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The octane rating is a measure of the resistance of gasoline and other fuels to detonation (engine knocking) in spark-ignition internal combustion engines. The higher the octane rating, the slower the fuel burns. High-performance engines typically have higher compression ratios (and hence longer piston strokes) which requires higher octane (slow burning) fuel to allow the piston power stroke to complete before the fuel is completely burned. If the fuel burns faster than the downward movement of the piston allows, then the pre-detonating fuel results in a loud "knocking" sound and vibration. A lower-performance engine will not generally perform better with high-octane fuel, since the lower compression ratio is fixed by the engine design.

The octane number of a fuel is measured in a test engine, and is defined by comparison with the mixture of iso-octane and heptane which would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of iso-octane in that mixture is the octane number of the fuel. For example, gasoline with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90.[1] This does not mean that the gasoline contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties. Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers higher than 100.

Octane rating does not relate to the energy content of the fuel (see heating value). It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner.

It is possible for a fuel to have a Research Octane Number (RON) greater than 100, because iso-octane is not the most knock-resistant substance available. Racing fuels, AvGas, liquefied petroleum gas (LPG), and alcohol fuels such as methanol or ethanol may have octane ratings of 110 or significantly higher — ethanol's RON is 129 (102 MON, 116 AKI). Typical "octane booster" gasoline additives include MTBE, ETBE, isooctane and toluene. Lead in the form of tetra-ethyl lead was once a common additive, but since the 1970s, its use in the United States, and most of the industrialised world has been restricted, and its use is currently limited mostly to aviation gasoline.

Source: http://en.wikipedia.org/wiki/Octane_rating