For many classes and national events, both the fuel and often the fuel supplier is specified. In that case, there is no fuel choice. However, for many others such as local bracket racing gasoline setups, that is usually not the case. The selection of gasoline is first dependent on leaded or unleaded. Racecars driven to the track are usually unleaded by ‘highway law’. Sole purpose racecars can be set up that require leaded gasoline with higher anti-detonation characteristics.
The next choice for gasoline fuels is the octane rating. Octane need would be dependent on the engine compression. That is static compression or, for forced induction, a combination of static compression and boost. An engine with the minimum octane that is needed for anti-detonation will usually not run better with a higher octane fuel. That fuel simply burns slower. In some cases, a bump in the octane will allow more ignition timing. Sometimes that increases power, but not always.
Gasoline racing engines have an air/fuel ratio window of about 10% between a lean and rich limit. That is relatively narrow and gasoline choice can affect whether you are in or out of that window. Many gasoline racers we speak with do not know the weight of the gasoline they are running. Some gasoline fuels vary in weight with the season, altitude, or location. Jetting needs can change based on variations in gasoline weight. Air/fuel ratio management is a good way to manage weight as a variable. The fuel can be weighed or the specific gravity can be measured to determine that value.
Many NHRA classes can run alcohol fuels. E85, a blend of ethanol and gasoline, is readily available at the highway pump in some locations around the world. It is usually a low cost alternative fuel for high compression or high boost application. E85 can be run 25% rich or more to retard detonation. Air/fuel ratios are richer than gasoline. That is a lower numerical air/fuel ratio number. However the best air/fuel ratio is dependent on the blending ratio. We saw blends varying from 80-20 to 90-10 not too long ago, although they are starting to become more consistent. An adjustment of a couple points of air/fuel ratio is needed for those differences if they vary from one tank full to another.
Methanol is a good choice in many NHRA classes where it is legal. It absorbs a lot of heat when it vaporizes. As a result, it tends to keep an engine cooler. This is an advantage for drag racing where many racecars do not have a cooling system. As long as it is not contaminated, it is 99.9+% methanol with no blend variation issue. Air/fuel ratios are richer than E85 or gasoline. That means it is a lower numerical air/fuel ratio number. Methanol racing engines have an air/fuel ratio window of about 30% between a lean and rich limit. That is a wide window that makes tuning a lot easier.
Nitromethane is the ultimate power making fuel. Nitro carries a lot of oxygen in the fuel, and extra fuel from excess enrichment can run in the monopropellant mode. Certain racing classes limit nitro to certain percentages mixed with methanol. For example, in exhibition nitro classes, different percentages of nitromethane can be added to a methanol engine to make more power especially in cost-effective lower percentages.
In our research, we provide suggested air/fuel ratios for different percentages, developed from consultation with Don Jackson Engineering. Those are reported in several of our publications available through our online store.
Air/fuel ratio control of different nitromethane percentages is the best way to setup and tune a combination. Trial and error from experience or lack of experience is done instead by many, and we saw a lot of engine damage during the infant mortality testing of many new combinations where the wrong air/fuel ratios were tried out.