A number indicating the resistance of fuels to spontaneous ignition, known as knocking, in spark-ignition engines. The higher the octane number, the greater is the fuel’s resistance to spontaneous ignition. The rating is derived from comparison between a fuel and a benchmark blend of iso-octane (C8H18) with normal heptane (C7H16). Higher-octane ratings are more suitable for higher-performance engines whose compression ratios are more likely to make the fuel detonate. Compare cetane number.