I've got the same engine in my plane, it's an O-320 D2C, but there are
other number series with the same compression ratio in the 160hp
class. That engine is listed on Peterson's mogas STC site as one that
is approved for premium unleaded autofuel. So you shouldn't run into
any problems as far as detonation goes, vapor lock is another issue,
and is different in each aircraft type. Just the other day a pilot at
my local airport said that if the gas has the common 10% ethanol
added, it's vapor lock potential goes up a lot. I'm not sure about
those facts, but I think it has more to do with fuel system design.
Gravity flow systems seem almost immuned to that problem. With wing
tanks below the carb, the best setup is to have fuel pumps at the tank
"pushing" the fuel to the carb. Rather than the more common setup of
the fuel pump in the engine compartment sucking the fuel from the
tank. Peterson's website has a good dissertation about homebuilts
worth reading, in regards to fuel system design to help prevent vapor
lock.

On Mon, 28 Apr 2008 21:13:16 -0700, "Stuart & Kathryn Fields"
wrote:

Well it has happened. Local 100LL is $5.60/ gal and is not expected to
decrease. Is there anyone out there using Supreme MoGas in a Lycoming 0320
with 8.5:1 compression? If so what ignition timing changes were needed if
any?

thanks
Stu Fields