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The octane number of a gasoline is a measure of its resistance to knocking. Gasolines with high octane numbers burn more smoothly and are thus more effective fuels. The more highly branched alkanes (carbon and hydrogen compounds) have higher octane numbers than straight chaine alkanes. The octane number of gasoline is obtained by comparing its knocking characteristics with those of "isooctane" (2,2,4-trimethylpentane) and heptaine. Isooctane is assigned an octane number of 100, and heptane is assigned 0. Gasoline with the same knocking characteristics as a mix of 90% isooctane and 10% heptane is rated as 90 octane.
Octane levels are improved by adding antiknock agents.
That is one of the few things I remember from two semesters of chem in college, one of those classes I don't wish upon anybody.
Ultimately though, unless your engine requires the higher octane fuel, you aren't really doing anything other than burning your money. Your engine gets optimal mileage at whatever the book recommends (most likely regular).
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