Octane / knock resistance

A fuel's resistance to auto-ignition, which sets how much timing and boost it can support.

Octane rating measures a fuel's resistance to knock, that is, how much heat and pressure it can withstand before auto-igniting. In the US, pump fuel is labeled by AKI, the (R+M)/2 average of Research and Motor octane numbers, shown on the yellow pump sticker. Higher octane does not add power by itself; it raises the knock ceiling, letting you run more spark advance and boost before detonation, which is where the power comes from. This is why a tune is fuel-specific. HP Tuners reflects this directly: strategies carry separate high-octane and low-octane spark tables and blend between them based on knock activity, and many tuners build distinct calibrations for 87, 91/93, E85, or race fuel. If you tune on premium and the vehicle later runs regular, the engine relies on knock retard to survive, so always tune for the worst fuel the car will actually see and leave knock margin.