89.5 AKI was enough

With the recent announcements from both Lycoming and Continental of new engines capable of running on 94UL and autogas, we wondered why, in both cases, a minimum octane was specified that was significantly higher than that required from autogas STCs for very similar engines that have been used safely by thousands of pilots since the 1980s.  We asked Todd Petersen, whose company Petersen Aviation owns most of these STC’s, for an explanation:

“After completing testing on 87 octane engines, Petersen Aviation began test programs on what were considered ‘high compression’ engines. The FAA drew that line at 7.2:1 compression ratio. Our testing of the 0-235, 160hp 0-320, 180hp 0-360, 250 and 260hp 0-540 and on the I0-520 (285 and 300 hp) and I0-470 (260hp) was all done on the lowest octane premium fuel we could find, which was Conoco 89.5 AKI. No detonation was experienced except on the I0-520 and I0-470 and that was safely resolved by using water/methanol injection.

“At the time, that testing was the only testing that had been done on these engines to determine operating characteristics at the low end of the octane scale. The FAA wrote the resulting STCs for 91 octane in order to have some margin for error. Had it been left up to us, we’d have wanted the STC to specify 89.5. Lycoming is building in even more margin for error by writing their recent approvals for 93. However it is clear from our test program that these engines function safely even down to 89.5 AKI.”

Does this mean that STC holders and others should operate their engines on 89.5 AKI autogas?  No, the law specifies a minimum AKI and pilots must adhere to this.  But the fact that 89.5 AKI was enough octane during Petersen’s exhaustive testing should provide plenty of evidence for even the biggest skeptics that autogas, an FAA-approved aviation fuel since 1982, is safe to use in aircraft approved for it.

The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.



  1. Kent Misegades says

    Dennis, the testing above dealt with high-compression ratio engines.  Many others are covered by STCs that require only 87 AKI.

  2. Dennis Reiley says

    Why wasn’t automotive non-premium fuels tested. It has been proven that other than the prevention of detonation there is no mileage/cost advantage to using lower octane fuels. The savings in lower octane fuels is greater than the range lost.

    • Dennis Reiley says

      Let me try that again: There is no mileage/cost advantage to using higher octane fuels. The savings in lower octane fuels is greater than the range lost. As for power, the slight lost should not be that noticeable, if it is the powerplant is undersized for what you’re asking it to do.

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