That depends on the BTU content of the fuel. The higher octane will allow
higher compression and tolerate heat better before knocking. Part of that
heat is the release of BTU energy. When I used to roadrace motorcycles long
ago in a mis-spent youth, I used a fuel made by ERC in Hayward. CA. The air
cooled bikes used stuff around 106 octane with about 18K BTU's. The water
cooled stuff could tolerate more BTU as the cooling system removed heat
before it became destructive. The fuel used for drag racing was rated at 22K
BTU. The engines would tolerate this heat release as it was for very short
times with an engine that was not fully warmed up. If this fuel was used in
an endurance event with one of our air-cooled engines, it would result in
holes in the pistons and seizures.

Now, what would 22K BTU stuff do in an 7 1/2 to 1 aircraft engine? Probably
nothing as the lower compression would limit the amount of work actually
being done. But it will still burn hotter than a fuel with a lower BTU
content. Might have to richen the mixture a bit if CHT or EGT were being
monitored, but knock and/or detonation won't be a problem.

One of the main benefits of using higher compression is the amount of power
available at part throttle. You can get more power out of a higher
compression engine at very low power settings than a lower compression
engine. The result of this is a lower fuel burn, unless the mixture is
richened to cool the cylinder. But cooling at part throttle shouldn't be an
issue all things being equal.

Dale Alexander


Eh? Where's this "extra heat" come from? There is no practical unit
energy difference between different octane fuels.
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Exactly. "Octane" is, by definition, a measure of a fuel's resistance to
knock under specific conditions. "High Octane" fuel does not burn any
hotter, generate any more power, or improve your fuel economy (note: see
exception below). Higher octane fuel lets the engine designer use a
higher compression ratio, or more spark advance, etc. without triggering
knock. It is the compression / spark changes that result in more power,
etc.