The octane rating of gasoline tells you the amount that the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, it causes knocking in the engine, which may damage an engine.

An octane number is a measure of a gasoline’s ability to resist knocking as it burns in the combustion chamber of an engine.

Lower octane gas can handle the least amount of compression before igniting.

The compression ratio of your engine determines the octane rating of the gas you must use in the car. One way to increase the horsepower of an engine of a given displacement is to increase its compression ratio. A “high performance engine” has a higher compression ratio and requires higher-octane fuel.