MOGAS STC

[old man emu]

I was going through the POH of a friend's plane and found that it had the Paterson (?) STC for Mogas, issued in 1988. (Lycoming O320-E2G). Has anyone heard of, or takes advantage of, this STC?

 

The STC specifies that the fuel to be used had an Anti-Knock Index (AKI) in the low 80s. So what's this AKI? It's a measure of a fuel's resistance to detonation. It is a calculated value based on the average of the values of the Research Octane Number (RON) and its Motor Octane Number (MON).

 

The research octane number (RON) describes the behavior of the fuel in the engine at lower temperatures and speeds, and is an attempt to simulate acceleration behavior. The motor octane number (MON) describes the behavior of the fuel in the engine at high temperatures and speeds – a full-throttle range. 

 

The values published by BP for its Ultimate fuel are RON = 98; MON = 86. Therefore the AKI is (98 + 86)/2 = 184/2 = 92 AKI.

The values for BP's Regular unleaded are RON = 91; MON = 81, giving an AKI of 86.

 

Since both fuels are ethanol-free, and the Lycoming O320-E2G has a compression ratio of 7:1 is there any point in paying the extra for 98 octane fuel?

 

NOTE: The STC advises that if unleaded Mogas is used, a lead additive is also used. This can be achieved by using Avgas for 10% of each 100 hours' running. The Avgas can be mixed with the Mogas, but the mixture must be described ar Mogas.

[pmccarthy]

In my O360 I ran 95 or 98 usually blended with at least 40% avgas. Paterson STC. I'm sure this was quite acceptable.

[old man emu]

This is a fascinating topic. Petersen have created a very informative website to provide information about the use of mogas in aero engines. https://www.autofuelstc.com/ which is well worth reading through, especially the FAQs in the side columns.