Testing the difference in running temperature between car gas and av gas

There’s a lot of dispute as to which is better to run in your paramotor, avgas or mogas (as in motorcar), especially with the higher ethanol content of mogas. But one claim is significant: that avgas runs cooler than mogas. That’s big because excessive heat is an engine’s mortal enemy. So I plan on getting some numbers together to see.

I consulted with Chris Bowles of Southern Skies, a long time proponent of Avgas, on the proper way to do the test for a fair comparison. I purchased the Digitron RPM/EGT/CHT temperature gauge for the test, mounted in according to the directions, and test flew it on my Black Devil paramotor mixed at 35 to 1.

The first test was to run the motor on mogas as it’s running right now—perfectly. I adjusted the mixture for highest RPM, which actually allowed quite a range of settings so I opted for the rich side but no more than 30 rpm rich of peak. With the motor warmed up, I started doing readings.

Just after the first test, I noticed a muffler bolt coming loose. That would skew the results so I’d have to go back and do the mogas test again.

By the way, the idle mixture is best set rich for starting but that will make it run somewhat rough. It starts surprisingly easy when rich but I struggled until figuring that out. And I started it a bunch of times.

Results were: Mogas 91 Octane, Shell or equivalent

7570 rpm, 1030 f, until stabilized

7540 rpm, 1050 f, :60

7520 rpm, 1030 f, :30 cht from 330 – 393

Then I put the fuel pickup in a 5 gallon can of avgas, ran the motor a minute to clear out the avgas and did the test again. Importantly, I tightened the very loose exhaust bolts.

I went to full power and adjusted the mixture to peak RPM which was about 100 less than with autogas. Go figure that one out. It took a lot of adjustment to actually get the RPM to change, at least a half-turn.

7330 rpm, 1180 f, until stabilized

7260 rpm, 1190 f, 400 f after :60

7330 rpm, :30 second run starting at cht 330, it went up to 384.

Then, since the exhaust bolts had gotten tightened between tests, I ran it up again with Mogas to see what I got.

7450 rpm, 1275 f.

I didn’t even go further because it was obvious the mogas was running hotter than the avgas.

So the verdict is out. At least on my Black Devil motor, Avgas does run cooler but also seems to get slightly less power when both are leaned to peak RPM.