Racing Fuel - Fueling The Equation

The engine's design has a direct correlation to the fuel requirements. Pre-ignition can be a result of the design of the engine and/or the selection of components that are assembled to make up the engine, not solely a function of the fuel used in the engine. The problem is when we use fuel not originally intended for the engine. Or, as in the case of the Saturday night racer, we race with an engine that was not designed for racing. Then we start to make modifications to the engine to make it work "better" for our application. The first thing we do is make the inlet and exhaust systems work better. Now the engine is getting more air and fuel, and suddenly the dynamic compression ratio increases. The improvements to the inlet and exhaust allow more air to be pumped into the same volume as when it was a stocker, and the compression ratio goes up. Also, we are creating more heat, and the cooling system may become a bit more marginal, so the internal temperatures start to rise above what the engine was designed to endure. In addition, we now are running the engine at wide-open throttle as much as we possibly can. And the final straw is that we crank more ignition timing into the engine, trying to make more power. Something has to give; the fuel is the first warning signal.

If we look at the newer cars, many are coming directly from the factory with compression ratios that are a throwback to the muscle cars of the '60s and '70s. How are they getting away with that? They have a complete package: well-designed combustion chambers, very exact fuel distribution due to electronic fuel injection, and computer-controlled ignitions. The spec sheets on some new cars have 11:1 compression ratios, and the cars run fine on premium pump gas. It is all about the package.

If we just look at the fuel to reduce the engine's tendency to detonate, we have to look at the chemistry of the fuel itself. This can be accomplished in several ways, as we can alter the chemistry of the fuel by adding octane or several other chemical compounds. Yes, octane is a chemical, just like benzene, tetraethyl lead, and isooctane. The formula for octane is CH3 (CH2)6CH3. From a molecular level, octane is a chain of 8 carbon atoms surrounded by 18 hydrogen atoms. It is a measure of the resistance of fuel (in this case gasoline) to knock or pre-ignite. The desired result of combustion is an even, controlled burn of the fuel in the combustion chamber. We want to avoid multiple areas of burn at different rates across the face or crown of the piston.

We may need to use a better grade of fuel in our engine to preclude the pre-ignition we are sure to get when we improve the volumetric efficiency of the engine or when we crank in a few more degrees of advance. How high of an octane rating do you need? I would be very surprised to see a Saturday night racer with an OEM-based engine needing racing gasoline with a 110 octane or greater. In fact, most of the racing gas companies out there make 100-octane gasolines that will handle any OEM-based "racing" engine. Most 100-octane racing fuel is usually good enough to work in engines with compression ratios up to 12:1.

Let's talk about burn rate or speed for a bit. Higher octane fuels do not resist pre-ignition due to a slower burn rate. While that is a seemingly plausible reason, there is no basis in fact for that explanation. The truth of the matter is that fuels with higher octane levels are blended to handle higher temperatures and pressures without pre-ignition. It is just that simple. For example, a Nextel Cup engine has a compression ratio on the tall side of 12:1, and the engines can turn in excess of 9,000 rpm when the track conditions merit high rpm. I do not think these engines use "slow-burning" fuel. They need to burn the whole of the fuel they are using to get the kind of power they are getting.

I am not suggesting or advocating that racers blend their own fuel. Stick to what you know. You probably know tires, shocks, and stagger more than the advanced chemistry required to blend fuel. The average racer does not have the infrastructure required to make fuel. The gasoline companies do a great job, so let them handle the fuel manufacturing. As racers, we have bigger problems to solve.

Do you see a pattern developing? It is not just about fuel, although the fuel may be the fix. Controlling pre-ignition is about more than just adding higher-octane fuel to the tank. If your car runs well on pump gas, is there a reason to spend the money on racing fuel? That is a question you will have to answer yourself. I suggest you consult the stopwatch and see what the data says.