Originally Posted by Jordo!

Octane refers to how easily the fuel will spontaneously ignite, as in from heat or pressure (which also increases heat); the higher the number, the less likely to auto-ignite.

Ideally, we want the fuel mixture to only ignite when there is a timed spark event. If it ignites at the wrong time, the force can turn the crank the wrong way (bad), and otherwise put tremenous shock and stress on the engine internals -- this is essentially what knock (preignition or detonantion) refers to: unplanned auto ignition of the fuel that can damage the motor from the stresses it creates.

Higher heat and cylinder pressure (affected by boost and piston CR, as well as timing advance -- spark events tend to have flame kernels that keep high heat on the piston, thus more timing, also means more in-clyinder heat that doesn't have as much time to dissipate before the next rotation) increase the likelihood that the fuel will auto ignite, so for applciations with higher CR's, boost (or even excessive heat under load), a higher octane is generally necessary (DI sytems can get around this somewhat, thanks to the super fine atomized high pressure fueling mix it creates and the ability to spray directly and precisely into the cylinder -- but that's another story).

There is a bit of a down side -- really high octane fuels are harder to ignite even when you want to -- thus a 120 octane fuel in a DD might be hard to turn over on a cold morning, and be more prone to misfires, whereas on a boosted, high CR track car that might work very well.

In general, there are no direct benefits running a hgiher than needed octane -- again, octane is all about preventing knock. The extra power is all based on cylinder pressure in the piston or engine speed, so unless you are running more timing, boost, higher CR pistons, etc, that might require a higher octane to run safely, it's not needed.

As a precaution on a very hot day (again, in-cylinder temps increase the chance of knock), running the car at constant high load on the track, the extra octane might be a reasonable precaution, in that it will prevent power loss/damage from possible knock events, but it will not give you any additonal power than you were already capable of making given your tune.

Incidentially, that is also why tuners will find better power on boosted cars by running a bit richer a mixture -- the extra fuel is not fully burnt, rather it just cools the piston, reducing heat, and therefore quenching possible knock.

Anybody want to add to/amend that?