Octane Rating

In general, the octane rating indicates how much the fuel can be compressed before it ignites, the spontaneous ignition of fuel before the spark from the spark plug is applied is the basis of 'knocking'. Therefore, the higher the octane rating, the greater the amount of compression required for ignition and the less likely that 'knocking' will occur.

If the octane rating of a fuel is 87 then this means that it contains 87% octane and 13% heptane. It is found that heptane will ignite spontaneously with little compression whilst octane requires a much greater degree of compression before it will ignite.