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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've 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.
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