"Matt Whiting" wrote in message
...
Bob Fry wrote:
"Scott" == Scott writes:

Scott One thing I think "might" be a concern is that burning
Scott 100LL (can't get 80 octane avgas these days) in an engine
Scott built for 80 octane is the extra heat. Eh? Where's this
"extra heat" come from? There is no practical unit
energy difference between different octane fuels.

Some believe that higher octane gasoline burns more slowly and thus can
put more fire past the exhaust valve. However, most of my reading on the
subject suggests that this is a myth and there is no substantial
difference in burn rate as a function of octane.

Matt

I have not had occasion to test this premise on an aircraft engine--since
only 100LL was available when and where I flew. Therefore, no comparison
was possible.

However, back in the "bad ol' days", running a "regular gas" automobile
engine on "hi-test" had much the same effect as running with the ignition
timing retarded relative to that called for in the manual--the engine ran
noticeably warmer. Therefore, I am fairly certain that Big John is correct
in his assessment (elsewhere in this thread).

There is also the oft' stated issue of lead fouling, and it is possible that
the 100LL valves have some greater immunity to that as well; although I am
neither an engineer nor a wrench turner and I really have no idea how that
might be accomplished.

Peter