Most people believe that the octane number of a fuel will determine which is the best fuel for them, believing the higher the octane the more powerful the race fuel. The octane number is not a value of power, and therefore many people will be surprised to know that a 104 octane unleaded fuel can easily outperform a 109 or 110 octane unleaded fuel in many cases!
As each engine, each racing application is different, selecting the right race fuel, is determined by a wide range of factors.
If you provide us with the information we need, we can provide you with expert advice from our many years of experience in the supply of racing fuels into the West Australian Market.
Specific Gravity – What Is It And Why Does It Matter?
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.
This does not necessarily mean that a “Heavier” fuel is less suitable. You need to take into account the application and engine design. Is it a naturally aspirated circuit car, supercharged V8 at the Drags or a Go kart running ultra high RPM’s. These are just some of the factors to take into account when choosing your fuel.
So in addition to paying attention to fuel metering changes when switching race fuels, you also 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.