Let’s pick up right where the idea of octane rating comes from — Page 85, Lesson 111

Let’s pick up right where the idea of octane rating comes from. I want to walk you through how that rating is actually measured, because it’s a clever comparison test, and then we’ll look at why aviation fuel carries two numbers instead of one. The octane rating is found by comparing your test fuel against two reference fuels. Those two references are iso-octane and normal heptane. Iso-octane is the high-quality, knock-resistant fuel; normal heptane is the poor, knock-prone one. You run the engine on the fuel under test, and you run it on a blend of the two references, and you adjust the blend until it produces the same degree of detonation in the engine. The test is done in a special single-cylinder engine, under standardized conditions. So if the blend that matches your fuel is 95% iso-octane and 5% normal heptane, then your fuel gets an octane rating of 95. That number is literally the percentage of iso-octane in the matching blend. So the octane rating is a measure of the fuel’s anti-knock value — the higher the number, the more resistant the fuel is to detonation. Now, here’s the catch. The original tests were run at an air/fuel ratio that gave maximum detonation. But that condition is not truly representative of the working range of the engine. Maximum detonation actually occurs with the economical mixtures used for cruising. For take-off and climbing, you use rich mixtures — extra fuel — and that suppresses detonation. So a single rating based on the worst-case mixture doesn’t tell you how the fuel behaves across the whole flight envelope. That’s why aviation fuel has two ratings. This is sometimes called the performance number or performance index. Let me give you the classic example: AVGAS 100LL. That’s a 100 octane fuel with a performance number of 100/130. The lower figure, 100, is the weak mixture detonation point. The higher figure, 130, is the rich mixture detonation point. So the fuel is rated for both the lean cruise condition and the rich take-off and climb condition. Now the practical rule that follows from all this: if an engine is designed to use a certain grade of fuel, you should never use a lower grade, because that would cause detonation. And if the correct octane rating isn’t available, you must use a higher octane rating instead. Going up is safe; going down is dangerous. So to tie it together: octane rating is the anti-knock measure from a reference blend, aviation fuel gets two numbers for weak and rich mixtures, and you always err on the side of a higher grade, never a lower one.