An indicator of a fuel's ability to burn at a controlled rate at high temperatures and pressures without causing a detonation (very rapid burning or exploding). This number is expressed as a percentage by volume of iso-octane in a mixture with a normal heptane that has the same knocking characteristics as the fuel. Fuel with an 87 octane rating is a mixture of 87% iso-octane and 13% heptane. Also called the knock rating
or the performance number
when the octane rating is above 100.