Lycoming to approve 93 octane auto gas for O-360 & IO-360

JGalban via AviationKB.com wrote:


I'm curious where you got the 88 aviation octane number. If that is
accurate, O-360s and IO-360s will likely be blowing holes in the tops of
their pistons on hot days.

John Galban=====N4BQ (PA28-180)


Temp has almost nothing practical to do with the octane required. What
matters is the internal cylinder pressure. For example the IO-520 in my
Bo develops about 800 psi, sea level and standard day, at max rpm and
full throttle. The GAMI guys have instrumented aviation engines and
gathered this data. Neither Lyc or Continental have done so. At 800
psi the 520 is on the ragged edge of detonation. That's at 31 inches of
MP, what you'd get at sea level. They also determined that at 26 inches
of MP the 520 develops 400 psi. This is also why owners with constant
speed props are told to always reduce manifold pressure first. If you
are at 800 psi with everything wide open and you reduce rpm you have
made the situation worse. The engine stays at the high pressures
longer, and at a different spot relative to top dead center, because
you just made the engine turn slower. All bad. So the fact is that it
is the internal cylinder pressure that determines what octane you need.
While reading the article he wrote I got to thinking that I never see
over 26 inches of MP at my elevation of 3650 and of course the higher
you go the lower it gets. Therefore here at 3650 MSL 100 octane is not
needed, 90 would be more than adequate. And go to those mountain strips
that are higher yet and your octane requirement drops even lower.