The Simple Definition

An octane rating (87 vs. 89 vs 92, etc.) is a measure of a gasoline's ability to resist detonation, which manifests itself in a pinging or "knocking" noise. Higher numbers indicate that the fuel can be compressed to a higher level before detonation / knock occurs in an engine, which occurs when. As described in "Detonation, Knock, and Pre-Ignition 101", detonation / knock occurs when air and fuel that is ahead of the combustion flame front ignites before the flame front arrives.

The Complicated Definition

Octane is actually more than just a rating - it is a hydrocarbon just like methane (single carbon atom), propane (three carbon atoms), butane (five carbon atoms), and heptane (seven carbon atoms). Octane (C8H18) is a hydrocarbon with eight carbon atoms and eighteen hydrogen atoms. 100% octane fuel is remarkably resilient to compression (i.e. it does not combust when compressed) and is thus resilient to detonation / knock. This resilience is derived from the branching of octane's carbon chain (see figure). Because of the nature of octane as being resilient to detonation, all fuels are compared to 100% octane as a benchmark fuel, from which an "octane rating"can be obtained. Heptane, a hydrocarbon with seven carbon atoms, compresses very poorly and spontaneously combusts even under small amounts of compression. In other words, Heptane's behavior when compressed is diametrically opposed to Octane's behavior under the same conditions. For this reason, Heptane (which has an octane rating of zero) is the other benchmark fuel used in the octane rating system to determine a fuel's octane rating. A fuel that spontaneously combusts (knocks) under the same amount of compression as a fuel composed of 87% octane and 13% Heptane would have an octane rating of 87. This is not to say that 87 octane gasoline is made up of 87% octane and 13% heptane, rather that the 87 octane gasoline "knocks" in a laboratory knock engine at the same compression ratio as a fuel composed of 87% octane and 13% heptane.

The coomposition of an octane hydrocarbon.

Unfortunately, it gets even more complicated. Because various fuels respond differently under varying engine loads, a gasoline may get a different octane rating on a free running engine and one under load. For this reason, the octane rating label that we see at the pump (monitored by the U.S. Cost of Living Council) is actually an average of two octane ratings - the motor method rating (where the engine is run under a load) and a research method rating (where the engine is run freely). The formula used to get the CLC Octane number on gas pumps in the United States is thus: (Motor Octane Number + Research Octane Number) / 2.

What's the benefit of higher octane?

Higher octane fuel has only one beneficial feature - it allows an engine to run at higher temperatures with more advanced ignition timing under higher levels of compression witout detonating / knocking. Higher octane fuel does NOT have more potential energy and will not make an engine perform better unless that engine is knocking. On modern engines with knock sensors, higher octane fuel may make the engine run better if the knock sensors are retarding the ignition timing, which hinders performance. High octane fuel does not burn cleaner, it does not clean your engine, it does not increase horsepower or torque (unless you are experiencing knock), it does not smell better, it does not increase fuel economy (unless you are experiencing knock) and is not better for the environment. If you buy higher octane fuels for any of the above reasons, STOP!

When should I switch to a higher octane fuel?

First off, never run lower octane fuel than is recommended by the vehicle's manufacturer. If the vehicle manufacturer recommends 89 octane gasoline, this means that the engine has been tuned to perform optimally without detonation on 89 octane fuel. Once you've done some modifications to your engine, the manufacturer's recommended gasoline may no longer suffice. Obviously, if you can hear detonation inside your engine in the form of pinging or "knocking", try a higher octane fuel. You will also need to run a premium grade fuel (91+ octane) if you have a supercharger, turbocharger, or if you have an ignition programmer that advances your ignition timing.

Why is higher octane fuel more expensive?

Higher octane fuels are more expensive because they must go through more refining steps that increases the octane rating. These additional steps do not make the fuel better in any other way.

How is it possible to have 100+ octane gasoline?

There are some fuels that are even more resilient to compression than 100% octane. Some additives, like tetraethyl lead, increase the gasoline's ability to operate without knock. Some racing and airplane fuels have octane ratings of 110+!