Fuel Grade Requirements for Lycoming Engines

Marc J. Zeitlin


During the discussion after my presentation at the Columbia Fly-In, the topic of Fuel grade requirements came up (I have no idea what tangent caused that, but whatever). Given the new 94UL, 100R, and other unleaded aviation fuels that either are or will soon be available, it behooves us to know what fuel we can use in our engines, as 100LL slowly starts to disappear (and good riddance, for many reasons). John Caulkins had a couple of good articles recently in the COBA (nee CSA) newsletter, discussing the issues around the new unleaded fuels, but he didn't discuss what could be used where. An article in this month's AOPA magazine does discuss it, but at a high level - not "you can use this if that".

At any rate, when I pull out my Lycoming O-360 operator's manual, in the "Fuel and Oil" section in Chapter 3, it lists what fuel is required depending upon your engine's model #. I have an O-360-A2A, so theoretically, even with a compression ratio of 8.5:1, I am limited to 100/100LL. HOWEVER, if I go to the Swift Fuel 94UL FAQ here:

and the Lycoming Service Instruction here:

both clearly indicate that my engine is approved to use Swift 94UL fuel in lieu of 100LL. When 100 octane unleaded fuels become available, one assumes that those would be acceptable as well, given the identical octane level.

So you should be able, if you know what engine you have, to easily determine whether available unleaded fuels are acceptable for use in your engine. And given our E-AB nature, we do not need STC's to do so.

All bets are off for 94UL, however, if you've installed high compression pistons in your engine - anything higher than the operator's manual compression ratio makes any approvals void, as it's the compression ratio and octane level that work together to eliminate the threat of detonation or pre-ignition. You'll have to wait for 100 octane unleaded fuel before switching. Just another reason not to screw with the compression ratio unless you're racing for $$$.

Hope this is useful.

Marc J. Zeitlin                      marc_zeitlin@...
Copyright © 2022                     Burnside Aerospace