September: Tuning with Temperature
Lately a lot of questions have come in regarding temperature - more specifically cylinder head temp, 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 race engine. 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 heat range, exhaust pipe diameter and length, flywheel screens and tape, 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 right. 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 in karting, 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 significantly during the course of any one heat. However, quite often we qualify or hotlap under sunny conditions and warm temps and race under a setting sun or cooler evening air. 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.
If you feel you cannot afford one, don't want one, or forgot it at home - whatever - consider that hot outside temps provide less cooling effect across the cooling fins on the head and cylinder while generally warmer outside air is less dense, thus requiring less fuel for proper fuel-air ratio. Follow me now, this is very important -- When it is hot outside you genterally run a smaller, yes leaner, jet in your engine --When it is cool out you will need to run a larger, richer, jet. I would venture to guess that about 50% of experienced karters still get this wrong. No kidding - If you understand this simple idea you will be faster than half the field - well at least you should be.
Let's say your engine builder set you up with the fastest combination available - the dyno proved it. It's timed right, has the proper jet, best pipe, etc. Now you take it to the track and it's 90+ degrees out. You're sweating like a pig in the old kart suit and your engine won't come up to your engine builder's specs - I like 385 degrees CHT on cool bore (alum cyl) and 400 on I.C.'s. Assuming first that the pipe, ignition timing, kart set-up and gearing are correct for track conditions, the first place I would suggest a change is to tape up the blower housing and flywheel screen. This can be good for as much as 25* temp gain over the course of a short 8 lap heat race, and can be eadily torn off if the engine starts to run too warm. Remember to dog-ear the ends of your duct tape so that your driver can grab it during the race if necessary. Make your tape in several short pieces, (2 - 3 inch strips), so you can "tune" your amount of air entering the flywheel area. Remember, the tape is not so much insulating as it is blocking off the cooler outside air that enters the flywheel and blower housing. Next I would consider changing the spark plug. Going to a hotter plug will gain between 10 and 15 degrees CHT - Most karters use ND plugs and generally start with a ND W24FS-U or ND W27FS-U plug - I don't use the fine wire or racing and marine plugs - I think they are overkill. A spark plug's job in life is very simple - make a spark between two points when the coil and flywheels tells it to. I also use NGK plugs on my personal engines. They are only a few cents more, but they make a great product and I have never been let down by them. If a more expensive plug made a noticeable difference, believe me, I would recommend it to all my customers! Either brand, ND, or NGK, will work fine. Unless you are running a hot coil, don't fool wtih wide plug gaps - I like .023 on ND & NGK plugs - I use to use .028 on Autolites and Champions. A spark plug is a little factory worker. It does what it is told when it is told to do so and when it stops performing it is replaced. Simple, period, end of story. For more detailed information on spark plugs, including a handy heat range and cross reference chart, please see the other tech articles on our website. If I can stay in the power band of the engine without sacrificing corner sped, I will next drop a tooth or two of gear from the rear sprocket. This will lower the engine rpm, making the engine "lug down" and build more temperature. As a last resort, to gain even more heat, I next look at going down in jet size. Keep in mind though that reducing the amount of fuel going into the engine can significantly reduce the amount of power produced.
Let's say now that your engine is running hot, (over your engine builder's recommended CHT.) I first remove tape from the blower housing and flywhel screen. go to the jet - push more fuel up to the point where you overfuel the engine at peak RPM - Lean the needle 1/2 turn to where it cleans up on top end and then recheck temp problem - If you're still too hot, next I'd go to a cooler plug . If excessive temp is still a problem, add a few teeth to the rear axle sprocket. More RPM helps cool an engine --Lugging it down heats it up. Don't adjust yourself too far away from the optimum gear ratio though. - While engine temp is very important, proper gear ratio should be given an equal or greater level of importance. While you're engine builder may provide you with target RPM and temps, a stopwatch never lies. Another point that your engine builder is likely to get hot about, no pun intended, is changing pipes - He has taken many hours to come up with a cam, pipe combination that works best for your engine and the track it was built for. On the other hand, tuning temps with a variety of exhaust pipes has been a widely accepted form of tuning at the track for years - Not only can a pipe produce more bottom end, torque, HP, what have you; it also affects the cooling of our engines. Fact: Larger i.d. pipes offer less exhaust restriction and allows cooler outside air to be more easily drawn back into the cylinder. Thus cooler CHT. So short, big (i.d.) pipes cool an engine more than long, small (i.d.) pipes. Understand it, Remember it, and Use it to your advantage. Keep in mind that while short big pipes help cool, they kill bottom end power and long small pipes eliminate top end rpms. This could be a great subject for a future article and probably will be, so we'll continue with temp tuning.
This last area in which I look to make a change at the track is ignition timing - It's tough if not impossible to correctly time an engine at the track. I strongly discourage my customers from timing changes at the track - they are guesstimates at best. However, sometimes, like in the case of a sheared timing key, or a flywheel that has inadvertently come loose and changed the timing from what the engine builder originally had set it, a change is necessary. Fact: Timing (advance) builds heat. Fact: Too little timing (retard) reduces heat or more appopriately does not build sufficient heat. What number key should you use? Personally I wish offset ignition tming keys would come with no number at all. We've all been told at one time or another that the number stamped on the key referes to the amount of degrees of advance it will provide - A stock motor with straight up key and typical crankshaft and flywheel keyways aligned will time 24 degrees BTDC so a #4 key will time the engine at 28 degrees BTDC. Sometimes, but probably 90% of the time the crank and flywheel provide different advances - Don't believe me, try a different flywheel sometime or replace the crank and recheck your timing. How about a 4 degree key that advances less than a particular 3 or more than a 5 key - You betcha - As far as I'm concerned that number is a rough (very rough) reference number only and proper ignition timing is to be set with a timing light, then referenced with the cylinder head off and a depth mic down the cylinder to the deck of the piston at the actual firing point. 28* BTDC or .185 in the hole is most common I think, and is a good starting point. Some engine builders refuse to use an offset key because of their inconsistencies and the fact that if the flywheel slips, the key will shear and most likely junk the keyway in the crank itself or trash the flywheel - Not using a key also encourages the racer to return to the engine builder to correctly time the engine. Personally I prefer to not use a key. I feel that I can much more accurately time the engine exactly where I want it, and saves a lot of time trying several different keys. - Anyone can replace a key - it takes some knowledge, tools, and experience to correctly set the ignition timing.
A QUICK OVERVIEW - Tuning With Temps:
Leaning an engine (turning needle in or decreasing jet size) makes temp increase
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Check with your engine builder to get his recommendations, but don't be afraid to experiment with tuning. These certainly are not the only ways to tune your Briggs with temp., you may even know better ways (for you) but many beginning as well as veteran karters need to better understand the tuning process.
Although I'm college educated, I don't claim to be a metallurgy expert, chemistry professor, internal combustion engineer, or meteorologist - I do understand the importance of maintaining a constant temperature in the combustion chamber to optimize performance, and you should too.