SPECIFIC GRAVITY

It is important for the racer to know is the specific gravity of the fuel. This is vital for tuning purposes. Specific gravity is very critical to your tuning. The specific gravity is essentially a measure of the weight of the fuel. Specific gravity is measured against the weight of water, which is arbitrarily assigned the weight of one. So, if you have a specific gravity of gasoline of .72, which means that 72% of is density is water, it would weigh approximately 45 pounds per cubic foot. Water weighs 62.4 pounds per cubic foot. In order to find the density of the gas you must multiply the specific gravity, which in this case is .72, by 62.4 pounds per cubic inch

The ideal air/fuel ratio for spark ignition engines is generally quoted as being 14:1. Therefore, you need 14 pounds air for every pound of gasoline that enters the combustion process, through the carburetor or fuel injector system.

You need to know the weight of the fuel so you can adjust to the proper air/fuel ratio. Keep in mind that, normally, people tune to between 12:1 and 13:1 air/fuel ratio. You want to be more to the rich side of the stoichiometric 14:6:1.

In most cases the weight of fuel will vary from one another. This is again where the consistency of the product is very important. For instance, if one week a race team uses a fuel that has a gravity of .75 and the next week they use the same octane fuel with a different gravity it can significantly change the tuning setup. It may cause the engine to not put out as much power as it did the previous week.

If you are racing at two tracks and buy fuel at each track, you may be buying two different products. You must be aware that the specific gravity of each product may be different. Always check the manufacturer’s brochure for the gasoline’s specifications and adjust your tuning accordingly. Most Saturday night racers can run a fairly wide range of specific gravity gasolines. The secret for maximum performance is to tune the engine exactly for the specific gravity of the fuel you are using each night.