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It's an octane rating. It's changed by adding and removing octanes, a family of hydrocarbons. Octanes affect the tendency of fuel to self-ignite under pressure, or to burn too quickly when ignited. High octane fuels do not necessarily contain any more or less energy than low octane fuels. High octane fuels do, however, allow engines to run at higher compression ratios without unwanted self-ignition or too-fast combustion of the fuel.
The higher compression ratio is where the performance improvement is found; higher fuel octane ratings are just a means to that end.
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