Data Support Plausible Mechanism in Certain Lycoming Engines

General Aviation Modifications, Inc., (GAMI) announced at EAA AirVenture that after performing extensive testing of 94UL avgas in a Continental IO-550 engine operating at the equivalent of 73% power, the data indicated moderate to severe detonation that was absent when operating with 100LL.

The detonation observed with 94UL provides a reasonable explanation of the cause of valve seat recession reported in Lycoming engines in the training aircraft at the University of North Dakota (UND).

George Braly, GAMI’s Head of Engineering said, “It’s not the lack of lead in the fuel, it’s the lack of octane needed by those high-compression engines with advanced spark timing. This, at least, is a competent cause for the reported issue.”

Lycoming noted that its R&D has not yet definitively determined the cause of the issue, but suggested the higher aromatic content of the Swift 94UL fuel is being investigated as a contributing factor. 

However, GAMI tested a wide range of high octane formulations during development of its G100UL, some of which had aromatic content well above the 18-20% reported by Swift.

GAMI’s detonation test data noted above was obtained during operation of a Continental IO-550 engine with 8.5:1 compression ratio and spark timing set at 22 degrees before top dead center (DBTDC) running at 73% power and mixture set at peak EGT consistent with the operating procedure at UND. Tests performed under those conditions showed no detonation while running on either 100LL or G100UL, but after switching to 94UL significant and continuous detonation exceeding limits was observed.

FMI:  gami.com/