: than 87 octane car gas. I believe, for most of them, they could get by with
: less than 100, but would certainly need super unleaded as a minimum.

True. The high-compression (8.5:1) O-360/O-540's were certificated on 91/96 avgas. The Petersen STC (the
only one applicable to high-compression engines) requires mogas with the appropriate ASTM ratings and to have an
anti-knock-index (Average of Research and Motor octane... "R+M/2") of 91 or higher. When I bought my STC, Petersen
said that even in the high-temp, altitude, CHT setup, they were unable to induce knocking or detonation even with 89.
The certificiation folks wanted a bit of safety margin.

It should also be pointed out that the R+M/2 rating does not directly compare to the avgas lean rating (i.e.
the "91" of the 91/96 or the "100" of 100/130 100LL). The avgas lean rating testing setup more closely resembles the
Motor method of the R+M/2. Typical point spread ("sensitivity" IIRC) is +-4 to 5 points on either side. In other
words, 91 AKI is more like 96 Research, 86 Motor... i.e. a bit dicey given the 91/96 rating of the aviation engine.

That said, I haven't experienced any troubles running hundreds of gallons of 93 AKI through my O-360 180hp...
summer, winter, etc.

-Cory


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* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
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