Racing Automotive Tuning
Excerpts from Circle Track Magazine, April 2004, by Henry Olsen
Jetting a carburetor is one of the few black arts in the automotive world that is still a mystery to most racers and tuners, most tuners look at the spark plug, the exhaust port and the first 6 inches of the header for proper color and make a guess at what jet size change is needed. One of the disadvantages of this method is that the header and spark plug can only indicate what the mixture was at the rpm and load condition at the time. The content of the engines exhaust can show what the air/fuel mixture is and how efficiently the engine is burning the fuel. Races are often won or lost by getting more laps out of a tank of fuel than the competition, how efficiently the fuel is burned by the engine can be a major factor in the difference between winning and losing a race.
The proper tuning of a race engine can make the difference between being the winner or having never ending troubles while trying to just keep up with the competition. For most racers, one of the biggest mysteries is how do you jet the engine in order to obtain the correct air to fuel ratio necessary for your race engine to not only supply drivable horsepower under all race load demands but also while cruising during caution laps, having the air/fuel mixture correct for the engines needs while you are cruising around the track at caution speeds is often ignored. If the air/fuel mixture is too rich for the engine while you are running at caution speeds, the engine may tend to load up and foul the spark plugs, while if the air/fuel mixture is too lean the engine may tend to run hot. Having a air/fuel mixture rich enough for all racing conditions will allow you to get all the horsepower out of the engine while getting as many laps as possible from a tank of fuel without overheating or doing any engine damage from having too lean of an air/fuel mixture, this is one of the many tricks it takes to beat the competition to the finish line. This may sound impossible, but the new advances in exhaust gas analysis technology have made it possible to read and/or record what the air/fuel mixture actually is under almost any driving condition. In the past exhaust gas analyzers have tended to be large and expensive, but we have been using one of new modern units from the PerformanceGas series of infrared exhaust gas analyzers from OTC/SPX; these units are not only compact and portable, but also affordable for a grass roots racer.
Most race carburetors sold today have a generic tune-up or jetting unless the carburetor is built for a specific engine package and fuel. Just adding mufflers or any header/exhaust system change such as adding an H pipe into the exhaust can cause the air/fuel mixture to change making it necessary to re-jet the carburetor. A carburetor not built and tuned for a specific engine, exhaust system, and fuel must supply an air/fuel mixture rich enough for a variety of engines. If the carburetor is supplying too lean of an air/fuel mixture, the engine will run sluggish, overheat or the lean mixture could cause engine damage. If the carburetor is supplying an air/fuel mixture that is too rich, the engine may tend to load up, foul the spark plugs, run sluggish and lack power.
Ignition Advance Curve
Before checking the air/fuel mixture, the ignition timing and advance curve must first be correct. Any distributor, performance replacement or original equipment, must have the mechanical and vacuum advance curves checked and then tailored to the engine and the fuel being used. (Note: MSD distributors come with a very conservative mechanical advance curve and included in the box are the bushings and springs to get the desired curve.)
Air / Fuel Mixture
A lean fuel mixture (too little fuel for the amount of air in the cylinder) can cause an engine to stumble or give a rough idle as well as to run too hot, overheat, and cause a lack of power as well as engine failure. A rich fuel mixture (too much fuel for the amount of air in the cylinder) can cause an engine to load up at idle, foul the spark plugs, and also lack power or run sluggish...An optional method of checking air fuel mixtures is by using a wide band oxygen sensor installed into the exhaust header, the...