Carb Class: Best Jetting

Carb Class: Best Jetting

Question: I hear a lot of theories about best jetting.  How do I know what the best jet is for my combination?

Answer: The short and quick answer is whatever jetting produces the best MPH on your time slip.  The long answer is a little more involved but the best MPH for given conditions (assuming a series of runs are being made the same day for testing purposes) is indicative of amount of fuel a given engine combination requires to produce the greatest horsepower (under the same weather conditions with no other changes).  Disregarding the Elapsed Time for a moment, once the car gets settled down and accelerating to the finish line, variances in E.T. have very little effect on the MPH.  Especially in the quarter mile, the mile per hour recorded has a direct correlation to the horsepower.  Since we are jetting for best power, MPH is the point of reference we use.  General recommendations are to go up or down, two jet sizes from your baseline runs. The baseline needs to consist of at least two runs. The safe approach initially 

is to increase the jet size. After changing jet size, record how each change affects the MPH.  An  increase indicates the engine needs more fuel, a decrease means the engine needs less fuel. Continue to go up or down until the MPH drops below peak, then simply return to that jetting combination that produced the best MPH. 

Question: When are the best weather conditions to do this kind of testing?

Answer: Weather conditions, more specifically Density Altitude can have a tremendous effect on both your engine’s performance as well as how much fuel it needs.  Our recommendation is do your testing in the conditions you most frequently race under.  If you live in a climate that is very hot during the summer and the on-set of night does not change the corrected altitude then the best jetting under those conditions should keep your engine happy in weather you most frequently race in.  This step may require a separate Spring/Fall and Summer tune up. These separate tunings will help make it easier to predict your car’s consistency and repeatability.  Pro competitors and Competition Eliminator tuners jet to the weather conditions because they are trying to extract every ounce of horsepower they can for the next round of competition. This isn’t necessary for bracket racing of course but when the jetting is very close to optimum the car’s performance even with nominal changes in weather, should be easier to predict.

Question: My buddy and I each bought EGT (Exhaust Gas Temperature) sensors for our engines this past year.  I’m little confused we have similar engines but my engine runs better with a lower temperature than his.  What’s the deal?

Answer:  EGT’s are a great tuning tool and reading the temperatures after every run can be invaluable for diagnosing problems when the readings deviate a great deal from your norm.  Unfortunately, your “norm” might be different than your buddy.  There can be some real variances because the exhaust temperature is rising or building heat as you are making your run.  Setting aside differences in the engines, like camshaft profile, timing, compression, etc. one engine can be running richer than the other at low RPMs and therefore starts out the run at a lower temperature.  Naturally that richer idle and off idle system is going to build heat faster than the leaner engine.  Additionally, when you get out of throttle at the end of the run, the richer idle system will keep the exhaust gas temperatures cooler.  If there is only a small variance between the two engines at the finish line, don’t worry about it too much because that is where the greatest load occurs.  Look at where in the RPM band the differences occur and that might be a tip off your engine is running richer (therefore the lower exhaust temperature) and perhaps concentrate your tuning efforts in at area.    

Dave & Donna Mullin