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

This forum has been developed to discuss Bush Flying & Specialty Air Service topics.

Moderators: ahramin, sky's the limit, sepia, Sulako, North Shore, Rudder Bug

Post Reply
Message
Author
Skylark60
Rank 1
Rank 1
Posts: 15
Joined: Sun Nov 14, 2010 11:27 am

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

#1 Post by Skylark60 » Sun Sep 02, 2018 5:21 pm

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?
---------- ADS -----------
  

User avatar
TheRealMcCoy
Rank 2
Rank 2
Posts: 62
Joined: Mon Apr 23, 2018 7:58 pm

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

#2 Post by TheRealMcCoy » Sun Sep 02, 2018 6:29 pm

Lower octane than 100
---------- ADS -----------
  

rigpiggy
Rank 10
Rank 10
Posts: 2220
Joined: Sun Jan 16, 2005 7:17 pm
Location: west to east and west again

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

#3 Post by rigpiggy » Sun Sep 02, 2018 8:38 pm

Ding ding ding, winner, winner.... chicken dinner
---------- ADS -----------
  

User avatar
PilotDAR
Rank 10
Rank 10
Posts: 2628
Joined: Sun Sep 30, 2012 6:46 pm
Location: Near CNJ4 Orillia, Ontario

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

#4 Post by PilotDAR » Sun Sep 02, 2018 10:22 pm

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.
---------- ADS -----------
  

FADEC
Rank 2
Rank 2
Posts: 90
Joined: Wed Feb 16, 2011 7:31 pm

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

#5 Post by FADEC » Mon Sep 03, 2018 11:05 am

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.
---------- ADS -----------
  

Posthumane
Rank 7
Rank 7
Posts: 534
Joined: Sat May 09, 2009 6:16 pm

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

#6 Post by Posthumane » Mon Sep 03, 2018 2:06 pm

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.
---------- ADS -----------
  
"People who say it cannot be done should not interrupt those who are doing it." -George Bernard Shaw

FADEC
Rank 2
Rank 2
Posts: 90
Joined: Wed Feb 16, 2011 7:31 pm

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

#7 Post by FADEC » 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.
---------- ADS -----------
  

Posthumane
Rank 7
Rank 7
Posts: 534
Joined: Sat May 09, 2009 6:16 pm

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

#8 Post by Posthumane » Tue Sep 04, 2018 5:27 pm

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
---------- ADS -----------
  
"People who say it cannot be done should not interrupt those who are doing it." -George Bernard Shaw

Skylark60
Rank 1
Rank 1
Posts: 15
Joined: Sun Nov 14, 2010 11:27 am

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

#9 Post by Skylark60 » Fri Sep 07, 2018 8:33 pm

Thank you to all for their input
---------- ADS -----------
  

Post Reply

Return to “Bush Flying & Specialty Air Service”