Actually, as the octane in gasoline increases, the energy content (BTU/gallon) drops slightly. It isn't enough to make a measurable difference in performance or mileage.
Since higher octane gasoline contains no more energy, it can only increase performance by allowing an engine to run more efficiently, usually via higher compression and/or advanced ignition timing.
An engine with a higher combustion head will develop more power then an engine with low combustion of the same size. Hi octane fuel is used only to keep from pre ignition and the added power is only because of the higher compression not the fuel octane. During WW II the p-51 mustang had quite a bit more power (hp) then the ME 109 even though the 109 had larger displacement. The P 51 Had a higher compression engine and had to run on 150 grade fuel while the 109 used 87 because of the inability of Germany to manufacture better fuels. High octane fuel actually have a higher ignition point. I try to use ethanol free fuels because I live right on the coast and it help avoids absorbing moisture into the tank.
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