We have to use higher octane in our cars because the engines are such high compression. When using lower octane fuel, the fuel ignites at a lower temperature, sometimes without the spark, and before top dead center. If this happens, then you get pre-ignition, and you hear the knocking sound. That why deisel fuel is like 56 octane (something like that) and deisel engines don't have spark plugs. the fuel ignites because of compresion.
