0.003" oversize. The ring end gaps before honing were: Before Hone: Top 0.016" 2nd ring 0.009" Oil ring 0.016" After Hone: Top 0.002" (using a 0.010" in a std. bore) 2nd ring 0.018" (same ring w/0.003 more gap due to hone) Oil ring 0.090" (ring thickness ground down from 0.130 to 0.095) Cylinder had 0.0035" clearance on the piston with .002 out of round and 0.001" taper. After Hone: 0.0065" piston clearance, 0.000" out of round and 0.000" taper. RPM Torque HP Change %Change
3600 - 7.08 0.060 0.85
4750 1.550 7.92 0.190 2.20
5000 1.480 7.96 0.360 4.74
5250 1.380 7.79 0.260 3.45
5500 1.290 7.63 0.620 8.85
5750 1.180 7.30 0.530 7.83
6000 1.060 6.84 0.930 15.74
Average 1.320 7.57 0.480 6.77
The engine picked up at the high rpm due to the decrease in ring drag and reduced internal friction. The engine runs the longer races, these lower temps will result in reducing the internal friction and engine wear.

Synthetic Oil

Up until now, all runs had been made using 21 oz. of 30W Petroleum oil as Briggs specifies. On this run, 14 oz. of a light synthetic oil generally used by most karts was substituted.
RPM Torque HP Change %Change
3600 - 7.10 0.020 0.28
4750 1.585 8.03 0.110 1.38
5000 1.505 8.03 0.070 0.88
5250 1.395 7.81 0.020 0.26
5500 1.320 7.75 0.120 1.57
5750 1.230 7.55 0.250 3.42
6000 1.120 7.17 0.330 4.82
Average 1.359 7.72 0.150 1.98

Not a huge gain here - maybe if we'd used some of the "trick" (i.e. illegal) oils here, we might have seen more gain. In fact, Checkered Flag sells some of the base substances to mix with synthetic oils to make it "hot." Their own dyno testing shows little if any real gain on the dyno.

Ignition Timing

The stock timing from Briggs for all previous dyno runs had been 21(. Using a #5 offset timing key, it was advanced to 28).
RPM Torque HP Change %Change
3600 - 7.25 0.150 2.11
4750 1.600 8.16 0.130 1.62
5000 1.500 8.05 0.020 0.25
5250 1.430 8.06 0.150 1.90
5000 1.340 7.91 0.160 2.06
5750 1.280 7.90 0.350 4.64
6000 1.170 7.54 0.370 5.16
Average 1.387 7.94 0.220 2.85

Again, just small gains with the timing advanced, all under a 5% gain. A Limited Modified engine with its more radical cam timing will show more results from the advanced ignition timing.

Camshaft Change
The stock Briggs cam was replaced with the ZX3 cam from Checkered Flag. This specifically profiled cam works the best in all their stock class engine blueprints.

RPM Torque HP Change %Change
3600 - 6.88 0.370 -5.10
4750 1.670 8.52 0.360 4.41
5000 1.620 8.70 0.650 8.07
5250 1.560 8.79 0.730 9.06
5500 1.460 8.62 0.710 8.98
5750 1.380 8.52 0.620 7.85
6000 1.310 8.44 0.900 11.19
Average 1.500 8.60 0.660 8.31

The engine initially lost power at 3600 rpm with the cam change because that rpm was out of the power curve for this cam profile. This cam provided a good power increase from 5000 to 5500 and another great power surge at the very top rpm of 6000.


The best horsepower number for this test engine came on the final run at 5250 rpm where it made 8.79 horsepower. Joey says the average engine out of their shop is 8.75 to 8.9 horsepower, so our test engine is right on target. The best WKA legal engine they have seen on their dyno is just over 9 hp!

Reviewing the step-by-step blueprinting process:
Modification Avg. HP Peak HP Gain
Exhaust Change 5.12 5.79 59%
Menthanol Fuel 5.38 6.27 5%
Blueprint Carb 5.70 6.66 6%
Deck Block, head 6.42 7.16 13%
Port Work 7.09 7.73 10%
Hone & Clearance 7.57 7.96 7%
Synthetic Oil 7.72 8.03 2%
Reset Timing 7.94 8.16 3%
Change Camshaft 8.60 8.79 8%
Total Blueprint 170%
Your results may vary according to how your dyno is set up and operated. It is very evident that the biggest gain was in the exhaust pipe and in the machine work and porting. The WKA has sought to restrict what can be gained from other things like camshafts and carbs, trying to even the playing field for all racers.

A blueprinted engine from Checkered Flag just like the one we have here would cost about $600. Just the machine work, decking the block ($20), porting the cylinder ($50) and milling the head ($10) would cost $80 and are the items the average karter would not have the equipment to do in his garage at home. But then, the experience and expertise of shops like Checkered Flag are sometimes worth the price of buying the total engine from them.

There are many local engines builders in your area that can preform the work that you have just read about. You can find a list of some of them at http://www.gakarting.com/dealer.htm , give them a call.


Proper Engine Break-In

It is important that when breaking your motor in that you do so under load. - Under load means that the engine can't free-wind RPM - The best way is to bolt the engine on your kart just as you would to race it. Be sure kart is sitting on the ground. NEVER attempt to run engine on kart stand --this is a quick way to destroy your engine. Turn the engine (kart) completely upside down for a second just prior to adding fuel and starting. This helps get oil to the top end of the engine before startup.
Be sure that the piston is at top dead center (that's where the PTO shaft keyway is at 12 o'clock straight up.) Turning the engine upside down now allows the oil to coat the lower end of cylinder walls, piston, rod and crank jouirnals as well as the top end parts like springs, valves, and valve guides. Use alcohol fuel as usual and fire it off. Let the motor idle for 2-3 minutes to gain some temperature. Do NOT use a hot (high heat range) plug to gain temperature quickly.
Keep the motor running - Don't let it stall. If it does, quickly turn the engine over two complete revolutions by hand and then refire. When engine is about 200 degrees and increasing, start driving the kart in a smooth large circle keeping steady RPM under 3500, always keeping the engine under moderate load. Run engine for ten minutes in this manner, then take a couple cool down laps, just coasting.
After a couple of cool down laps, bring the kart in. Shut off the motor. Hand turn the crank over two revolutions and stop on the compression stroke at TDC; cap the exhaust off with a tennis ball, duct tape or whatever you choose - This allows the enigne to cool down slowly so as not to crack or warp valve stems, etc. Turning the crank over to TDC also allows the valve springs to relax at full length rather than cool down while they are compressed. This will retain good spring tension which results in increased spring life. Now it is very important to drain all oil while the engine is still good and warm. There are magnetic drain plugs in the motor so the magnet picks up any metallic particles floating around or settling in the oil. Clean off the end of the magnetic drain plug every time you drain oil.
Re-torque bolts - New motors tend to shake loose any improperly tightened bolt as well as some that were just fine. Especially check carb-to-tank as they are affected by fuel swelling the gasket; and carb-to-block, as they loosen with vibration. Do NOT overtighten. Due to improved gasket technology, head bolts usually do not lose torque. Do NOT re-torque hot. It is very important to do this after the motor has cooled. I generally check the spark plug for good combustion. Check for lean versus rich condition, etc., but replace plug with another quickly so that cool air doesnt' rush in on top of the piston.
Refill crankcase with remaining 16 ounces of break-in oil. Refire engine, and repeat previous ten-minute break-in procedure.
As temperature increases start using more throttle coming off of turns and more or less coasting down straights. After just a few laps your engine should be to 365 or greater temperautre; now go full throttle off the corners - really lugging the engine down and coasting with NO throttle down straights. With each lap, increase full throttle on time until you are nearly up to race speeds and braking. Take one or two more laps just coasting at moderate speed to cool the motor some - If temeprature becomes a problem, keep richening the carb adjustment to keep temps below 390 degrees for beak-in. Try to keep temperature at 385 degrees optimal. (This is generally the temp we race also.)
Drian alcohol and run engine on gas for at least two minutes to clean alcohol out so carb pick-up tubes will not clog. Drain oil, and re-torque bolts. after all oil is drained, I like to slosh some fresh oil around in the crankcase and drain it as well. This helps get rid of any additional metal particles which did not drain with the hot used oil. Then put in your 14 ounces of Cool Power (Green Synthetic) oil. By putitng it in NOW, you won't forget it.
This entire break-in oprocedure will last around 30 minutes total, so be sure to allow yourself sufficient time to break-in your engine as intructed. THIS IS NOT SOMETHING TO BE DONE ON RACEDAY. Proper engine break-in is essential for increased performance and durability.

Dyno Myths and Truths

Now while I certainly don’t claim to be an “expert” on dynamometer theory, I do have quite a bit of experience using different types of dynos. Even the most simplistic hydraulic dyno can yield very valuable information if that information is gathered and analyzed correctly. A simple hydraulic pump with a pressure gauge can be used to measure engine output directly. Your kart mounted Digatron gauge will record the engine’s rpms for you. At least you will have a number for measurement comparison. But without a baseline for comparison, this newfound number is absolutely worthless. The #1 problem with dyno numbers is not the dyno itself, but rather, the user. Just like in most airliner crashes, almost always the fault will be that of the pilot over something mechanical. You see, a machine doesn’t think, or “over-think”. That is the biggest hurdle most newbies in the dyno room have. The most important part of keeping useful information from your dyno is that it is all recorded consistently. That’s right, consistent! If you tested one day when the hydraulic oil was 140^, then always test at that temperature. If the ambient air temperature was 70^ when you tested, then make ALL tests at 70^! If the barometric pressure was 29.90” when you tested previously, then you must always test at 29.90“! Now, obviously it is difficult, at best, to match every circumstance, but you honestly have to be as close as possible. Then, and only then, do you want to run calculations that “correct” to the previous test session standards. To make corrections on a test session is essential to most closely simulate the previous testing that you have done. That way you know you are comparing apples to apples. But how do you correct everything? First try to eliminate as many variables as you can. In other words, build yourself a dyno “cell” or chamber so that the room conditions can be monitored very easily. A small room is easily heated or cooled, so ambient air temperature will be the easiest to keep repetitive. A simple thermostat in the room will keep the room within a degree Fahrenheit. Humidity is somewhat more difficult to change through humidifiers and dehumidifiers…honestly, we don’t try to make the humidity level stable in our test cell, although we do keep very accurate and timely records of the humidity levels in the room at all times. Barometric pressure would be the most difficult to create without pressurizing the room, etc…This would be a very expensive venture, and I seriously doubt that any kart engine builder has this capability. We will undoubtedly have to correct for these variances from test to test. Be certain to record actual aneroid barometric pressure measured in the room, and check it periodically throughout your testing, as it does change slightly. You will also need to know your exact elevation in feet above sea level. While The Weather Channel is a great source for most families, you have to realize that much of the information given there is standardized, or “corrected” already. Keep in mind that it is corrected for the elevation, etc, of the location in which the broadcast is being made, not necessarily the location that the information was reported, or for your shop’s location either. Often times a call to your local airport can yield some helpful information. Pilots are always interested in elevation and aneroid (actual, not corrected) barometric pressure, as it gives them a more reliable form of air density. Air density is important to a pilot because the horsepower generated by his plane’s engines is dependent upon how much oxygen it can swallow, and how to most efficiently match the fuel curve of that particular engine to maximize horsepower and fuel economy. Remember, that it requires horsepower to move a heavier object, and extra fuel load on a plane is just extra weight it must power around in the sky. While thousands of dollars are spent on titanium and lightweight composites in a plane’s construction, it would be useless to just “top off” the tanks before every flight. A plane will only carry the amount of fuel necessary to make that particular flight in most cases. This is particularly true in the ever competitive commercial airliner industry where an extra mile per gallon can mean more profit. For corrections for atmospheric conditions, we will utilize a Society of Automotive Engineers, or SAE, chart that will correct for atmospheric changes, and we can follow this chart religiously every time we record results. Ok, so now we have the room and atmosphere accounted for, but how about the dyno itself? If you are using a hydraulic dyno, this is pretty easy to monitor and influence. Fluid temps are handled quite easily by a heater and cooler (heat exchanger or radiator).. if you are into making your own stuff...consider a 4 core radiator out of a Ford E350 or pick-up, that has a transmission line run internally through the radiator. Fittings are already made onto it, and it is a pretty straight forward idea...blow a box fan on it when the temp hits your dyno's hydraulic fluid temp number, or if you are creative enough with your time, put an electric solenoid or servo on it to switch it on and off at a given temperature. Biggest thing is to keep your fluid temps stable, and always make your power runs at the exact same temperature. Use high quality fluid as well, and keep it fresh and clean. Hydraulic oil does wear out over time. Funny side story... We had a customer that was very happy with our engines and was running top three every night out. Another local builder used the "inflated numbers" sales pitch and lured the customer to take our engine to this engine builders shop to have it dyno'd. This "other" engine builder uses a hydraulic dyno with a pressure gauge and Digatron tachometer gauge. He bolts our engine down to it, and blasts a couple quick runs on it. While our engine is sitting there idling, he claims his engines make "tons" more power. He then places his engine on the dyno, and "wha-la" -- more power..."See", he says. It measured almost 2 HP more - and that's a BUNCH folks! Hmmm.... this other engine builder talked the customer into buying an engine from him right on the spot. With numbers like that - can't go wrong, right? The customer takes this new engine to the track, all excited to have this extra-extra power, only to find out he's only a fifth place kart at best now. The customer brings the engine down to me to ask what might be wrong. First I told him that if there is something "wrong" with the engine, to talk to the "builder" of it...but I think at this point of the game he's realizing he was snookered. Seems our engine was only used as an oil "pre-heater" for this engine builder's hydraulic dyno in his unheated garage. That’s’ right, it takes less power to move thinner warm hydraulic fluid than it does to move cold thick fluid. Oh yea, I forgot to mention that no corrections were ever used! At least a calculator and SAE chart were never even mentioned. So...you can't really "fool" a dyno...but you can have a "foolish" operator and "fool" a naive customer with a hydraulic dyno. Oh yea...some flowbenches work similarly. ;) My take on inertia dynos. Inertia dynos seem to be all the rave today. Inertia dynos are similar to chassis dynos in that they provide some "interesting" data. Clutch engage, etc. The biggest advantage of them, that I see anyhow, is that they are fairly simplistic by design...have very few variables, or parts to go wrong, and are CHEAP to build. Comparably speaking anyhow. :) Just the load cell for our dyno runs in the $1000+ neighborhood. I am certainly not an expert on inertia dynos so I can't vouch for their assets. I have my doubts, for sure, about some of the claims of being superior to a well designed hydraulic or water break dyno with decent data acquisition software. My biggest skepticism lies in the fact that we are working with an engine that does not throttle smoothly to begin with. A Briggs engine, in WKA stocker form, has a serious lull, or “flat spot” in the fuel curve a part throttle. This is most noticeable at about 4000 rpm. Interestingly enough, that is exactly where we are stalling our clutches at with the newer style cams today. This hesitation, or lull, can be audibly heard, and definitely shows up on an acceleration dyno. The bad thing is, that the rpm lull is never consistent from one run to the next. So you have to wonder just how much this little stumble at part throttle is affecting the total acceleration run. Obviously it will take longer to accelerate to the same target rpm if the engine stumbles worse around 4000 during one test than another. It is my personal opinion, and shared by some other talented engine builders, that this is the sole reason you see the use off offset throttle shafts so heavily today. The offset throttle shaft itself does not make any horsepower, nor does it flow any better than the standard one that came in the carb from the factory. In fact, it is often worse! What it does do, however, is create more pulse pressure, or vacuum signal, at the short stem of the carburetor. How does this help? Well, at full throttle it simply does not help, in fact, it often hurts. However, at part throttle, around 4000 rpm, the offset shaft will manage to pull a little extra fuel into the carb bore and it helps reduce the “lull at part throttle problem” that all Briggs have. This is especially true on a small .425 restrictor plate engine. Another point to consider, is that an inertia dyno is simple in design by spinning a known fixed weight to a target rpm. What rpm you start making pulls on the engine makes an enormous difference. Even 50 rpm one way or another can make a significant change. Ever tried to hold the rpm on your Briggs at exactly the same rpm for any length of time at all? While inertia dynos might be a great idea for translating horsepower from an electric engine or another source that goes through its rpm band smoothly and repetitively every time, it is not, in my opinion very valuable for accurately measuring the output of our little alcohol stumbling Briggs racer. Keep in mind that you are swinging about a heavy flywheel on the end of a shaft hooked to the business end of your Briggs race engine. Most inertia dynos have a flywheel that is huge and heavy. That flywheel has to be precision ground on all edges, not only to make it safer, but to make it as accurate as possible. If the flywheel is even a few thousandths out of round it will be worthless. Remember that steel is easily effected by temperature variances as well, and that under cooler temperatures, steel contracts, and under warmer temps it will expand. Albeit only thousandths, you have to take that into consideration when using a flywheel type dyno as well. Bearing drag, brake drag, shaft flex, and any other outside influences, including air density or turbulence created by the flywheel itself, can and will effect your results. As far as problems with any dyno...again, I think the biggest variable is the operator! Now, if you have seen accurate dyno numbers on your engine, you are probably wondering why in the world all your competitors are turning all these unbelievable high rpms! When looking at the dyno sheet of a typical stocker, it is not uncommon to see peak horsepower to be around 53 or 5400 rpm even using the newest cam designs. “What’s this? Outrageous! I told the cam grinder / engine builder to give me an engine that would turn upwards of 6500 rpms! And I get this pretty little print-out that says peak hp is 1000 rpm or more less than what I expected.” Well, welcome to reality. In oval track racing it is widely accepted that you will need to turn your little Briggs engine at 1000 or more rpm above peak hp that it produces. Why, you ask? Because of drag, loss of rpm in the corners, creating a wider power band, etc, etc. There’s lots of reasons why we all do this, but to make it seem clearer, consider this. If your chassis AND driver were to not scrub any engine rpm whatsoever in the corners. The tires, the bearings in the kart, etc, rolled without any friction or drag whatsoever, if there were no wind resistance at all, then you would theoretically be able to run the same engine rpm the entire way around the track. With this being the case, you would now want to run a much lower rpm and much higher mph around the track. But what about the starts and restarts, and that guy that got into the side of you down in turn two? You got it, you have to compensate for all these by turning your engine harder. In reality, we’re not too overly concerned with the peak horsepower that a particular engine makes. In fact I look more closely at average horsepower over the rpm band that the customer will be running the engine. I try to build an engine that suits his/her driving style and the track that it will be competing on. This is why so many engine builders would prefer to “custom build” your engine, rather than have them sitting on the shelf for the next person who walks in the door. You, as an educated racer, by now should expect this treatment as well. While anyone can take an engine off the shelf and sell it, it takes a knowledgeable engine builder to build your engine specifically for your needs. This is even more critical when looking at restrictor plate engines. While we don’t concern ourselves with peak HP numbers, isn’t it strange that nearly every engine builder “touts” or “claims” big high horsepower numbers. When in fact, it is not difficult to build a high HP number creating engine, that same engine has a very narrow power band and is about worthless to 95% of the racers out there today. In fact, a good consistently high average horsepower throughout the engines upper part of its power band is what we measure an engine against. Of course everyone has their own standards of expectation, and we are no different. To hear an engine builder claim 10+ horsepower on his WKA legal stocker is simply absurd. Well, on our dyno it is anyhow. While on his dyno, he may reach those elevated numbers, there is no way mathematically that you can achieve such a high efficiency of horsepower on a flathead engine that is 13 cubic engines, and flowing at less than 30 cfm at 15”! Physically it is simply impossible. That’s not to say that this other engine builders’ claims are false, simply that he is not comparing apples to apples. Take that same engine builder’s pride and joy and put it on our dyno, and you are likely to see a 8.25 HP engine, NOT 10 HP! Likewise, I’m sure that if that engine builder took one of our engines and put it on his dyno, he would likely see similar numbers to that shown of his own engines, 10+. Typically, a WKA legal stocker will make 8.2-8.5 hp between 4900 and 5400 rpm. We can always move the power curve up and down slightly with exhaust configuration, etc, but the number will be predictably similar. Bottom line... Don't buy numbers...buy performance!

RAPTOR III Piston Tech

Recently, a karter who builds his own engines asked for some help setting up his Raptor III piston. The following is a reply to his e-mail asking what cylinder clearances etc. for use on the Raptor III piston.

Honestly I think that will be up to builder preference as always. Here's what I can tell you though...I have tried the new pistons at .002, .004, and .006. FM reccommends that you run as tight as .0005 to .0015. The one thing that I think they fail to realize is that most of us run methanol fuel which has a tendancy to wash our synthetic 0-weight oils off the cylinder. Coupled with the fact that we don't necessarily run these engines in the cleanest environment, ie air filtering is not as important to racers as air flow. Racers also have a tendancy to run these engines on the warm side if there's only a few laps to go, and they are leading a race. Thermal expansion and contraction has always been a problem with the cylinders and pistons. Although I saw no problem with running as tight as .002 clearance, I also saw no gain. I think that in a high rpm application there might well be. Even at .006 I saw no scuffing of the skirt or top ring land as was typical on the older style pistons. Also no chatter marks at the top of the cylinder. This is probably due to the new ovality design that the piston has. So far I have really liked the new piston. I have seen slight power gains, specifically on top end, and I am sure that there is some increase in acceleration, although I have no accurate way to measure acceleration on our dyno at this time. Biggest problem that I see right now is the inavailability of rings, specifically over sized top rings. I feel that the low tension bottom and second ring are a welcomed addition, however, I was never really a fan of low tension compression rings. Sure they'll rpm quicker, but I bet we'll see some ring flutter after just a couple races, the same as with the current aftermarket soft top rings. With no ring sets available yet, you can't play around with ring end gap at all. We're kinda stuck with what comes with the piston, or steal rings out of another piston box. Briggs is working very hard filling back orders as we speak, I assure you.

I will continue building our customer engines at .004 clc until I find something that works better. But the true test will be when guys get a few races on these and they come back for rebuilds. We'll magnaflux some pistons and see what they look like after some hard miles. It's difficult to duplicate racing conditions and strains on a dyno alone.
As is everything in our sport, to each their own.

Tuning of Polar Clutch

Information from an article "Cold, Hard Facts" Part 2: On the Track. Written by Stephen Payne

In "Cold, Hard Facts, part 1" (JDR January/February '96), we learned that the Polar Junior clutch is a centrifugal system: The drive clutch shifts by using centrifugal force to move a set of rollers down a set of ramps. The weights on the ends of the rollers can be adjusted for tuning (the heavier the weight, the greater the load on the engine). The driven clutch uses a combination of springs and cam angles to control how quickly the clutch will shift.

Theory is one thing. Using the Polar Junior on the track is another. Following is a typical tuning situation to help you know what to expect the first time out with the Polar Junior. If you've already been running it for a while, keep reading. The following information may help you work out a few bugs. First, let's presume you have installed a Polar Junior clutch on your Jr. Dragster. You made or bough a jackshaft motor mount and everything bolted up fine. The next step is to set center-to-center distance and offset. Use the special tool available from Polar Performance or a piece of flat bar and a tape measure. Measuring center-to-center distance The center-to-center distance be about 8 15/16 inches. If it's right, the belt will deflect only about 1/2 to 3/4 inch in the middle. To set the belt tension hold the driven clutch in your right hand and squeeze the sheaves together. With your left hand, push the belt backward around the driven clutch. If the belt backward around the driven clutch. If the belt is too loose, it will go very easily. When the belt tension is right, it will just go backward. At idle, the car will roll ahead slightly by dragging on the belt. A new belt might squeal a bit, but after it's broken in. It won't squeal unless it's too tight. Proper belt tension makes for easier staging.

Measuring offset For the belt to be in perfect alignment between the drive clutch and the driven clutch, the two parts must be offset by 5/16-inch. Offset is measured from the back of the moveable sheave on the driven clutch (with the belt off) to the backside of the fixed face on the drive clutch. Proper offset makes the belt last longer and helps the clutch perform at its best. If the center-to-center distance and offset are correct, putting the belt back on should be easy. With the belt already on the drive clutch, just push and turn slightly on the cam of the driven clutch so that you can slide the belt right onto the driven. (Tip: Removing the belt when towing with also help it last longer.) Getting ready The clutch comes with a 12 degree stall and a 20 degree shift ramp with four gram weights on the arm. Weights ranging from three to eight grams are remommending for tuning a Jr. Dragster because no two race cars are the same. Polar Performance does not recommend using weights heavier than eight grams. A tachometer that the driver can see will help when dialing in the clutch. Note that the clutch stalls at about 4,500 rpm. If the belt tension is correct, the driver will feel a small tug when the car moves. Reviewing the data Let's say the engine in your Jr. Dragster should make the most power at about 7,500 rpm. The car ran an eighth-mile e.t. of 10.19 with the old clutch system, but the time slip on the first pass with the Polar showed an e.t. of 10.15. Polar claims that its clutch will cause a car to lower its e.t. by two- to three-thenths of a second. What happened?

A view of the Polar Junior from above shows the drive cluch should be offset from the driven clutch by 5/16 inch***** The tach showed that the engine was running at 8,500 rpm most of the way downtrack. This is past the powerband (remember? 7,500 rpm). You know that adding weight to the clutch arms will reduce engine speed, so for the next pass, you replaced the four-gram weighs with six-gram weights. With the heavier weights, the e.t. was much better: 10.04. The engine was in its powerband at 7,600 rpm, and the driver felt the car pull all the way down. More adjustments With the old clutch, the gear ratio was changed to help load down the engine. Polar Performance believes that gear ratio should be changed only to help a car reach the speed it is capable of, not to increase load. And with a 54-inch-circumference tire, a 16-tooth top gear, and a 75-tooth bottom gear, that speed is 81.8 mph. Here's the formula, where TG equals Top Gear, BG equals Bottom Gear, and TC equals Tire Circumference:

Changing to the Polar clutch increased the car's speed to 58 mph. Switching to the six-gram weights brought it up to 59.5 mph. What happens when you trade the stock 16-tooth top gear for a 12-tooth? The car runs a 9.96 at 60.8 mph. With engine rpm at 7,650. Lets say the last problem left to solve is that the engine over-revs to 8,000 rpm up to the 60-foot mark but runs fairly constant after that. Changing the ramp angle slightly for the first shift will solve the problem. You change to combination of 12/24/20: 12-degree stall (same as stock), 24-degree first shift angle (loads the motor more), and 20 degrees stock). On the next pass, rpm remain constant at 7,600 rpm. The e.t. is 9.93 at 61.1 mph. The next several time slips show consistent runs. That's it - a typical tuning situation with a new Polar Junior clutch. You may go through a similar process when tuning your own. Just remember that a good understanding of any component will always help you get maximum performance from it.

General Polar Tuning Tips


Heavier Weights= Less top end RPM's.
Smaller Weights= More top end RPM's.

Heavier Weights = Loads Motor more on the launch and makes it shift sooner. A bigger motor (stroker) requires more weight to load the motor for better 60' times.

Smaller Weights = More (Quicker- RPM) off line (lighter car-driver combo) allows motor to RPM more before shifting, therefore letting it shift farther down track.

TIP: More weight on roller arms and/or steeper ramp angles loads up engine more (I.E. slows down rpms) less weight or shallower angles speeds up the engine.

The purple spring in the drive unit (front half) with an 18° ramp angle will raise stall speed to about 5200 RPMS - the stiffer black & yellow spring will take you to about 6000 RPM stall.

The drive unit uses the amount of weight on the roller arms to raise or lower the shifting point of the motor (more weight shifts faster centrifugal force). Most cars with (good) motors - bigger stroke good porting etc. will use the 18/28/22 ramps with a purple spring and number 5 or number 6 weights.

Driven Unit (Rear) - The spring in the rear can be used to control how quickly the clutch shifts. (Small effect) If your engine is basically stock I recommend placing the spring in the fourth hole in the cam modified engines use the fifth hole. The driven unit uses a cam angle also to control the shift. (Big Effect) (Stock is 34°) Light car (combo) - 38° to 42° cam (quicker et)

Light car (combo) - high revving motor (46° cam) I recommend 42° for light combos

I tune by launching car and making a 250' - 300' pass then slow down and drive to end slowly or shut off. (Read max recall on tach) Should be 7500 - 8000 RPM (or in your cams may power zone) because that's where you want it to shift at, so the torque stays up. (It will have already shifted by 300') if tach reads much less it means it shifted too soon and is lugging your motor - wind rear spring tighter - or change rear helix to attain your proper shift RPM. (Remember steeper helix angle for light cars that need less torque to move, less angle for heavy cars for more torque to get going.

Steep - shifts quickly,

Shallow angles - shift slower

Take front drive unit apart by using two bolts in the threaded holes in cover - turn each a little at a time.

The clutch bushing (square gray plastic) MUST be clearanced for proper engagement & release & consistency - use a feeler gauge to check clearance between the bushing and the square hub - 0.007 to 0.10 clearances.

** Scrape with a sharp knife till all sides are OK everything should slide freely on its own weight.

1. Use a 12" piece of 5/16" square stock to set the offset (front to rear) see diagram) (rear sites outside front by 5/16')

2. 2. Set centerline to centerline of crank to jackshaft 1/16" less than belt 8 15/16" or 9 5/16"

Polar Clutch Assemblies

Polar Primary (Driver)

Polar Secondary (Driven)

Polar Pro

The New Polar Pro is the latest innovation in Junior Dragster Clutching. The Pro is a completely redesigned clutch from the Innovative Clutch Experts. Like all Polar Clutches, it has hardened ramps, hardened rollers, billet arms, and chrome silicon wire primary and secondary springs. The Pro also features hex, aircraft quality bolts and nuts. With strategically placed cutouts, the Polar Pro is the most user friendly clutch on the market today. The Ramps and Weights can be changed while the clutch is assembled and attached to the car. By removing the six faceplate hex-headed bolts, the spring can be changed with ease, the spider can be removed, and the entire clutch can be disassembled. The Polar Pro is more than user friendly; it is performance based, as well. Pro features overdriven, dual angle 5.75” billet shieves. The dual angle shieve will better grip the belt as the car launches, and the dual angle shieve design was very successful in the Polar Snyder Eliminator Clutch. The Pro comes with a shim kit, which allows the tuner to adjust the belt to shieve distance and ramp to roller relationship. The adjustability of the shieve to shieve distance allows the tuner to get the perfect spacing for better reaction times and quicker launches. The starting distance of the ramp to roller relationship is also fully adjustable, which will adjust the reaction times and the engagement of the clutch. The Pro Primary comes race ready with 15-degree hi-per radius ramps, 7-gram weights (14 total per arm), and a purple spring. The Polar Pro is not just a primary, but rather a complete clutch system. The Secondary has larger shieves at 7.25” for quicker launches and better 60-foot times. Pro Secondary has a 38-degree helix with a yellow spring in hole 6. With the larger Secondary, the center-to-center distance will change slightly. This center-to-center change but should not be enough to cause the need for a new motorplate. The Polar Pro System also comes with the newest innovation from Polar the Outlaw belt. With the latest technology and insight from the World’s Leading Belt Manufacture, Polar has developed a totally redesigned rubber compound and belt profile. Tests have proven this innovative belt to be the most consistent to hit the junior market to date. Boasting a new denser rubber compound (for consistency throughout and longer belt-life), a low profile ‘bottom-cog’ design (for an increased belt contact area and reduce belt slippage), and new ‘top-cog’ design (for a less resistant turning radius and improved belt flexibility), this new 9-inch Center-to-Center belt is truly a work of innovation. The Polar Pro was specially designed with the Tuner and horsepower in mind. With the ease of adjusting the Pro, the Tuner can effortlessly get the maximum out of any combination.



A Crew Chief asks, "What is a gear ratio, and why do I care?"

Let's say you are new to a track, and your engine builder or fellow racer in your class recommends that you run about a 4.42 ratio. Your next question is, what clutch and gear makes a 4.42 ratio.

4.42= 53 divided by 12. It's just a mathematical equivalent. A ratio is just like a fraction but in decimal form so that it is easier to compare. example... 14T clutch, 56T rear gear. 56 (divided by) 14 = 4 This would be the same ratio as 15T (divided by) 60T = 4. 56/14 = 4, 15/60 = 4
Likewise 12/53 = 4.42. It's just 6th grade math and pre-algebra that many of us took for granted back then. Whoever said algebra was useless! Math is extremely useful in racing. Something you can try for yourself...lets say your gear chart only goes up to 19T, but a buddy of yours has been running a 20T clutch for street races (not uncommon). He is running a gear set of 20T clutch, 58T rear gear. You only have an 18T clutch with you.
What do you do?
A.) Pack up and go home cuz you can't buy a 20T clutch drum at the race track.
B.) Put the smallest gear you can find on your kart and hope not to overrev your engine.
C.) Grab a pencil and paper and do the math yourself. -- better yet, use a calculator.

C.) 58/20 = 2.90 so your buddy is running a 2.90 gear ratio even though it wasn't on your chart. Knowing that you want to run about the same as he,
Y/18 = 2.90 Y/18 X(times) 18 = 2.90 X(times)18.
Y=2.90 X(times)18.
Y=52.2 (pretty close to a 53 tooth gear.)
Therefore, all other things equal, engine, weight, chassis set-up, and driving style...you should just about the same rpm as your buddy with an 18-53 on.

The ratio chart just saves you all this time in doing the math is all.
It's really extremely useful.

The following is a gear ratio chart. You are welcome to copy this and print it out for future reference. A nice idea is to take it to a local copy shop and have it laminated and put it in the lid of your toolbox, or on the wall of your trailer next to your gear selection.

Discovery by Fatality

Re: Crankshaft flex and side covers
By: Lynn Cooley
Date: July 8, 1998

We all are aware that a crankshaft will flex and bend. What we don’t know, or should I say didn’t know, is how much. We also didn’t know what kind of specific problems the flexing would create.
Obviously the .875" journal crank could flex more than the 1.000" journal crank but there is not as much difference as you might think.

As a lot of you already know, we have been conducting tests on our new .875" stroker rods. This has been ongoing for about 6 months and we've tried several engineering designs along with different alloys and finally arrived at the ultimate design and alloy. Now let the serious testing begin.
We have a dynamometer but felt that real testing is accomplished on our adult fitted Jr. Dragster and a custom built Kart.

Our first test engine was built using a Raptor block, .140" overbore, 4.225" ARC stroker rod with .875" bearing, 3.000" forged stroker crankshaft, ARC head, ARC flywheel and ARC billet side cover.
This engine assembly was carefully balanced by us.

Now, to be fair and honest, our goal from the get go was to break the rod. We really abused this engine. Our RPM limit was 9000, but we did hit 9300 a couple of times.
The first problem was a high RPM miss and then coil failure. The clearance between the flywheel and the coil was set at .020. When the flywheel rubs the coil the first thing that happens is an interruption of the magnetic field and the coil misfires. The next thing is that friction creates heat and fries the coil.

The flywheel was checked and was running true and the coil was not slipping on its mount. We finally had to set the air gap on the coil at .030 to keep it from contacting the flywheel. This seemed strange and, little did we know, our next lesson would teach us a lot.
About 10 more minutes on the Kart and we blew that sucker all to pieces. The only parts that survived were the carburetor and the ARC cylinder head. The ARC stroker rod looked like it had been through a war but survived - unbroken.
The crankshaft had broken straight down at the radius of the rod journal. This is exactly were it would and should break if the crankshaft bends and flexes. Incidentally this crankshaft is one of, if not the best on the market.

The first symptom of this flex problem was the high speed miss and then the coil failure. By examining where the flywheel had rubbed the coil the hardest, we determined that most of the flex occurs at BDC (bottom dead center). The second place was just after combustion or TDC (top dead center), both of which really make sense.

Think about it - cylinder pressure is at its highest just after combustion and when the piston reaches the bottom, there is no resistance to cushion the abrupt change in direction of the reciprocating weight. As the piston travels upward, it has compression to offer resistance in one stroke and exhaust gases to offer some shock absorption on the next.
On the same note, have you ever wondered why additional deck clearance is needed the more you increase engine RPM? Most engine builders think of rod stretch the same as you do in automotive.
In a Briggs application, crankshaft flex is the main problem. In fact, we checked every rod we had tested and didn't find any permanent stretch.

Our next test engine was built using a BlockZilla block, .174" overbore, 4.225" ARC stroker rod with .875" bearing, 3.000" forged stroker crankshaft, the new ARC BlockZilla head, ARC flywheel and a billet side cover (no name mentioned). The ARC side cover was still working its way through engineering.

With the air gap set at .030" between the coil and the flywheel, we ran the engine very hard for 10 minutes, stopped and immediately tore it down. The temperature of the crankshaft got our attention real fast. After 45 minutes the crank was still to hot to handle (have you ever bent a piece of wire back and forth and felt the temperature just before it broke?). We felt that if we had run it any longer, we would have broken another crankshaft.

One other problem that showed itself on the billet side cover (no name mentioned) was that the 0-ring seal had scuffed and chaffed itself almost into nonexistence along the top of the cover. Along the bottom and up each side seemed to be OK (remember, this was only a 10 minute run).

Ball bearings, by nature, have a certain degree of self-alignment built in. The inside dimension between the 2 ball bearings in the block is 3 ½ inches. It's not hard to flex the shaft .020" in one direction and still have a free spinning shaft. This translates to at least .040" total flex in the shaft and could probably go to as high as .050" and still free spin.
To add insult to injury, the manufacturer of the forged cranks we tested leaves entirely to much clearance on the slip fit bearing area on the shaft. This dimension should be .9995" not .998". This sloppy fit can allow another .015" flex and still free spin. Remember this lesson if you ever have to slip fit a bearing on a stock Briggs crankshaft.

To summarize all of this, it's no wonder the flywheel rubbed the coil and that the crankshaft broke.

Since the ARC BlockZilla side cover was still in the engineering stage, we added a second ball bearing to it and held them in place with 3 recessed button head screws. This created the first DUAL BEARING SIDE COVER.

After installing our new duplex bearing side cover on the same BlockZilla block with .018 clearance between the coil and the flywheel, we had no problems. The engine was run hard for 20 minutes then disassembled and with a shorter cool down period, the crankshaft was noticeably cooler. This was a huge improvement even though the crankshaft had the sloppy slip fit on the bearings.

One other plus over the (no name mentioned) side cover that we tested, is that the ARC BlockZilla side cover is truly light weight, and the ribbing supports on the outside of the cover and, more importantly, the outer perimeter ribbing reduce flex.
Our 0-ring seal showed just a slight sign of scuffing.

The testing was so successful, we decided to use this same technology on our Raptor side covers, and they will be available by the time you read this.
You should see a longer life of the Raptor blocks as well.

The primary reason for all of this testing was to see if our new stroker rod design would live, and IT DID. Both sizes are now in production and available.

See our "Product Showcase" for details of these and other new products.

We try very hard to keep our prices competitive, so look closely at what is furnished with this new ARC side cover design :)


Rolling Resistance
By: Tom Cole
 Date: January 17, 2002

Every rear wheel drive car or truck on the road or racetrack uses tapered roller bearings in their front hubs.  The reasons for this are simple, less friction and combined radial (perpendicular to the axial) and thrust (parallel to the axial) load capacity.  So why are kart racers using ball bearings which are designed only for radial loads on their karts?  I can understand why Jr. Drag racers don’t care about thrust load.  They just go fast in a straight line.  But kart racers go fast and turn fast.  Usually, the one who gets through the turns the fastest wins.  So again, why are so many kart racers using ball bearings?  Endplay!  Somebody has actually sold people on the idea that karts with tapered roller bearing hubs have so much endplay that you can’t set the toe-in.   What a pile!  How is it that those NASCAR and F1 boys can set the alignment on their cars?  Never mind that endplay tolerance for a wheel with tapered roller bearings is only .002” (that’s half the thickness of a piece of notebook paper).  How precise are the widths of the inner and outer races of a .65-cent ball bearing?  How precise is the relationship between the inner and outer races?  Are they parallel?  Are they precisely on the same plane?  How precise is the bore of the hub?  Is the center sleeve exactly long enough?  Well guess what! None of this matters if you use a properly installed tapered roller bearing. The guy beating you doesn’t give a rip how the wheel rolls on the rack.  How the wheels roll on the track, in the turn under thrust load and down the straightaway under radial load is what matters in wheel bearings.  And he is not going to tell you why he’s beating you.

All ball bearings roll on a “point” of contact between the ball and the races.  A tapered roller bearing distributes the load over the length of the roller in a “line” of contact.  This greatly reduces the friction coefficient allowing tapered roller bearings of the same diameter as comparable ball bearings to carry a greater load and achieve a much greater fatigue life.  Simply put, it will roll easier than a ball bearing.  The angle of the races along with the taper of the bearing rods allows a tapered roller bearing hub to roll equally well in the turns or on the straight-aways.  Standard ball bearings DO NOT roll as well through a turn as they do down the straightaway, and they do not handle the demands of a kart racer as well as a  tapered roller bearing.


By Design

By: Carl Amundsen
Date: July 20, 1998

ARC Racing, by design, will not be the first to the market place (at least not very often) with new, earth shattering ideas or products. We're not slow by any means, we just want to bring you, our customers, a well thought out product that has been thoroughly engineered and tested. We won't take an idea, make the part, then sell it to you for our testing purposes.

When we test a part, a motor is built and then run under the most undesirable conditions. We literally try to break it and, if we fail to break it, we try again.
When testing the different alloys for our connecting rods, we even conducted the "Dirt Road Red Neck Test". This is done by taking rods of different designs and alloys and putting them in a vise and using a pipe to see how far they would bend before breaking, and we did break a few.

For the most part, when you buy an ARC Racing product it will be stronger, lighter and cosmetically more appealing than our competition offers and you'll be proud to own it.

We have never - and will never advertise that "this part will give you one more horsepower", or "this part will shave another seven tenths" and so on. The parts you buy from us are true racing parts and in the proper combination (with a little common sense) will make you go fast and, with a little luck added, maybe faster than everybody else.


Remove 400 lbs. from your crankshaft !
July 2, 1998

How - you ask ? With an ARC billet connecting rod.
Compared to some connecting rods that are on the market today, the ARC rod produces 400 lbs. less centrifugal force (on the crank journal end) at 9000 rpm.
That's a lot of unnecessary pressure on the rod, rod bolts, bearings and crankshaft.

Have you ever wondered why additional clearance is needed in the air gap between the coil and flywheel and piston to head clearance on a high rpm race engine?  It’s crankshaft flex!
The less weight the crank rod journal "feels" the less flexing, fatigue, wear and breakage you'll encounter.

How can a part be lighter, smaller and stronger?
After numerous hours of research into the physical properties, force, inertia, elasticity and extensive dyno pulls of connecting rods, we found that attention to details such as shape, type of material, size and weight are extremely important to the structural integrity of a connecting rod.

Just one more plus of the ARC billet connecting rod is that when it's used with most Briggs & Stratton crankshafts, the engine is better balanced. This is very important for the Kart classes that don't allow modifications to the crankshaft :)

The 10,000 rpm Bomb
Date: July, 22, 1998

Several times a week we get calls from people that have just exploded their engine and, for the most part, they're looking for answers as to why it happened.
On occasion, they'll send us some of the parts (or the whole mess) for evaluation hoping that we can shed some light on the problem. Well - sometimes we can and sometimes we can't.

Lets take a moment to consider where it all started and where we are now.
We took a 5 hp. lawnmower motor that was designed to turn a maximum of 3,600 rpm then bored, stroked, welded, ground, filed, polished, fitted, crammed, invented and generally violated the entire book on common sense and ended up with a remanufactured 10,000 rpm BOMB and all of this was done in the name of fun. Well, of course it is.

We just finished testing an engine with a .174 overbore and 3.000" stroker crank and at 9,000 rpm this is what was happening inside that engine:
1. The valves were opening and closing 150 times per second.
2. The crankshaft rod journal was traveling 79.8 mph in a 3" diameter circle.
3. The piston & rod moved, stopped then changed direction 18,000 times per minute (300 times per second).

Think about this:
The piston is at top dead center (TDC) in a momentary stop position, we've already had combustion, the piston travels down the cylinder 1.3671" reaching a top speed of 84.3 mph. while the crankshaft has rotated 75° and all of this has only taken 1/720th of a second to happen.

Another way to look at it:
The piston and rod start and stop 300 times per second reaching 84.3 mph. between each cycle.

Now, we ask the $ 64,000 question:
Could anything go wrong in this kind of environment ?
The answer is - Everything.
It's truly an engineering miracle that this (or any) engine ever gets to 9,000 rpm just once let alone sustaining it.

Calculation Formulas

Stock Briggs & Stratton 5 hp. specifications
Deck height: 6.2835"
Rod length: 3.8750"
Compression height: 1.1900"
Rod bore (crank): 1.0010"
Rod bore (wrist pin): .4910"
Cylinder bore: 2.562"
Stroke: 2.4370

Measuring Stroke
Stroke is measured from the center line of the crank bearing journal to the center line of the rod journal multiplied by 2.

Measuring Deck Height
Deck height is measured from the center line of the crankshaft bore to the deck of the block.

Measuring Compression Height
Compression height is measured from the center line of the wrist pin bore to the top of the piston.

Calculating Cubic Inch Displacement (cid)
To calculate cubic inch displacement (cid):
multiply bore x bore x stroke x .7854
example: 2.562 x 2.562 x 2.437 x .7854 = 12.5633 cid

Rod Length & Compression Height - Made Simple

Calculating Rod Length
To calculate rod length:
subtract deck height - (stroke divided by 2) - compression height.
example: 6.2863 - (2.437 / 2) - 1.190 = 3.875 rod length

Calculating Compression Height
To calculate compression height:
subtract deck height - (stroke divided by 2) - rod length.
example: 6.2835 - (2.437 / 2) - 3.875 = 1.190 compression height


The Crankcase Vacuum System (CVS)           

By: The ARC R&D Team                                          
Date: January 7, 1999
Re: Crankcase pressure management

For as many years as I can remember, crankcase ventilation went like this:
If you were blowing oil and having gasket and seal problems, you just added another line from your motor to the catch can. I have seen as many as 6 lines running from a motor, but the problem was still there.
In case you didn’t know it, we were just adding insult to injury.

Viewed from the crankcase, the single cylinder motor is an excellent air compressor. The more air you take in, the more you have to push out somewhere. Consequently, the more air that is being passed through the crankcase, the more oil that will follow the air out.

All small engine manufactures have done a fair job in their crankcase ventilation system, but it only works up to about 3,000 rpm.

NASCAR and the Drag Racing industry have long used a dry sump oiling system that also creates a negative pressure (or partial vacuum) in the crankcase.

A multi-cylinder engine is a little simpler to deal with because one piston going up is canceling the pressure of the other one coming down. The basic thing you are dealing with here is called blow-by.

Without getting into a long technical and complex discussion on the subject, let me just make this statement. The rings on the piston are designed to work at their maximum with pressure from the top and vacuum from the bottom.

Common sense will also tell us that the piston will function much better and develop more horsepower if it is being pushed down in a negative pressure environment. In fact, with vacuum in the crankcase, the piston is actually being sucked down the cylinder wall.

Any positive pressure in the crankcase will allow a certain amount of oil to get passed the rings and contaminate the fuel charge. This can and will reduce horsepower.

Our Crankcase Vacuum System is very complex in design and every hole, groove, passage and vent are critical to its successful operation. Even the length and size of the tubing used in the catch can model for Kart racing are critical.

While the design is very complex, the operation is very simple to explain.

Without the CVS, the tappet room (valve spring area) is continually being flooded with oil and, contrary to popular opinion, this volume of oil is NOT being caused by the length or design of the dipper on the connecting rod - the cam gear is the culprit.
A little side note right here on horsepower. If the tappet room is flooded with oil and the valve guides are a little on the sloppy side, oil can easily be sucked by the valve stem and contaminate the fuel charge.

Our CVS works like this:
As oil and air are being pushed up to the tappet room, the first baffle is atomizing the oil and lubricating the valve stem with a mist. The second baffle is now starting to separate the air from the oil.
The 2 check valve discs are sensing the blow-by of each down stroke of the piston, regardless of how minute the amount, and are opening and closing on each stroke.

When the piston is at the top of the stroke, we will have our maximum vacuum. When it nears the bottom, vacuum gives way to the amount of blow-by pressure from the rings. The best calculations we can come up with is that we have vacuum 95% of the time and little or no pressure 5% of the time.

As the mist of oil and air move through the CVS, we are continuing to separate the two in our maze of holes, grooves and passages.

In the final step of this unique process, we are now using the vacuum in the crankcase to pull the oil, which is heavier, back into the crankcase, and the oil free air is vented.

While the operation is simple, the R&D on this project has been the most intense of any project we have ever undertaken at ARC Racing. The Dyno testing has been extensive; not only by us, but other engine builders as well along with track testing that has been on going for months.


See ya next time :)


Does It Matter ?
A common sense approach to engine building
Date: November 11,1998

Historically,  human nature has taught us two basic things about the question "Does It Matter ?"
1 - If a problem exists, and we have the capability to correct it, we tell the world "It Does Matter".
2 - If we don't have any interest or desire to correct a problem, we either keep our mouths shut or simply say "It just doesn't matter".


This article could have been greatly expanded into a full length novel but would have gotten boring with illustration on top of illustration.
What's written here is not fiction, and mixed with a bucket full of common sense and imagination, you will run faster and longer.
If you disagree with our opinions, let us know. We can learn too.
If you have any questions or problems, send us an e-mail or call 1-800-521-3560. We'll be here if you need us.

If the base of a block is not perfectly flat and true ?
If the surface of a motor mount that accepts the base of a block is not true and flat ?
Through no fault of the chassis builder, it’s virtually impossible to have the mounting rails for the motor mount perfectly aligned.
Does the weight of the driver flex or move the motor mount rails on the chassis ?
Do the motor mount rails move and flex during a race ?

In other words, can you take a perfectly good motor, bolt it to a motor mount and then to the chassis and totally mess it up ? And the answer is - YES !

We took a block and precision align bored it perfectly round and straight from top to bottom and bolted it down to a rigid, perfectly flat surface plate. Under one corner we put a .050 shim. This shim could now represent a distortion caused by any one of or combination of the problems mentioned above.
Next, we took a good tenth (.0001) reading dial bore gauge to examine the bore.
Looking at the top of the block, and considering between the two valve seats would represent 6 o’clock, we took 2 readings. One aimed at 11 o’clock and one aimed at 1 o’clock. Starting at the top of the bore we moved down a ½ inch at a time. As we moved down the once perfectly round bore, it started taking the shape of an egg.
The distortion was .002.

This test was performed on a Kool bore motor bored .030 over.
The crankshaft bores had also moved and were slightly tighter in the DU bushing.
A dual bearing block is more forgiving when this happens but either way it is a problem.
The deck of the block also moved and became distorted.

Here’s a shocker:
Before we did the test, we bolted a side cover on the block without a gasket and could see day light between the two surfaces. Then we took feeler gauges and it was easy to find where a .002 gauge would fit but we also got a .004 gauge to fit in one place.

Have you ever wondered why sometimes you have problems with oil leaks and blown side cover gaskets ?

These are some problems that can happen because of this:
• Scuffing or galling pistons.
• Abnormal wear in the cylinder wall.
• Rings not seating.
• Loss of compression.
• Loss of horsepower.
• Increased blow by in the crankcase.
• Oil consumption.
• Oil contaminating the fuel charge.
• Engine life short lived.
• Blown head gaskets.
• Side cover gaskets leaking.
• Blown side cover gaskets.
• Blocks cracking because of stress.

The question is: DOES ANY OF THIS MATTER ?
The answer is: ABSOLUTELY

Briggs blocks are not equal and will vary from block to block.
Kool bore blocks will distort more than IC blocks.
Big overbores will distort more than stock bores.
Pre-Raptor motors, with thinner castings, will distort more than the current castings.
Large overbores for sleeves and then boring the sleeve for large pistons will distort even more.
Over a period of years we have heard the following question asked several times a week from our customers:
"I built two identical motors, 1 runs super and the other is a real dog, what’s wrong" ?
or "I dyno tested this motor and it was great, then put it on the chassis and it wouldn’t fall out of a tree by itself. Why" ?

I wonder if anything we have discussed so far would shed any light on this subject ?

To solve one of the problems, the base of the block must be surfaced perfectly flat. The side cover must be on the block when it is surfaced. If you ever change side covers you must check the new one and make sure it does not stick down below the base surface of the block. If it does grind this material away.

The two drain plugs should be installed in the block TIGHT before it is surfaced. The reason here is that the drain plugs swell the block and create a knot on the base surface. Either before or after surfacing the base, with the drain plugs installed, take a die grinder and remove about .050 of material. The area to work on is 1 inch to the right and the left, and into the center of the block from the drain plug.

One last important thing:
You must change your habits because you never thought about protecting the base of the block before.
A special fixture (part number 7718) for milling or grinding the base of the block is available from ARC. This fixture also aligns the crankshaft parallel with the base of the block and aligns the existing bore 90 degrees with the base front to rear and is also excellent for use prior to boring a block on a vertical mill.

Ask yourself this question. Would you go to a race and add some oil to your fuel and loosen your spark plug just a little so you would have less compression? Obviously not. But you might be accomplishing the same thing and here’s how.
Remember this rule of thumb. For every .001 wear on the diameter of the rings, and or the cylinder wall, the end gap of your rings grow by .003.

The following are some common examples of imperfect cylinder bores:

This cylinder measures the correct size at the top and the bottom of the bore but it is .004 bigger in the middle.
You set the end gap of the rings at the top of the cylinder correctly but the end gap of the rings open up an additional .012 when they get to the middle of the bore.

This cylinder measures the correct size at the top of the bore but is .006 bigger at the bottom.
You set the end gap of your rings at the top of the cylinder correctly but when the piston gets to the bottom of the stroke, the end gap opens up an additional .012.
These examples may or may not be exaggerated, but its all relevant.

The end gap of the rings and the correctness of the bore is a very controllable loss of compression to the crankcase and oil is being forced past the rings to dilute the fuel charge. ITS JUST HORSEPOWER BEING WASTED. Now stir this problem in with the first problems of distortion and you might as well call in the dogs.

Two more examples of imperfect bores:

The cylinder measures the correct size at the top but is .006 smaller at the bottom.

This cylinder measures the correct size at the top and bottom of the bore but is .004 smaller in the middle.
You set the end gap of your rings at the top of the bore correctly but as the piston gets to the middle of the bore we have what is known as a CRUSHED END GAP. The cylinder wall galls, heat builds up and rings lock onto the piston. Need I say more.

Perfection in any area is highly improbable but, as a rule of thumb, it should be within (.0005) ½ of a thousands. Holding the tolerance to less than this is certainly possible, so never give up trying at perfection.
You should own a very good dial bore gauge that reads at least in the ½ thousandths (.0005).

When a motor is ready to compete in a race, the cylinder wall should be as perfect as possible. The bore should be round, straight from top to bottom and aligned exactly 90 degrees form the crankshaft. The cylinder wall should be as slick as possible with the rings already seated to the cylinder wall. The ring end gap should be at a minimum of .004 to .005" and the clearance between the piston and the cylinder wall (depending on preference) can be between .004 and .008.

There are many methods and theories on breaking in that new motor. The end result is that all the parts, especially the rings and cylinder wall, must wear a small amount to become compatible. WE MUST SEAL THE CYLINDER. Anything that is moving inside the engine is going to wear to some degree. During this break in period, all the metallic debris is being splashed back on these new parts causing more wear.

The big difference between these two methods:
An engine running under power is under far more stress because of the pressures created by combustion. Therefore the metallic debris is under more pressure as it circulates over the moving parts. Fuel contamination of the oil is another problem. Fumes and noise are certainly a negative. Depending on motor design, engine RPM is sometimes hard to control. Ring end gap must be set at .004 to .005 because of the combustion heat. When the rings and cylinder wall have seated the end gap on the rings will now be .008 to.009.

An engine running on an electric motor run in stand is not under the stress of compression because we are running without a spark plug. At approximately 900 RPM we have a much more friendly environment to allow these parts to seat in. We are developing heat in the motor and it can run for several hours unattended with no noise or fumes. You can stop and change oil conveniently. The motor can even run without the valves, camshaft or lifters.
The biggest benefit: Your engine can run with the ring end gap set at .001" which allows the rings and cylinder wall to wear and seat with a final end gap of about .004 to .005".

As you can see, by now, we believe that ring end gap is very important. Some of you are going to take issue with us and say that it should be .007 to .008 or more.
The only thing that you have to worry about when you have the end gap at .004 is, MAKE SURE YOUR MOTOR IS AT OPERATING TEMPERATURE BEFORE YOU RACE OR LOAD THE MOTOR. Ring end gap is a controllable leak of compression.

Lets stop here and discuss a very important process.

After you finish hone your block to size with a nice cross hatch pattern and could look at it under a magnifying glass, you would see little peaks and valleys.
These sharp peaks are going to be scraped of very quickly when the motor is first run. All of this metallic debris is going to be circulated through the engine.

Plateau honing is nothing more than wrapping a fine piece of wet or dry sand paper around your hone and with very light pressure make 2 passes from top to bottom. We now have Plateaus and valleys.

This is really important if you are using cast iron rings. These rings are porous and softer than chrome rings and the fractured material coming off the cylinder wall will become imbedded in the ring. When this happens it becomes increasingly difficult to seal the cylinder. Chrome rings are not effected by this material.
In any event, plateau honing is well worth the time and in fact should be done on every motor.

Another subject worth mentioning:

Most parts will come clean enough in a good solvent bath. THE DEFINITION OF CLEAN FOR A CYLINDER WALL IS, A BUCKET OF HOT SOAPY WATER AND A SCRUB BRUSH. Then when you think you have it clean take a white rag dampened in solvent and wipe the bore. Now, is it clean ?

As we have discussed earlier, seating happens when the cylinder wall and the ring wears enough to seal the bore. If you inspect the bore after this has happened you will find that the cross hatch pattern has partially worn away and the bore is a lot slicker and the rings are polished the full 360 degrees.

Cast iron rings are easier to seat, are porous and will retain oil.
Your final honing should be done with 320 grit stones and plateau honed with 600 grit sandpaper.
Chrome rings are harder to seat and the surface finish will not retain oil.
Your final honing should be done with 280 grit stones and plateau honed with 600 grit sandpaper. It may take a little longer run in time to seat them also.

The rings have already done a good job of reducing friction, but we can take this a little further. This, however, is another one of those subjects that there are many opinions on and we are probably going to step in you know what. But here goes…

As we stated earlier, chrome rings will not retain oil on their surface but cast iron rings will. So it is imperative that the cylinder wall has a texture that will hold oil.

In theory, after the rings are seated there should never be a metal to metal contact between the rings and the cylinder wall. It takes a thin film of oil between the cylinder wall and the rings to maintain the seal and keep the two parts from wearing. If this doesn’t happen, you can call in the dogs, the hunt is over.

Take a piece of 600 grit sandpaper and wrap it around your hone and with very light pressure, hone the block. This should only take a couple of minutes. The cylinder wall should remain bright and shinny. If you use to much pressure, or spend to much time in the bore it will start to look dark and you are now burnishing it. That finish will hurt oil retention.


We are going to use an electric motor run in stand, a leak down tester and a crankshaft locking bar.
First, assemble all the internal parts in the motor, set the proper valve lash and install a breather plate. Do not install the cylinder head at this point.
Fill the motor with your favorite engine oil.
Secure the motor on an electric motor run in stand, bring the piston to top dead center and install the crankshaft locking bar.
Install the cylinder head. Install the Leak Down Tester.

What we are going to do is a leak down test before we ever run the motor and document the results. Now as we go through the break in we can test periodically and measure our progress.

Remove the leak down tester and the crankshaft locking bar.
Do not install the spark plug.
Set the timer on the run in stand for 1 hour.

Now, make another leak down test and document your results.
Run for another ½ hour, test and document your results.
At some point, there will be no improvement and its time to stop.

The documentation will be a handy reference for future testing.

Now its time to disassemble the motor and do our final honing for friction. After that is done the parts need to go through the cleaning process and be reassembled for the final run in test.

Repeat the previous run in test (it should only take about half the time).

Document the results for future reference.

The following is a good rule of thumb:
8 % - Something is really wrong
5 % - Just OK
4 % - Good
3 % - Very good
2 % - Excellent
1 % - Unbelievable
0 % - Almost impossible

This break in process can be done without the leak testing and for that matter without the cam and lifters. Your first run should be about 3 hours and the second about 1 ½ hours.

This subject is probably one if the most misunderstood areas we can discuss, and some will take us to task for our theory and opinions but here goes…
Most people think a stress plate bolted to the top of a block pulls and distorts the cylinder bore, this is not quite true. However there is one exception were it possibly can, and this is on a very wavy and untrue deck surface. Were not going to build a motor with this kind of problem anyway, so it really doesn’t matter.


When a head bolt penetrates the threaded area of the block nothing happens until it is tightened down to the proper torque specifications. What happens then is a knot or swelling occurs in the bore of the block at the exact depth of the head bolt. And that’s all there is to it. Simple right, not quite.

Equal bolt penetration and torque on the bolts moving from the stress plate to the cylinder head should be a good match and this is the key.
Quite naturally, all blocks are not equal and the amount of swelling into the bore is different. Kool bores, IC blocks, old style blocks, overbores and sleeved blocks are all going to react differently.
Clean, undamaged bolt holes and bolts are also important.
If you don’t have a stress plate, you can use a short piece of tubing and head bolts to accomplish almost the same swelling, but a stress plate is the best.


One last thing on stress plates. There are a number of people who won’t grind the valve seats without a stress plate on the block. My opinion is that it does no good.

This is another subject that is controversial and I’m sure there will be some disagreement on our opinion.
I do not like studs and can see no benefit from using them. Plus, they can cause a lot of problems.
While there are many illustrations I could give you, the following is a classic one.
You have a block ready to build and, for whatever reason, you decide to use studs.
The block has been bored and honed properly - and it is perfect.

Now, you install a set of studs and tighten them down. If you stop just when they run out of threads, and they are nice and snug, you probably have already pushed a knot in the top of the bore. This lump only goes down about .125 but it is different than the one caused by the head bolts. This one is caused by the tapered conclusion of the threads ending on the stud.

Lets assume the piston fits and this goes unnoticed. The head is installed, and the proper torque is applied, but the stud decides to turn a little more and push’s more metal into the cylinder wall. No need to go further because this ball game is over.
I could give many more illustrations, but I hope you got the message.

Before you do any boring or honing:
• Clean the threads in the block and the studs with alcohol and let dry.
• Screw the studs in with a good coat of Lock-Tite.
   (it can take up to 24 hours for the Lock-Tite to cure)
• Now, using a stress plate, do your boring and honing.

The valve guides that come in a new Briggs block are loose and sloppy and one thing is for sure, the valve will never stick. It is not uncommon to find them misaligned with the lifter bore. This can cause abnormal wear and valve seating problems. As far as using them for a racing engine they certainly come up short.

A full length brass guide is better, such as stacking 2 Briggs guides on top of each other. However the full length phosphorus bronze guide from ARC is the best.
The ARC Bronze guide can be run with .0015 clearance on the exhaust, .001 on the intake and out last several sets of valve stems.
A must with either brass or bronze guides is a smooth finish on the valve stem.

There are many people who feel a loose valve guide is the best because it reduces friction, but an ARC Bronze guide properly fit at the above tolerances will have little or no friction.
Remember: Loose valve guides will contribute to oil contamination in the fuel charge which reduces horsepower.

The biggest problem with a loose valve guide is the poor little valve trying to find the seat at high RPM. Think about this: at 6000 rpm a valve must find the seat and seal it perfectly 50 times a second. Seems to me it needs all the help it can get.

ARC has complete valve guide installation equipment available that’s quick and easy, and also corrects any mis-alignment between the lifter bore and the valve stem.

Leak down testing is a very handy tool to use for determining if you have a problem with a motor or if it is still race worthy.

The compression ratio of an average stock Briggs motor is about 6.02 to 1, it would be very hard to increase it much with out killing flow and flame travel.

If cranking compression (with an electric starter) is 150 psi, on a great running motor, the compression at 6000 rpm will only be about 20 psi (or less).

Without a sealed cylinder, compression will just disappear at high rpm and guess what, the motor slows down.

If your motor is not running up front,  and you make a header change, gear change, carburetor change and nothing seems to help. Do a leak down test and you’ll probably find the problem.

When you drive a new valve seat into your block, you will distort the top of the bore.

The average racer spends much more time tweaking and worrying with a camshaft than he does with sealing and maintaining the cylinder. The cylinder should come first and foremost.

A ball hone should only be used for freshening up or re-ringing a motor.

Be careful of "trick of the week" parts that claim to perform magic. They just don’t work.
Just remember Kart Racing can make you moderately wealthy, if you start out filthy stinking rich.


Dual Plane Balancing
Date: January 7, 1999

We receive lots of calls about balancing and one of the first questions is always, "What is DUAL PLANE BALANCING".

If you read the book on it, and you had a background in engineering and physics, you could get a good understanding of it. The two terms that are used in the explanation are Force and Couple balancing. What we are going to attempt to do is reduce all of this down to a non-technical explanation.

Lets go back many years to tire balancing. Your rim and tire assembly was mounted on a shaft and then placed on a frame with the shaft resting in a ball bearing V fixture. The tire assembly would then rotate around until the heavy part came to rest at 6 o’clock. A wheel weight was then placed at 12 o’clock on one side of the tire. You kept adjusting this weight until the tire would not move regardless of how you repositioned it in the V fixture.
This process is called, Static or Force balancing.

Then an improvement was made in this process. Instead of putting all the weight on one side of the rim, it would be split with ½ of the weight going to the inside. This was the first form of Couple balancing in the tire industry. Not perfect, but an improvement.
As years went by, and the advent of much wider tires came into being, the need for Couple balancing increased.

We know have electronic tire balancers that spin the tire assembly and calculates the amount of weight needed to Force balance. Then it calculates what amount goes on the inside of the rim and what goes on the outside. This is Dual Plane balancing of a tire.

How does Dual Plane Balancing apply to the Briggs Crankshaft ?

A bob weight is attached to the rod journal of the crankshaft that represents 100 % of the rotating weight and a percentage of the reciprocating weight. This assembly is then placed on the ball bearing V’s of the balancer. It is then spun up to the operating RPM. Now the balancer can read both sides of the crankshaft and precisely tell the operator the amount of weight that needs to be added or removed from the left or right counter weight.
This completes the Force and Couple (Dual Plane) balancing.

Special Note:
If you change the connecting rod and/or piston, wrist pin and rings, the crankshaft may need re-balancing.


What is Safe Peak Power RPM for an engine ?
Date: July 18, 1998

This is a question that is rarely asked before an engine blows, but always asked after the second one is in a basket.

The formula is simple: divide 22,000 by the stroke.
Why 22,000 you ask ?
Well, the automotive industry came up with this number after decades of engineering research using literally hundreds of stroke lengths - and we believe it's probably correct.
Example: The engine we just finished testing had a 3.000" stroke so 22,000 divided by 3 equals 7,333 safe peak power rpm (which we slightly exceeded for our test).

Now that you know - just keep on truckin.


Probable Cause of Most Rod Failures
Date: June 1, 1999

To help prevent engine failure, here's a few tips:

For the most part, a quality billet connecting rod doesn't just break. There are several things that can really shorten the life of a rod of which bearing clearance is probably the most important and too much is worse than not enough.

Visual inspection (or eye balling) is not good enough. The only way to check a crankshaft, rod or bearing is with micrometers and dial bore gauges. Dial calipers or digital calipers are just not accurate enough. Even with the ones that have .0005" graduations, the accuracy is generally + or - .001" which means outside to inside measurements could have a total error of .004".

The Crankshaft:
Just because a crankshaft looks good, doesn't mean it is to size and round.
Using a 1" micrometer, measure the rod journal of the crankshaft in 4 places. A crankshaft with more than .0005" wear or out-of-round will probably not last very long in a 9500 rpm race environment.
Keep in mind, what you might get by with in a stock engine, will not be as forgiving in a high rpm, high horsepower engine.

Surface finish of the rod journal is also very important especially when a babbit bearing is used. Most of the new Raptor III cranks I've seen and checked are way too rough.

One of the most common things we hear is "The bearing looked ok, so I reused it"
Again, looks can be very deceiving. Take a 1" ball micrometer or use a ball anvil attachment on your mic to measure the bearing thickness in several places. The ball anvil is necessary because of the curvature of the bearing.
The measurement should be .075" and, on a crankshaft that measures .998" on the rod journal, this should give you .0025" clearance.  

On a bearing that has been run or has been honed or sanded for clearance, carefully mic each half on the outside edges and in the middle.
Remember, a new bearing should measure .075" which should give you .0025 clearance. If the top bearing measures .073" and the lower bearing measures .074", you now have .0055" clearance.

Some engine builders use a ball type hone to clearance bearings and this can cause a major problem. The Babbitt material is very soft and easily removed. What happens is that as the flex ball hone enters and exits the bearing bore, it removes far more material from the edges than from the center producing an hour glass shape. Using a plastigage or measuring the bearing in the center may give you a false and possibly fatal reading.
You may think you had .003" clearance (and you did in the center) but, it will be a very narrow contact area and wear rapidly and as this clearance increases, it compounds the problems.
This additional clearance pounds the bearings until the rod & piston assembly becomes a 9000 rpm slide hammer. This is when you have a failure.

Special note: Never file or grind the ends of a bearing.  If the installed i.d. of the bearing is too large (measured inside diameter of the bearings installed in the rod and torqued too 150 inch/pounds), gently and slowly remove material from the parting edges of both halves with fine emery cloth on a flat surface until you get an installed i.d. of .9995" to 1.0015" for stock .998" cranks and .8765" to .8785" for stroker .875" cranks.  It is better to sneak up on this slowly.

These bearings are designed to crush in the bore of the rod which holds them in place and prevents them from spinning. The tangs are primarily for location.  If your crank measures less than .997" or .874", it is probably a good idea to replace it.  In any case, it is not a good idea to try to compensate for a worn crank journal by reducing the i.d. of the bearing.

Inspect and/or replace the bearings regularly. By examining the bearings, you can set up a schedule of how often to replace them - plus - by measuring and looking for smearing of the Babbitt material, you'll be able to tell if your oil is doing its job.

Remember: Always torque the rod bolts evenly and equally to 150 inch/pounds.

Treat your engine as the piece of precision equipment that it is. The environment in which it operates is extremely harsh and attention to detail along with precise measurements is absolutely necessary.


Rod Length Ratios
Date: July 18, 1998

There are as many theories about rod lengths as there are any other subject that deals with racing.

As strokes get longer, rod lengths get shorter and at some point in time, this will create a problem. We don't know if we've crossed the line yet, but we're definitely leaning on it. At the same time, there's a point where rod lengths can get too long for a particular stroke and we're leaning on that line also.

Rod length ratios are calculated by dividing the rod length by the stroke.
Example: a 5.000" rod length divided by a 3.000" stroke equals a 1.67 rod ratio.

For a good illustration of what increasing the rod length does for an engine, we'll use a 350 Chevy.
A stock 5.7" rod divided by a stock 3.480" stroke gives us a 1.637 rod ratio.
Now, put a 6.00" rod in it with the same stroke and the ratio increases to 1.724 and the engine produces more power and rpm.. This is a known fact that's been around for some 25 years.

A stock, 5 hp. Briggs & Stratton engine uses a 3.875" rod and has a 2.437" stroke which equals a 1.59 rod ratio.
We know that by changing the rod to 4.475" and using the same stroke, the ratio increases to 1.863 and the engine produces more power and rpm.

The most important thing that a longer rod does is increase the dwell time of the piston when it's at top dead center (TDC) and this will make more power.
The second most important thing is that it improves the leverage the piston exerts on the crank journal and this also increases power.

Another feature of using a longer rod is that it creates a much friendlier environment for the piston, cylinder and crankshaft to operate in.
Consider this: The piston is moving up and down in the cylinder trying to make a   crankshaft rotate in a circle. When the piston is in a down stroke, the resistance of the crankshaft is trying to push it out the front of the block and in an up stroke, with the resistance of compression, the crankshaft is trying to push it out the back of the block.
Our recent test engine had a 4.225" rod with a 3.000" stroke equaling a 1.408 rod ratio and we believe we may have gone beyond the short rod ratio limit but, the engine made a bucket full of power and survived even at 9,300 rpm.

Rest assured that one day we'll reach the point of sheer stupidity.





Briggs & Stratton 5 hp. Specifications

Pistons & Rings

Piston clearance
Ring end gap (stk.)
Ring end gap (Dykes L)
Compression height (stk.)


.006 to .007"
.005 to .006"
.010 to .012
1.1870" +/- .002

Connecting Rods

Length (stk. ctr to ctr)
Wrist pin bore
Wrist pin diameter
Crank journal bore (stk.)
Rod & piston (oal)


3.873" +/- .002
.491" +/- .0005
.4895" +/- .0005
1.001" +/- .0005
5.065" +/- .003


Journal diameter (stk.)
Rod clearance (journal)


.9985" +/- .0005
.003 to .0035"

Valve Guides

Stem clearance (stk.)

Stem clearance (brz)


.003 to .004" in
.004 to .005" ex
.0015 to .002" in
.002 to .0025" ex


Stem diameter (stk.)

Valve lash (stk.)


.247" +/- .0005 in
.247" +/- .0005 ex
.005 to .007" in
.009 to .011" ex

Torque Specs

Rod bolts (stk. rod)
Rod bolts (ARC rod)
Head bolts


100 in./lb.
150 in./lb.
140 in./lb.


Is 1° Costing You The Win?
By: Tom Cole
Date: April 12, 2002

Do you REALLY know if advancing or retarding the ignition timing of your engine 1° will increase horsepower?  I think most of us can agree that there is a good chance that we would find some improvement, if we checked.  How easy is it to experiment with your motor?  As I have said before, I am no expert.  I am just a father like many of you who likes to build his son’s racing motors.  I love the feeling when a motor I built takes the checkered flag, and I have felt the heat and disappointment from the driver (and the wife) when it is obvious that MY poor performing engine cost us the race. 

One fact I have learned is that perfect timing is when your motor runs at the proper temperature, responds the way you want it to respond, and you win races.  There are no magic numbers or exact calculations that can tell you where timing needs to be.  There are only beginning reference points.  Just like the carburetor, timing must be tuned to the conditions and variables specific to each motor, track, driver, and kart.  Throw in the altitude and the weather conditions, and you can see that the ability to adjust timing quickly and accurately is something that can set you apart from the crowd.

Because the degree wheel infuriates me, and offset keys are a joke, I set ignition timing by aligning the trailing edge of the flywheel magnet with the middle of the coil trigger button while the top of the piston (before top dead center) is at a measured depth from the top of the cylinder.  The depth of the piston in the hole varies with many factors, but your cam manufacturer will usually give you a good starting point.  (30° BTDC = .2115” in the hole when popup is 0.000” on a stock engine)  Once I have everything aligned, I carefully hand tighten the starter nut.  Then I recheck my measurements, torque the starter nut down to somewhere around 1,000,000 in./lbs. (I give it all I’ve got with hand tools) and recheck my measurements again.  Also note that I do not use an offset key.  I lap the flywheel and crankshaft together and then apply Locktite to the taper (not the threads) on the crankshaft before installing the flywheel.  Now I reassemble the motor, mount it up to the kart, and give it a test run.  This whole process takes about 30 minutes.

Now how many of us are going to take that motor and go through that same process at least three more times to check and see if +or- 1° makes a noticeable difference?  The method I use to set the timing has a margin of error of at least  +or- 2° because I don’t know the exact firing point of the coil trigger button.  I know I need to experiment, but what a pain!  And how much stress am I putting on the stock flywheel each time I knock it off and torque it back down.  Haven’t they been known to fly apart?  And how well is that flimsy stock animal flywheel going to hold up?  I have heard of engine builders breaking them when they are first installed! 

It wasn’t until I faced all this trouble with that touchy little blue-plate slapper-cam motor that the value of ARC’s billet finned flywheel with its adjustable timing hub really came into view.  I can literally be finished getting the motor’s ignition timing right with the ARC flywheel, before I can test the first timing change on a stock or fixed hub flywheel.  How much time do you have at a race between practice and the first heat?  I am too busy changing oil, cleaning and prepping tires, and making chassis adjustments to tear down a motor. 



Setting Ignition Timing
By Tom Cole
May 8, 2003

            Almost every day someone calls or emails us asking how to set the ignition timing on their engine.  It is an important topic because as little as one degree can be the difference between an engine that runs up front and an engine that sputters and pops its way to last place.  In this article, I am going to describe what I believe to be the most accurate and reliable method to set the timing on a Briggs and Stratton™ Engine.  If you are using an ARC adjustable hub flywheel, begin by setting the hub index mark in the middle of the degree marks on the aluminum body.  This will give you the maximum amount of adjustability after you set the timing based on the cam manufacturer’s specifications.  The adjustable hub gives you an “at the track” advantage, because it allows you to easily advance or retard the ignition timing to tune for variable conditions.

            The first thing you must do to set the timing is to identify the exact position of the flywheel’s magnet in relation to the coil when ignition occurs.  To do this right, you need a plain old induction timing light and a car battery.  Some folks will tell you about aligning the trailing edge of the magnet with the center of the little button just in front of the left leg of the coil, and for the most part, they are correct.  But there is no such button on the Animal coil and factors such as coil gap make this method only a close approximation.  So here we go with the timing light.

  1. Install only the crank, its bearings and its timing gear in the engine block and put on the side cover.
  2. Install the sheet metal guard that goes behind the flywheel.
  3. Install the flywheel with a standard key on the crank and snug it up with the starter nut. (No need to torque it, you are going to take it back off)
  4. Install the coil with the proper gap.  (It will not be removed so tighten it up)
  5. Attach a spark plug to the plug wire and tape it to the block so as to ground it and create a spark.
  6. With the magnet at 12 o’clock under the coil put a white line on the outside rim of the flywheel at about 3 o’clock.  This line needs to be plainly visible when looking at the block from the front.
  7. Attach the timing light to the battery and clamp the induction lead on the plug wire.  Be careful that all wires are away from the flywheel.
  8. With the timing light pointing at the front of the block, turn the crankshaft clockwise with a drill or starter and you will see the timing strobe light up the white line.  You need to spin it faster and more consistently than possible with a pull starter because a magneto has a retarding effect at higher rpm, and you want to compensate.
  9. While the strobe is flashing and the flywheel is spinning, make a white mark on the sheet metal guard or block that aligns with the white mark shown by the timing light on the flywheel.  You can now place the magnet exactly where the spark is triggered when and if you ever remove the flywheel.  BUT, if you move the coil or the metal guard, you have to start all over again.
  10. Remove the flywheel, side cover and crank, and install the piston, rod, crank, cam, etc (leaving off the cylinder head) getting to the point where you are ready to install the flywheel and set the timing.


You are now ready to set the timing.  Truthfully, it is more accurate to set the timing by fixing the piston at a measured distance before it reaches its highest point on the compression stroke.  But, although everyone knows the “in the hole” distance for a pure stock setup, (30deg is .2115”) it is difficult to calculate the distance needed by different length rods since the distance traveled by the piston per degree of rotation varies with rod length and/or stroke.  You can calculate it, but it just really isn’t worth the effort.


So, since the cam manufacturers generally provide you with a recommended ignition timing expressed in degrees before top dead center (BTDC) of the compression stroke, it is going to be best to set your timing using a degree wheel.  (This is a good time to degree your cam too)  On to the dreaded degree wheel…

  1. Using coarse grit sand paper, rough up the tapered part of the crank and then make sure it is clean.  If you are un-willing to lick it, it isn’t clean enough!
  2. Similarly, rough up and clean the inside of the hole in the flywheel hub.
  3. Set the crank so, according to the degree wheel, you are at the cam manufacturers specified degrees of ignition timing before the piston reaches the highest point of its compression stroke BTDC.
  4. Put a few drops of Loctite™ on the tapered part of the crank and install the flywheel with the white timing lines aligned.  DO NOT USE A KEY!  Keys do very little to prevent a flywheel from spinning, and they will hinder your accuracy.
  5. Carefully tighten down the starter nut.  Check and recheck the timing several times as you tighten to be sure that the white timing lines are still aligned and that the degree wheel is still where it is supposed to be.  Then tighten the starter nut, A LOT.  You are shooting for tight enough to hold it together, but not so tight as to split the flywheel.
  6. If you have an ARC adjustable hub flywheel, you can fine-tune your ignition timing using a dyno or a stopwatch and a Digatron CHT/Tach at the track.


That’s it!  Once you have the timing set and are done with any track adjustments, measure the distance from the deck to the top of the piston (with the white timing lines aligned) and record the measurement.  As long as you refresh the engine with the same parts, you can just set it in the hole and go when you refresh.  If you re-deck the block, just subtract the amount removed from your recorded depth figure.


Are you Unbalanced?
By Tom Cole
October 29, 2003

Several years ago when we were developing our crankcase ventilation system for the Tecumseh Star engine, I got some seat-of -the-pants experience with the value and need of crankshaft balancing.  I was accustomed to driving a 3” bore, 3” stroke test engine, which had one of our billet crankshafts in it.  The 3x3 crank had been balanced, and ran very smooth considering it was producing about twice as much horsepower as the Star.  The Star’s crank/rod/piston setup had not been balanced, and the difference took me by surprise.  As I made 8000 rpm, my vision was so blurred from the engine’s vibration that I had to slow down to see the turn.  My teeth and ribs felt like they were banging together and after my 15 lap stint at this ¼ mile asphalt oval, I was not interested in driving any more that day.  My body was directly reporting to me the increased pain and fatigue that an unbalanced crankshaft can do to an engine and driver.

Clarence Clark is a friend of mine and he is what I would call an engine-building guru.  For many years, Clarence’s company rebuilt the engines for the world’s largest fleet of racecars, the United Parcel Service.  He has since traveled the country doing seminars for rebuilding supply companies like Goodson and Cobra Products.  When I told Clarence about my experience, he went over to his tool box and produced a metal “H” which was made out of five, six inch long 3/8” metal pipes and two 3/8” pipe “T’s”.  He handed me this contraption and told me to gently hold the cross pipe in one hand like an axle and spin it.  Everything was fairly in balance and it spun easily making five or six revolutions.  He then removed one of the four, six-inch uprights from the “H” and said “now it’s out of balance, spin it again.”  I did and it only made one revolution!  When I tried to spin it real hard, it only made two revolutions.  This was an example of the need of both Force and Couple or “Dual Plane” balancing of a crankshaft.  He told me one of the most interesting things about an out-of-balance crank or cam is that this tendency to stop (or inertia) increases with RPM so it requires more and more horsepower to obtain the same RPM as a balanced setup!  It is hard to believe that we spend so much time and money on carburetors, valves, porting, flow and displacement and many of us ignore or are unaware of such a power robbing aspect of an engine.

Force and Couple balancing are technical terms used that really just mean top-to-bottom and side-to-side balance and are commonly referred to as “Dual Plane Balancing.”  They are easily illustrated with a little history in tire balancing.   Many years ago, when car tires were balanced, the rim and tire assembly was mounted on a shaft and then placed on a frame with the shaft resting in a ball bearing V fixture. The tire assembly would then rotate around until the heavy part came to rest at 6 o’clock. A wheel weight was then placed at 12 o’clock on one side of the tire. You kept adjusting this weight until the tire would not move regardless of how you repositioned it in the V fixture.  This process is called, Static, or Force balancing and it is the method that is used on most kart and Jr. Dragster wheels today.  Later, an improvement was made in this process by splitting the weight and putting half on the inside of the rim and half on the outside.  This was the first form of Couple balancing in the tire industry.  As years went by the advent of much wider tires came into being, so the need for more accurate Couple balancing increased because the part of the tire that was out of balance was often further from the centerline of the tire.  And since one side of the tire could weigh more than the other, it became necessary to be more precise than to just split the weight in half to achieve optimum balance.  We now have electronic tire balancers that spin the tire and wheel and calculate the amount of weight needed to Force balance. Then it calculates what amount goes on the inside of the rim and what goes on the outside. This is Dual Plane balancing of a tire. 

The crankshaft of an engine has counterweights to dynamically offset the effect that the movement of the reciprocating mass has on the rotating mass of the crankshaft.  The reciprocating mass is a percentage of the total mass of the top half of the connecting rod, the piston, wristpin, rings and circlips.  The percentage used comes from a chart, which is calculated from expected RPM and stroke length.  The rotating mass is the total mass of the bottom half of the connecting rod, the rod bolts, washers and bearings.  To balance a single cylinder crankshaft, a bob weight is attached to the journal of the crankshaft that represents 100 % of the rotating mass and the percentage of the reciprocating mass from the chart. This assembly is then placed on the ball bearing V’s of our Stewart-Warner crankshaft-balancing machine and spun up to the expected operating RPM setting. The balancer can read the change in weight on both sides of the crankshaft and precisely tell the operator the amount of weight that needs to be added or removed from the left or right counterweights to achieve optimum dual plane balancing. 

Balancing a crankshaft in itself does not produce more horsepower; it improves output potential by eliminating or greatly reducing a non-productive use of horsepower.  It also helps prolong the life of an engine by reducing damaging vibration.  In any situation where some grinding of the crankshaft or modification of the piston is permitted, it is a largely untapped source of performance gain.

Fastener Stuff For ARC Racing

-On Rod Bolts, By Mike Gifford
April 4, 2005

 My name is Mike Gifford; I'm "Outrider" in the 4Cycle.com forums, the guy that commented on how happy I was to see a fastener tightening procedure that made allowances for the differences in friction coefficient (and hence applied torque required) when different thread lubes are used on threaded fasteners.  At the time, I promised you that I would eventually send you some comments on fasteners and tightening procedures.

 By way of background, I started my engineering career with a BS degree in Engineering, an Ensign's commission in the Naval reserve, and an Unlimited Horsepower US Coast Guard 3rd Engineer's License for both Steam and Diesel ships.  After several years as a sea-going Marine Engineer, I came ashore in 1970 and eventually went to work for the Navy.  I ended up as an engineer in the Submarine Fluid Systems Division, and spent the better part of the next thirty years in various Engineering and Naval Architect billets in the Naval Sea Systems Command, all in the submarine community.  Got to ride/play with things most people only see in National Geographic specials and on the History Channel, and got paid for it.  And got to retire when I was 55.

 Submarines have thousands of bolted joints, many in critical applications, which, in the end, accidentally caused me to build a small empire within the Navy's engineering community as a fastener expert.  Amazing how a collateral duty (that's all it ever was) gets to be a big deal.


In the early 1970s, the Navy's submarine community realized that they had a bunch of bolting problems; both with fastener selection and joint assembly, and by 1978 published a Submarine Fastener Manual.  In 1981 I switched jobs and, as part of my new job's duties, inherited responsibility for that manual; after that, wherever I went in the submarine community, the job description for my new position would be rewritten to include the Submarine Fastener Manual and all submarine fastener problems as a collateral duty.  As one who had been assembling motorcycle and automobile engines since he was 12, and new how to use a torque wrench, I thought I had this one wired.  Within 3 days of inheriting the Fastener Manual I discovered that I about had the tip of the iceberg wired, and embarked on a very steep learning curve for the next two months.  The learning hasn't stopped to this day, but the curve is rarely that steep any more. My specialty was "in service" engineering - how to select a fastener that would live for a specific application, installation procedures, including procedures for tightening with a torque wrench, use of angular turn, and use of ultrasonic direct stress measurement; review of failed fastener analysis reports and developing solutions for preventing those failures in the future.  The less publicly visible (but probably more important) section handled fastener materials, manufacturing processes and QA/QC testing of materials and finished products, and contributed to my ongoing education on a regular basis.  So that's how I got to be a fastener geek/goon.


Random comments, in no particular order of importance:


1.  While there are many materials used for bolts, cap screws, machine screws and nuts, where there are not problems with corrosive environments or other special considerations, selecting from among the various steel alloys used for fasteners is a hard act to top.


2.  In the drive to reduce space and weight, ever-higher strength alloys are considered.  This is a good thing if not carried to extremes, but people need to remember that high strength is no good without toughness.  Just because a fastener is strong doesn't mean it is tough - VERY high strength fasteners (250ksi - 270ksi yield, as found in some heat treats of some of the Nickel/Cobalt alloys) tend to be brittle and not respond well when subjected to high shock loads.  Since they don't stretch much, they break, where a tougher (though less high strength) alloy will just stretch.  Of course, the tougher alloy would usually have to be a larger diameter or use more fasteners in the joint; everything is a compromise in the fastener world.


3.  Concerning toughness and resistance to high shock, there is an all to often ignored quality buried in fastener material chems and physicals called "Percent Elongation" which makes screening for toughness relatively easy.  The Navy's big hurdle is passing tests for resistance to "Hi Shock and Undex" ("Undex" is short for UNDerwater EXplosion).  The Navy flatly refuses to allow components (including the fasteners holding things together) in critical applications to use materials with a percent elongation less than 10%, and will only VERY rarely allow materials with a percent elongation less than 10% in a non-critical application by approval of an exemption specific to that component for a specific service on a specific class of ships (we're speaking combatants here; noncombatant ships are allowed a little more leeway in their design).  In general, to meet hi shock and Undex requirements, materials with a percent elongation of 15% or more should be chosen, over 18% is better, and, as heat treating processes for metallic alloys in production quantities has improved over the last 20 -30 years, many fastener alloys that were in the 15% - 18% range can now be reliably produced in the 20% - 25% elongation range, which is great for toughness.  Fortunately, by the nature of the beast and the general conservatism of mechanical joint designs used in shipbuilding, any fastener material with a percent elongation of over 10% will generally pass hi-shock/Undex in a properly designed bolted joint.  To protect its flanks, the Department of Defense has some specifications that it piggybacks onto commercial specs (MIL-DTL-1222, for instance) to get what they need in this respect, so an ASTM A574 4340 socket head cap screw for a critical application would be ordered with a minimum percent elongation over 10%, which is higher than the minimum demanded for 4340 socket head cap screws in A574.  And many engine building applications don't have the hi-shock loads present in Undex testing and can benefit from really high strength fasteners, even if their percent elongation is less than 10%, but you do have to match the alloy chosen to its shock environment carefully.


4.  Once the user the user settles on the basic fastener configuration (hex head cap screw, socket head, 12 point, etc.) and the correct material, the most important things (more important than material and basic configuration, unless grave errors are made in selecting those two items) are the geometry of the fillet radius where the unthreaded shank joins the head and the transition from the threaded portion to the unthreaded shank of the fastener.  Errors in design or execution in either of these two areas can result in fastener failure where it would not otherwise occur.  To eliminate problems in the threaded to unthreaded shank portion, use of rolled threads is usually sufficient.  For the shank to head transition, the proper fillet radius needs to be specified, and adherence to that specification needs to be checked religiously as part of the QA program.


5.  When doing qualification testing or random sample receipt inspection of fastener lots, the most effective single physical test (after a good visual inspection and dimensional checks) to verify the quality of a finished fastener (including the items in 4. above) is a wedge tensile test per ASTM F606.  The beauty of the wedge tensile test is that it is done on a finished fastener, not a machined tensile test specimen, so it picks up both material problems and manufacturing defects like an inadequate fillet radius.  Generally, if you purchase fasteners from an outside source, they will state what specifications are used for manufacture and random sample visual inspection and dimensional checks are sufficient.  Cash flow permitting, it's nice to send a few to a lab once in awhile for a wedge tensile test by an independent lab - I was spoiled; any Naval Shipyard had a lab with lots of neat machines, including a tensile test machine. 


6.  With regard specifically to rod bearing cap screws for ARC connecting rods, my inspection of a limited number of these fasteners (two rods from my engine builder's stock) left me impressed.  Their high quality was obvious, as was correct design and execution of the fillet radius and the thread transition (to the extent that this can be determined by a visual inspection, but I'm willing to bet that they would pass a wedge tensile test without breathing too hard), but more interesting was an extra little design feature, a reduced diameter section in the unthreaded shank of each cap screw.  That little feature actually improves resistance to shock and greatly improves performance in hi-shock situations.  Basically, it makes the fastener a better spring, mitigating shock damage by increasing fastener toughness with a mechanical trick, rather than exotic metallurgy.  As an example of what this feature can do, the Navy has subjected a group of fasteners (studs in this case) to a shock load that would make them fail; the average stretch was 1/16" before they broke.  Repeating the test with studs identical except for a slightly reduced diameter in the unthreaded section, the fasteners actually stretched an average of 3/16" prior to failure.  When not carried to extremes, that little feature is an excellent way to improve high shock performance, and it doesn't take much reduction to collect that benefit; obviously, too great a reduction in diameter in the unthreaded shank will reduce ultimate strength more than it benefits shock resistance, but it is usually possible to strike an effective balance without causing problems if the fastener design isn't right on the edge of failure due to the in service load profile to begin with.


7.  Excerpts from a Navy training lecture - "What your mother and the professors didn't teach you"


A.  Preload range of bolted joints assembled with a torque wrench:


Most people see a torque specification for the threaded fasteners in a joint assembly and think that (1), the desired preload is achieved with great precision, and (2) that the fastener-to-fastener preload variation within the joint is small.  Neither of these impressions is correct.  On the best of days, the fastener-to-fastener preload variation is about 35%, and often more.  It is not unusual to see the largest preload measured in the bolt circle twice that of the smallest in joints assembled with a torque wrench by an inexperienced mechanic without benefit of a proper installation process.  Although other standards have been (and when there is a specific reason, still are) used, most bolted joints found on submarines (that require use of a torque wrench for assembly) have a preload established at 2/3 of yield of the weakest element of the joint (150% of yield in the case of bearing stress), and, ideally, the limit will be reached as tensile stress in the bolt or stud.


So you have a torque from a drawing, Maintenance Standard, tech manual or whatever, based on, say, 2/3 of yield.  When the mechanic is done assembling the joint, all the bolts in the bolt circle would have (theoretically) a tensile load of 2/3 of yield, because the torque was chosen to give that result.  Unfortunately, there is a significant fastener-to-fastener variation in friction coefficient, AND a significant fastener-to-fastener variation in short term preload loss (the relaxation that occurs in the first 2 to 10 minutes after the wrench is removed from each fastener in the bolt circle for the last time).  And a few other things (all told, about 76 different things, according to the Air Force, which did an excellent study on the subject, like prying loads and cross talk.  The result is that all we know for sure is that each fastener in the bolt circle, having been tightened to a mean torque mathematically equivalent to a mean preload of 67% of yield, has an actual preload of somewhere between 40% and 90% of yield.  It sounds crude, but it's close enough, even for hull integrity/high shock/UNDEX/hazardous fluid, etc, services.  The distribution of this preload variation is more or less a bell shaped curve in a statistically valid sample.


B.  The value of process instructions:


The real case is not quite as bad as the above makes it look, as the methods incorporated into Navy/Navy approved process instructions are designed to reduce this preload spread.  The reality is that the multiple passes, check passes, etc, of Navy process instructions skew the curve.  The top value (90% of yield) doesn't change, but the number of fasteners below 67% is significantly reduced, and the amount by which they are under 67% of yield is also reduced, while the number between 67% and 90% is increased.  Since the best defense against long term preload loss is high initial preloads and minimization of fastener to fastener preload variation, the value of good processes and training in those processes is once again proven.


C.  Good shop practice and precision:


As far as fastener to fastener preload variation is concerned, the variation for fasteners in a joint tightened without a torque wrench, but in stages and with check passes, is 5% to 10% more than the variation with a torque wrench, on the average.  In reality, other than being cheap to use, the torque wrench only offers 3 advantages:


1.  It assures adherence to a specified mean torque, which in many joints is not a particularly significant item, except for record purposes, or where minimizing the chance of exceeding the yield strength of the material or the threshold stress level of an H2 embrittlement prone material is important.


2.  It results in slightly less preload variation than the use of "good shop practice" and a box end, open end or socket wrench, in the hands of an experienced technician.


3.  It offers visual proof to witnesses that each fastener in the bolt circle has been properly tightened, needed for certification records for "critical joints".


For what it's worth, the most accurate method of tightening the fasteners in a bolted joint without resort to expensive ultrasonic measuring devices, strain gauge equipped bolts or other cost increasing approaches is angular turn of the nut.  The fastener to fastener preload variation runs about 15%, much better than a torque wrench can ever hope for.


When fasteners are overtorqued severely during initial installation but survive the procedure without failure, the joint is generally OK for service without further action.  Various embedment phenomena and other short-term preload losses will reduce the stresses to a high but acceptable level.  The exception is where the fasteners are made of materials prone to hydrogen embrittlement.  They may settle out, after short-term preload loss, at a level in excess of their threshold stress level, leaving a high probability of brittle failure in the future.  Resolution where H2 embrittlement prone materials are involved should always favor loosening and re-torquing, one fastener at a time, to the correct value (Note to Tom:  Many bolted joints on Navy ships require hydrostatic testing for verification.  If the joint integrity is violated by loosening the fasteners, an expensive retest is required.  By common sense and resulting executive fiat, the Navy does NOT regard the integrity of the joint as violated if the fasteners are loosened and re-tightened one at a time, saving the expense of retesting, hence the importance of "...one fastener at a time").  If discovery is after the joint is buried by interferences and cost is too great, use of Level I certs may allow acceptance based on stresses adjusted for the actual yield rather than the min spec numbers usually used in design calculations and by PC Bolts.  Fasteners of materials such as Grade 5 steel and other materials not prone to H2 embrittlement may be left as is and torqued correctly at the next disassembly and re-assembly of the joint unless the activity is having a lot of such errors, in which case remedial retorquing is necessary to get production's attention.


G.  Notes on choosing thread lubricants:


2.  Once in awhile the subject of the range of friction coefficients that will be exhibited by a thread lubricant will come up.  When discussing this subject, insist that the range be tied to a specific material (nut and bolt or stud) combination.  The reason for this is that the same lubricant often has both a different mean friction coefficient and different extremes (and can have the same mean friction coefficient, but different extremes, the high and low values that establish the range) when you compare the mean and extremes for different combinations; alloy steel and alloy steel, CRES and CRES, Monel and Monel and KMonel with a monel nut, for instance.  The extremes for each material combination establish the range FOR THAT COMBINATION.  Taking the high for the combination that has the highest extreme and the low for the combination that has the lowest extreme, a method that will quite often be attempted by your Nuclear counterparts if you don't call them on it, doesn't give the range, it gives a number useless in calculations and of little interest in intelligent discussions.


8.  Notes on tightening procedures (the following is a slightly edited excerpt from the Navy's Submarine Fastener Manual, from the sections that are the basis for development of local activity's process instructions and instruction packages for individual work packages, where such detail is necessary for a specific work package.  As you can see, the basic approach is relatively simple and grounded in common sense.  It also would pass as the generic basis for ARC's specific procedures):


Before tightening any fasteners, the following should be performed:

a.    Examine fasteners for compliance with marking requirements.

b.    Examine the internal and external threads for burrs, nicks, metallic slivers, etc., that could cause jamming or excessive resistance to tightening.  Remove or correct as necessary.

c.    Ensure the threads and mating bearing surfaces are clean and free of rust, chips, or other foreign matter.

d.     Ensure the nut (or cap screw) seating surface is flat and contacts the mating surface all around.

e. Lightly lubricate the threads and bearing surface with the specified lubricant and remove excess lubricant to permit air to escape from under the nut (or head of the cap screw).  Flange spot facing should also be lubricated. 

f. If using torque measurement method, ensure the torque wrench has a current calibration sticker.  Select a torque wrench such that the required torque is between 20% and 90% of the full-scale range of the torque wrench selected.


FASTENER TIGHTENING PROCEDURES.  The following procedures are applicable to nuts, through bolts, studs, cap screws, and set-studs used on flat-face and raised-face flanges:

a.   Prior to applying final torque, perform the prerequisites described in steps a through f above.

b.   Assure proper alignment of the mating components.

c.   Where the application requires O-rings or gaskets, ensure the O-ring or gasket is in its proper position.  Make up the joint evenly by tightening diametrically opposite fasteners until the mating components contact each other.  This will normally be accompanied by a noticeable increase in torque when metal-to-metal contact is made. Check all fasteners to ensure that no fasteners are loose.  Continue to tighten fasteners sequentially.  Apply approximately ten percent of the specified torque to ensure solid part contact.  Finish torquing the joint in 25 percent increments of the specified torque.

d. For determining torque values used in this procedure, refer to paragraph 5-5, use Appendix E (PC Bolts), or seek guidance from your activity's Design Division.

e. When tightening nuts in set stud and nut type joints, check stud rotation by marking with a felt-tip marker on the nut end of each stud in a direction toward the center of the flange.  Check the mark on each stud after tightening to ensure the stud did not rotate.

f.  After completion of the last tightening pass, wait a minimum of 2 minutes, and execute a check pass or passes until the joint holds the specified torque setting.  This minimizes the effects of short-term preload loss and helps minimize fastener-to-fastener preload variation.


9.  Where hydrogen embrittlement is mentioned above, it shouldn't be a problem in the environment of a rod bearing cap screw, since you need high stress (above the material's threshold stress level) in the fastener, a source of free hydrogen and an electrolyte, and an electrical potential, like screwing a Kmonel stud into an HY-80 steel pressure hull, or a steel cap screw into an aluminum rod - the whole world is a battery waiting to happen once you introduce dissimilar metals.  High strength steels (140 ksi yield and above for purposes of H2 embrittlement discussions) are subject to H2 embrittlement and can be assigned a threshold stress level of 80% of yield (threshold stress level is a bit of a moving target, but 80% of yield is a good working value for high strength steels), but for embrittlement to be a problem, you have to have ALL the factors, not unlikely in the marine environment, but HIGHLY unlikely inside an engine, even if the target mean preload is 90% - 100% of yield for the rod bearing cap screws.


10.  Where PC Bolts is referred to above, it is a relatively simple to use computer program developed by the Navy for calculating fastener torques for bolted joints.  I can send you a copy if you're interested in playing with it - it covers through bolted joints, cap screws threaded into blind holes, and set stud and nut type joints.  The present version is a pleasant little calculating machine in 16 bit Dos code, so old it lacks mouse support, but it's lean, mean and gives you torque, preload and a large collection of joint component stresses.   Or you can input a torque and get preload and the stresses, or input a desired preload and get the necessary mean torque and stresses.  The Navy hands it out for free to anyone that wants it.  My one mule consulting outfit heads the Beta test program for the new 32 bit Windows version, which we will have out soon and which also will be given away free to anyone that wants it, once it's ready to release into the wild.  As a Navy employee I headed Beta testing for the three previous versions, so PC bolts is sort of the crown jewel of the contributions I've been able to make to solving fastener problems; that, and a whole bunch of friction coefficient testing that went into the lubricant library of PC Bolts, and which led me to take note of your rod installation instruction.


11.  Oh yeah, one other little item; all of the above discussions assume that when the designer chose the fastener size and alloy and the preload we are going to apply to the fastener, he/she had a reasonably good idea of what the worst case in service load will be and chose the preload to be well above that load after short term preload loss and a reasonable profile for long term preload loss.  If we blew any portion of that little set of choices, we have the potential for cyclic load reversal and good old fatigue failure.  Short term preload loss can run anywhere from 5% to 40%, though proper tightening procedures will make it unlikely that you will ever see the latter figure.  Our preferred approach is to take the max in service load and select a preload 4 times that amount, so that you can lose 50% of your initial assembly preload to any combination of short and long term preload loss and still have twice the preload needed to do the job - crude, but it works.  Most bolted joints work well because their design is conservative (if nobody goofed), and hence they are very forgiving of minor errors, abuse and neglect.  Of course, the more refined your design assumptions are from a standpoint of test results or other prior experience with a specific application on which to base your decisions, the closer you can cut it if space or weight considerations intrude, without putting the joint's integrity in danger.  In combatant aircraft design, there are bolted joints that are subject to fatigue failure in order to reduce size to fit in the space available.  With fatigue life to failure in a cyclic range that corresponds to 12 to 18 months of service, the aircraft maintenance plan calls for automatic replacement every 6 months.  That's a valid solution for a fighter plane, but probably won't hack it for a passenger car.


12.  I have repeatedly used the terms hi-shock and Undex in the discussions above.  To give you an idea of the magnitude of the forces we're dealing with when Hi shock and explosion testing are invoked, civil engineering design for earthquake involves designing for accelerations primarily in the 9G to 11G range.  Hi shock and Undex deal with G forces in the 450G range, give or take, and greater. 



Tech Center Printable Archive
By popular demand.  One document containing ALL of ARC's TechCenter Articles.

On Rod Bolts, By Mike Gifford
Learn what is so special about proper torquing procedures.
Date: April 4, 2005

Are you Unbalanced?
Why crankshaft balancing is important.
Date: October 29, 2003

Setting Ignition Timing
Date: May 8, 2003

Mike McCarty's Book
Want to understand your Kart Chassis?  Click here and buy Mike's Book from his website.

Kart Setup Sheet and Calculation Sheet
Microsoft excel and Works format spreadsheets.  Helps organize and calculate kart setup & race data. FREE
Instructions and explanations included.

Is 1° Costing You The Win?
Re: How easy is it for you to experiment with your motor?
Date: April 12, 2002

The Crankcase Vacuum System (CVS)
Date: January 7, 1999
Re: Crankcase pressure management

Remove 400 lbs. from your crankshaft !
Date July 2, 1998

Probable Cause of Rod Failure
Re: What accurate measurements can really mean
Date: June 1, 1999

Dual Plane Balancing
Re: What is it ?  Should you do it ?
Date: January 19, 1999

Does It Matter ?
Re: A common sense approach to engine building
Date: November 11, 1998

A 10,000 RPM Bomb
Re: Why some engines self destruct
Date: July 18, 1998

Briggs & Stratton 5 hp. Motor Specs
Re: All the basic specs for stock Briggs 5 hp. engines
Date: July 15, 1998

Calculation Formulas
Re: How to calculate just about anything
Date: July 15, 1998
Updated: November 11,1998

Cylinder Head Torque Pattern
Re: Proper torque procedure of a Briggs head
Date: July 15, 1998

Safe Peak Power RPM
Re: What it is & how to calculate it.
Date: July 18, 1998

Rod Length Ratios
Re: What they mean & how to calculate them.
Date: July 18, 1998


Definition of the Sport

In a drag race two vehicles go against each other over a straight, measured distance from a standing start. The standard distances are a quarter-mile (1,320 feet/402.3 m), 1000 feet (301.5m) (contested in the highest horsepower classes only), or an eighth-mile (660 feet/201 m). A standard drag racing event involves several classes, mom tires and lays rubber down at the beginning of the track, improving traction), then lines up (or "stages") at the starting line. Informal drag races can be started by any means, including flag-waving and arm-dropping. Professional drag races are started electronically, with a series of vertically-arranged lights known as a Christmas Tree. A Christmas Tree consists of a column of lights for each driver/lane. In each column, the top two lights are small amber lights connected to light beams on the track, which when broken by the vehicle's front tire(s) indicate that the driver has pre-staged (approximately 7 inches from the starting line) and then staged (at the starting line).[1]

Below the staging lights are two large amber lights, a green light, and a red light. When both drivers are staged, the tree is activated to start the race, which causes the three large amber lights to illuminate, followed by the green light. There are two standard light sequences: Either the three amber lights flash simultaneously, followed 0.4 seconds later by the green light (a "Pro" tree), or the amber lights light in sequence from top to bottom, 0.5 seconds apart, followed 0.5 seconds later by the green light (a "Sportsman" or full tree). If the driver leaves the starting line before the green light illuminates, the red light for that driver's lane illuminates instead, indicating disqualification should no further infractions occur.

Several measurements are taken for each race: reaction time, elapsed time, and speed. Reaction time is the time from the green light illuminating to the vehicle leaving the starting line. Elapsed time is the time from the vehicle leaving the starting line to crossing the finish line. Speed is measured through a speed trap near the finish line, indicating the approximate maximum speed of the vehicle during the run.

The winner is the first vehicle to cross the finish line (and therefore the driver with the lowest total reaction time + elapsed time). The elapsed time is a measure of performance only; it does not, per se, determine the winner. Because elapsed time does not include reaction time, a car with a faster elapsed time can actually lose the race if the driver does not react to the green light fast enough. In practice, it is advantageous for the driver to "jump the gun" by a fraction of a second, starting the car during the split-second interval between when the yellow light goes out and the green light goes on. However, if the car leaves the front light beam before the green light comes on, the driver has "red-lighted" (because the red light is lit on the Christmas Tree) and should no further fouls happen during the race, is disqualified. Once a driver commits a red-light foul, the other driver can also commit a foul start by leaving the line too early but would win because he or she would leave the line slower. A driver who gets a substantial lead at the start is said to have gotten a "holeshot". A win where a driver wins a race with a higher elapsed time but lower reaction time is known as a "holeshot win".

It is also possible for a driver to be disqualified for other infractions, depending on the rules of the race, including crossing the centerline between lanes, touching a wall, striking a track fixture, failing to stage, failing a tech inspection, or running faster than expected/allowed for the assigned class. In boundary line violations, if the offending driver have made a clean start, and the red-light driver does not commit the violation unless forced by the offending car for safety reasons, the driver who committed a red-light foul wins.

In the common Eliminator racing format, the losing vehicle and driver are removed from the contest, while the winner goes on to race other winners, until only one is left. In cases where a driver has no opponent for a round, the driver makes a solo pass or "bye run" (in order to at least partially eliminate the advantage that would otherwise come from the engine having one less run on it) to advance to the next round. In most Eliminator formats, the bye runs take place only in the first round. On bye runs, some drivers may choose to drive slowly so as not to stress the car unduly, though choice of lane in the each round is often determined by time in the previous round, making this strategy possibly detrimental. Unlike the NHRA, many European events feature a consolation race where the losers of the semifinal rounds race for third place, the final spot on the podium, and standings points.

During drag racing events, vehicles are classified by various criteria that take into account the extent of modifications to the car. These criteria include engine capacity, configuration of cylinders, frame type, vehicle construction materials, wheelbase, horsepower to weight ratio, number of cylinders, whether or not power adding devices such as turbochargers, superchargers or nitrous oxide are employed, vehicle type (such as car, truck, et cetera), or even make and model for limited entry fields. The aforementioned divisions are in place to ensure that the cars are evenly matched during the race. (not all of these can apply)

Drag racing vehicles are special in that they are modified to be lighter and more powerful than in their standard form. A lighter vehicle means that the power-to-weight ratio is increased and hence a greater acceleration will be achieved. Power increases vary depending on the extent of the modifications to the engine.

Flopper with body up. Note single-plug heads.

Racing organizations-
The National Hot Rod Association (NHRA) oversees the majority of drag racing events in North America. The next largest organization, Live Nation's International Hot Rod Association (IHRA), is about one-third the size of NHRA. Nearly all drag strips are associated with one sanctioning body or the other. The NHRA is more popular with large, 1/4th mile nationally-recognized tracks (although the two fuel classes have 1,000 foot races because of safety issues), while the IHRA is a favorite of smaller 1/8th mile local tracks (and offers selected races on their national tour under the 1/8th mile format. One reason for this (among others) is the IHRA is less restrictive in its rules, such as rules on nitrous oxide (legal in Pro Modified) and oversized engines (no 8.2 liter / 500cid engine restriction in the IHRA's Pro Stock category) and less expensive to be associated, as the IHRA is part of a publicly traded company.

Prior to the founding of the NHRA and IHRA, smaller organizations sanctioned drag racing in the early years. The first commercially sanctioned drag race on the East coast was reputed to have been held at Longview Speedway (now Old Dominion Speedway) in Manassas, VA. Old Dominion Speedway is currently sanctioned by the SBRA (Southern Bracket Racing Association).

Caterpillar-sponsored dragster. Note wide slicks and high-mounted wing, to assist traction.
There are literally hundreds of different classes in drag racing, each with different requirements and restrictions on things such as weight, engine size, body style, modifications, and many others. NHRA and IHRA share some of these classes, but many are soley used by one sanctioning body or the other. The NHRA boasts over 200 classes, while the IHRA has fewer. Some IHRA classes have multiple sub-classes in them to differentiate by engine components and other features. There is even a class for aspiring youngsters, Junior Dragster, which uses an eighth-mile, also favored by VW racers.

In 1997, the FIA (cars) and UEM (bikes) began sanctioning drag racing in Europe with a fully established European Drag Racing Championship, in cooperation (and rules compliance) with NHRA. The major European drag strips include Santa Pod Raceway in Podington, England; Alastaro Circuit, Finland; Mantorp Park, Sweden; Gardermoen Raceway, Norway and the Hockenheimring in Germany


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