From Wikipedia
"An octane rating, or octane number, is a standard measure of the performance of an engine or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating (igniting)."
IIRC they have more mid-range volatiles that allow them to do this.
No extra energy in the fuel (of note), but having a higher compression ratio in the engine design means that more energy can be extracted for the same amount of fuel.
Modern petrol engines with direct injection overcome octane limitations and have a higher compression ratio than port injected engines through introducing the fuel at a later stage of compression.
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