.svg)
Understanding Octane: How to Pick the Right Rating for Your Racing Engine
Sunoco is often asked questions in regard to mixing fuel and calculating octane when mixing fuel.
Mixing fuels is generally safe; Sunoco race fuels are all blended using the same components, just mixed at different concentrations to aim for specific properties. The fuels will not cause any reactions or unsafe conditions when mixed. It is important to be aware that mixing leaded and unleaded fuels is not recommended for environmental and compliance reasons.
Despite being safe, mixing your own fuels is not advised unless you have experience with fuel chemistry. Fuel properties do not always mix in a linear or expected way. Octane and RVP especially behave abnormally.
When mixing equal parts of a 100-octane fuel and a 110-octane fuel, you may not end up with a 105-octane blend due to the way the molecules interact and behave in an engine.
When ethanol is involved, things get even more unpredictable. All octane test engines, as defined in the octane rating procedures set forth by the American Society for Testing and Materials (ASTM), are carbureted. Air/fuel ratio adjustments on octane engine carburetors are limited and cannot accommodate the extremely different air/fuel ratio requirements of pure alcohols.
Blends of alcohols can be tested to determine what is called a “Blending Octane Value” or BOV. Basically, the octane rating of an alcohol/gasoline blend is compared to the octane rating of the gasoline without alcohol, and some math is done to calculate what the effect of the alcohol was on the octane of the gasoline. Thus, a BOV is determined. However, a BOV is not the same thing as a normal octane rating.
Ethanol's BOV is generally accurate when looking at an E10 type blend, but at higher concentrations the BOV does not hold constant.
In addition, because volume is temperature dependent and not a fixed property, a blend of 85% ethanol and 15% gasoline may not result in an 85% ethanol blend.
All of these unknowns can cause batch-to-batch discrepancies which could result in negative racing outcomes. Sunoco has decades of experience with blending fuels and a line of consistently dependable race fuels. For the best racing outcome, racers can rely on Sunoco race fuels rather than risky hand blends.
If you do decide to mix fuels together to create your own octane or ethanol blend, use an octane calculator rather than a hand calculation to take these non-linear properties into account.
