Matt Whiting wrote:



This is only true for engines that have very advanced ignition timing,
which isn't true for most GA piston engines, or engines with very high
compression ratios, again, not true for most GA piston engines. Keep in
mind that most were designed to run on 80 octane avgas...


I don't disagree with the fact that in normal conditions, and a normal
engine, the power of an engine running 100LL or mogas should be
virtually the same.


Yes, I can't imagine many GA piston engines where this wouldn't be the
case. Maybe a turbocharged engine running very hot.

Matt

There are plenty of high compression GA piston engines that can't use
mogas because the octane requirement is too high. The majority of GA
engines are lower octane (something like 78% of the engines IIRC). The
majority of the hours flown in piston aircraft however is done by the
21% of aircraft that have higher octane requirements than mogas can
offer, my 260 HP O-540-E4B5 included. It is a normally aspirated 6
cylinder engine, but with high enough compression that there is not
sufficient detonation margin with less than 96 octane fuel.