Ron Natalie wrote:
: You can (almost) always run a gas that has a higher octane than
: what your engine requires, so if you normally run 87 cargas (something
: like 82 avgas rating), then flying somewhere that only has 100LL, you can
: burn that without trouble.

: Well the octane numbers aren't directly comparable. The major reason you
: can run 100LL is because your aircraft was originally certificated for 80 or
: 100 octane Avgas. The STC doesn't undo the original certification, just adds
: the option of of using autogas.

That's true, they're not directly comparable. The sticker at the
pump is an "Anti-Knock Index" or A.K.I, which is the average of the octane
measured using the Motor method, and the Research method. For typical
autogas, the difference between the two is about 10 points, so for 87 AKI,
the Motor rating is about 82, and the Research rating is about 92. The
Motor rating is very similar to the avgas lean rating. Thus the comment
that 87 AKI cargas is roughly equivalent to 82 avgas. Also not a
coincidence that the new unleaded avgas is called 82UL... basically cargas
with traceable handling and ensured free of arbitrary additives. The
avgas rich rating comparison is anyone's guess.

-Cory



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