Matt wrote:
Hi everyone. I was reading about Petersen's STC to use mogas in the O-235.
Apparently the STC for the O-235 only allows premium (91 octane) to be used.
I would still be reluctant to use premium since it is a "step down" in
octane for the O-235. I would not have a problem if I was flying an O-200
and could "step up" to 87 octane since the engine was originally certified
to run on less.

Aviation fuel is rated with an octane rating that is not the same as
the (R+M/2) automotive octane rating. The traditional aviation rating
was more reliable as a guide with the WWII and 50's airliner operations
that ran such powerplants as the R-2800-CB17 at high BMEP low RPM
schedules. A normally aspirated Lyc or Continental (which would chuck
jugs through the cowling in that regime) is essentially a lightweight
farm tractor engine-it operates in a regime more similar to automotive
engines.

Premium mogas is still much cheaper than 100LL. If I had good
availablity of both fuels I would probably rin a mix of 100LL and
mogas-the effect of the TEL in the avgas will boost the mogas's octane
rating much higher than that of each fuel separately, even with as
little as 10% 100LL.

I have operated autombiles and motorcycles for years on just such a
mixture and still run my high compression BSA on just such sauce. I
know of many others running aircraft engines on this as well with good
results. Unless your STC says the tank has to be cleared of avgas
first, it is obviously legal under the STC and gives the FBO a small
revenue stream keeping him off your ass.