Octane – A gasoline’s octane rating measures its resistance to “engine knock.” Gasoline vapor heats up as the cylinder compresses it; if it heats up too quickly, combustion will occur before the cylinder reaches its peak and the spark plug fires. The mistimed combustion causes a vibration – a “knock” -- that results in engine wear. Finely tuned, higher performance engines generally require higher-octane rated gasolines.
As with RVP ratings, gasoline octane ratings vary from one refinery process to the next, but are generally lower than required for efficient engine operation. Refineries therefore add alkylate – produced from the light gases generated by catalytic cracking – to raise the octane rating of blended gasoline.»next