ORIGINAL: epegasus

Engine knock is actually called premature detonation. The fuel mixture explodes in the cylinder as the result of a couple of things; pressure increases as the piston moves upward; and/or hot spots in the cylinder that cause 2 flame fronts that bang into each other. The problem is that the spark is timed to happen as the piston starts it's downward stroke. When the mix explodes early the piston cap is jammed against the connecting rod & crank which are still moving upward. BAD Juju and it wastes power trying to force a piston down when it can only move up. It can also kill valves and rings. So the purpose of higher octane levels is to raise the fuel mixture's resistance to detonation under pressure so that it waits for the spark timing.

The ECM can compensate for knocking caused by hot spots by retarding the spark so that the flame fronts don't collide but you still have a pre-detonation problem that will waste gas and hurt parts so don't rely on the knock sensor and ECM to compensate for using the wrong octane. Hot spot predetonation can be caused by using spark plugs that are too hot or by carbon build-up that super heats and acts like a second spark plug.

A lot of people think higher octanes burn faster but that is not necessarily true. The speed of the flame front may be the same as lower octane but the pressure at which the fuel explodes is higher.