Racing Fuel - Fueling The Equation

From A Reader: I was just going through some earlier issues of Stock Car Racing and saw the article "Fuel 101" from the Jan. '07 edition. You must have been looking under my hood when you were describing the Hobby Stock motor. My question deals with spark timing and fuel octane. Is there a correlation between the two? When I started racing a few years ago, I was told by an engine builder and a few other racers that 110-octane fuel burns slower, so it needs the spark to happen faster, therefore the timing should be set at around 36 degrees BTDC. What I read in the Fuel 101 article was I shouldn't be using 110 for a stock engine with an aggressive cam. Should my spark timing change when I change to pump gas? Thanks for any guidance on this and for all the other guidance I have already received.
Jerry via e-mail, Crystal River, Florida

The simple answer to the question is, yes, there is a correlation between spark timing, fuel octane levels, and the ability to advance ignition timing. The reader stated in his letter that fuels with higher octane burn slower, but this is not correct. The question demands a more complete answer, as there are more complex issues that need to be defined, explored, and resolved.

Starting from a very high level, we know that as compression ratios climb, the likelihood that we will overextend the range of the fuel becomes more of a probability than a random occurrence, especially if we are using pump gas. This condition can be caused by more than just climbing compression ratios. If the racer applies too much ignition timing to the engine, a variety of things will happen. Cylinder pressures will rise faster as the piston approaches TDC. This means that the fuel will start to burn sooner, due to the advanced ignition timing, adding additional pressure to the combustion chamber as the piston is traveling up the bore.

We may over-pressurize and over-temp the fuel charge in the chamber to the point at which the fuel can't withstand the pressures created. Heat goes with that pressure, and detonation occurs due to the fuel's inability to provide stable combustion based on the chemistry of the fuel in use.

Several things will be happening that will add more complexity to the situation, and they will all be happening at the same time. The heat created will be greater, it will occur earlier in the cycle, and more of the cylinder wall will be exposed to this heat for a longer period of time, taxing the cooling system. If the cooling system does not have the extra margin to compensate for the greater heat loading, the result will be even hotter chamber temperatures, which can single-handedly cause serious engine damage.

The rapid rise in pressure from advanced timing will be accompanied by a rise in temperature, which is a good thing when controlled. In fact, that is what every engine builder is trying to accomplish-more heat and control over the results. But in our scenario, the engine cooling system may not have the extra margin, and that will cause a cascade of problems to bubble up, so to speak. The least of which will be a rise in the temperature of the incoming fuel mixture. This is a bad thing, as the rise in the inlet temperature can further promote pre-ignition.

In the case of many Saturday night racers, the designs used in the engines are more applicable to the daily driver than that of a well-designed racing engine. We need to remember that we are placing rpm and heat loads several orders of magnitude greater than these engines were designed to be operated. Consequently, we contribute to detonation or pre-ignition through more than simple timing changes. Deficiencies in the cooling and fueling systems (and that includes the fuel distribution to the individual cylinders) can contribute to detonation. Many of the chamber designs that are in use today in the hobby classes are, at best, '80s technology.

Simply put, pre-ignition is fuel in the combustion chamber that explodes or burns at a rate much faster than the fuel around it. Detonation or pre-ignition is caused by more than just the compression ratios. Chamber design-including surface finishes, general shape, squish, and squelch-has an effect on the engine's ability to avoid or promote detonation. Type and heat range of the spark plugs can have an effect on whether the engine detonates or not. Specific areas in the combustion chamber that are not at an optimum design can cause detonation problems. Even a dirty combustion chamber that may have chunks of glowing carbon can cause pre-ignition. The point is that pre-ignition can come from a number of other sources and not just poor fuel.

Any knowledgeable engine builder will tell you that the design of an engine package is a collection of systems designed to work in harmony. This includes an intake system to complement the port shapes and volumes, cam timing that works in harmony with the valvetrain, and compression ratios that will yield a good balance between power and durability. There is more to building an engine than just matching some parts and setting clearances.