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Octane (Fuel Octane Rating)

The 'Octane' or Fuel Octane Rating, of a fuel, is a measurement of the resistance of that fuel to autoignition. It refers to the percentage of iso-octane in an 'ideal' iso-octane/heptane mixture (i.e. 90 octane is 90% iso-octane, 10% heptane), that would exhibit the same auto-ignition characteristics as the fuel in question. Higher octane numbers means the fuel is capable of withstanding higher temperatures in the cylinder before spontaneously combusting.

The 'ideal' gasoline used to test octane is made of only two chemicals, iso-octane and heptane. The more iso-octane in the mix, the less readily the fuel will ignite. In reality, gasoline is made out of a cocktail of various petroleum distallates, which varies depending on the brand, season, and region. The fuel is compared to the 'ideal' two part gasoline test mixture for auto-ignition resistance and judged accordingly.

Other characteristics of the fuel such as energy content, flame propagation rate, and combustion temperature are not taken into consideration, so it can be said that in many ways all '87 octane' fuel is not all created equal.

A pre-spark combustion event in the cylinder is known as pre-ignition, knocking, or pinging. It is characterized by a harsh banging noise coming from the engine, on mopeds it will typically sound like marbles being shook inside a can.

If you are running high compression, aggressive timing, a kit or a hot plug, running a higher octane fuel can control pre-ignition. Other than that there is no reason to run higher octane fuel than required, aside from claimed benefits from additive packages.

Read this if you have a lot of time and want to read about gasoline and octane ratings.