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.

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.

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