Definition: specific gravity (noun) - the ratio of the density of any substance to the density of some other substance taken as standard, water being the standard for liquids and solids, and hydrogen or air being the standard for gases.
For fuels, specific gravity can be determined by dividing the density of the fuel (in units of pounds per gallon) by the density of water (8.325 pounds per gallon). Let’s look at one example.
Sunoco Supreme weighs 5.95 pounds per gallon. Applying some math: 5.95 / 8.325 = 0.715. So Supreme has a specific gravity of 0.715.
If Fuel A has a lower specific gravity than Fuel B, Fuel A is said to be “lighter” than Fuel B. Literally, a gallon of Fuel A weighs less than a gallon of Fuel B. Sunoco Standard has a specific gravity of 0.728, so it is said to be “heavier” than Supreme.
Why does this matter? Well there are two reasons.
First, specific gravity has an impact on fuel metering, especially for carbureted engines. A heavier fuel is of course denser, so the float in a carb’s float bowl will sit higher than if a lighter fuel was used. If the float sits higher, the fuel level will be lower. Fuel level affects fuel metering in a number of ways, so if you’re switching fuels, pay attention to the fuel level in the bowls.
For most race fuels, specific gravity is also an indication of the composition. Note that the words “most” and “indication” were used – there are exceptions. However, for most race fuels, a lower specific gravity suggests a faster-burning fuel, while a higher specific gravity suggests a fuel is slower-burning. This is because most light hydrocarbons used to make a race fuel are faster burning than most heavier hydrocarbons. This matters because faster-burning fuels usually require less spark advance than slower-burning fuels.
So in addition to paying attention to fuel metering changes when switching race fuels, you need to pay attention to ignition timing as well. We’re not talking huge changes here, but these changes are important to the proper tuning and consistent performance of your race engine.
One final thought – there are implications for pump gas as well. The specific gravity of pump gas will typically range from about 0.720 to 0.770. As you can now guess, this wide range is a reflection of the wide ranging composition. Pump gas composition varies by octane, by region, and by season. For these reasons, a race engine that can run on pump gas must be tuned conservatively just to prevent engine failure. You can imagine the consequences of an engine that is tuned to the ragged edge on one batch of pump gas, then another batch is used in a race and the motor runs lean and timing is over-advanced. Not good.
One of the most important attributes of a race fuel is its consistency. Even if you don’t need all the engine protection offered by a high octane race fuel, you may need its consistency. Specific gravity is one of the many parameters we monitor to ensure quality and consistency in all our fuels, batch after batch.