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Mogas in O-470k giving higher EGT than 100LL

Posted: Sun Sep 02, 2018 5:21 pm
by Skylark60
I began running Mogas recently and noticed an increased EGT of about 25 Deg F full rich cruise??

Can anybody help me understand why this is?

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Sun Sep 02, 2018 6:29 pm
by TheRealMcCoy
Lower octane than 100

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Sun Sep 02, 2018 8:38 pm
by rigpiggy
Ding ding ding, winner, winner.... chicken dinner

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Sun Sep 02, 2018 10:22 pm
by PilotDAR
The "energy content" of MOGAS is very slightly greater than that of AVGAS. The octane rating is not a measure of energy (power) of the fuel, but rather its resistance to detonation in higher compression engines. You won't be getting close to the detonation margin in an O-470K, unless you're horribly abusing it. Because MOGAS is a mixture of different constituents of fuel, it burns a little differently, and may give slightly different EGT readings, Don't be too concerned about absolute values, but rather the relative EGT values, and any anomalies. I've run MOGAS in a wide variety of engines and airframes, and with one Piper low wing exception, never had a problem whatever.

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Mon Sep 03, 2018 11:05 am
by FADEC
You should have longer cylinder life on Mogas; less chance of valve problems.
Remember; 80/87 in Canada never had lead in it; the Spec allowed lead, but it was not used because the octane was achieved without lead.
Lead is not good for any engine unless the detonation margin needs to be raised without using more expensive ingredients.

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Mon Sep 03, 2018 2:06 pm
by Posthumane
I used to run Mogas in my 172, but my current plane won't tolerate it. There are a number of differences between 100LL and Mogas besides lead content and octane rating. Mogas has less strict controls on the different blends that it can be made of, so different brands may have slightly different combustion characteristics. As PilotDAR said, the anti-knock rating of 100LL is not needed on most aircraft engines, but what did cause an issue for me was the fact that most Mogas blends have a lower distillation point (think of it as boiling point) than 100LL. I ran into a few situations where the fuel would boil over in the float bowl (hot summer days, heat soaked engine) and would flood the engine, sometimes during taxi and sometimes just after applying full takeoff power. Just food for thought - there may be other factors to consider when switching fuel types.

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Tue Sep 04, 2018 10:19 am
by FADEC
A Lycoming O320 in a 172 running on Mogas for years is giving cylinder life of over 2500 hours.
O360's give similar results; greatly increase cylinder life particularly in hard use like glider towing.
The fuel is Shell 91 octane.
No issues with heat.
The possible problem of heat could occur using "Winter" Mogas in hot temperatures.
Mogas Vapour Pressure is adjusted seasonally to improve winter starting.
"Summer" Mogas often has a lower Vapour Pressure than Avgas and is thus less likely to have heat problems.

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Tue Sep 04, 2018 5:27 pm
by Posthumane
FADEC wrote: Tue Sep 04, 2018 10:19 am A Lycoming O320 in a 172 running on Mogas for years is giving cylinder life of over 2500 hours.
O360's give similar results; greatly increase cylinder life particularly in hard use like glider towing.
The fuel is Shell 91 octane.
No issues with heat.
The possible problem of heat could occur using "Winter" Mogas in hot temperatures.
Mogas Vapour Pressure is adjusted seasonally to improve winter starting.
"Summer" Mogas often has a lower Vapour Pressure than Avgas and is thus less likely to have heat problems.
I also had over 2500 hrs on an O-300 which ran Mogas and didn't show any detrimental effects.

While it's true that vapour pressure of Mogas varies considerably between seasons, do you have a reference showing that summer Mogas has a lower vapour pressure than avgas?

From what I've been able to find, the spec for reid vapour pressure for 100LL is 38-49 kPa, as found here:
https://www.exxonmobil.com/english-US/C ... gas-Series
https://www.bp.com/content/dam/bp-air/e ... eaflet.pdf

What I've been able to find for Mogas shows a range from about 48 to 103 kPa, as found here:
https://www.chevron.com/-/media/chevron ... review.pdf
And a broken down by temperature ranges here, with the lowest being summer fuel at 45-60 kPa
http://www.globalautomakers.ca/files/pr ... %20NEW.pdf

Re: Mogas in O-470k giving higher EGT than 100LL

Posted: Fri Sep 07, 2018 8:33 pm
by Skylark60
Thank you to all for their input