Understanding Octane

The pump octane ratings are essentially averages of two numbers, the Research Octane Number "R" and the Motor Octane Number "M". Often, the formula will be expressed on the pump as:

(R + M) ÷ 2

You should note that two different gasolines with the same pump octane rating might behave differently. This is because the pump octane is an average of the two values R and M. R and M may be numerically close or differ greatly. If they differ greatly, the gasoline will most likely have better anti-knock properties than a gasoline with similar R and M numbers, even though both averages are the same. Unfortunately, R and M numbers are not readily available. So if there is a slight knock in your engine and you are using the correct grade of gasoline, try a different brand with the same octane rating. The knock may disappear.

SYSTEMS
FUEL
OCTANE ADJUSTMENTS
GASOLINE VOLATILITY