knock rating[′näk‚au̇t ‚rād·iŋ]
Rating of gasolines according to knocking tendency.
The standard scale of resistance to knock (the ability of fuel to remain at high temperatures and pressures without causing detonation) of reciprocating engine fuels, measured relative to isooctane. It is expressed as a percentage by volume of iso-octane in a mixture with normal heptane that has the same knocking characteristics as the fuel under question. Fuel with an 87 knock rating is a mixture of 87% iso-octane and 13% heptane. It was previously called octane rating.