Paying Homage to MoGas

timwinters

timwinters

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I've burned about a 50/50 mix of MoGas/100LL in both of the airplanes I've owned. (as an aside, I refuse to burn straight MoGas in an airplane for reasons I won't go into here).

My '57 172 had a continental O300 in it. My '58 182 has a continental O470 in it. Both engines like the mix. They run smoother. They don't foul plugs. And, in general, they're happier.

With the recent increase in fuel prices I've been experimenting with the ratio of the mixture. I've been slowly increasing the mix to nearly 75% MoGas and 25% 100LL. Actually, this pretty closely replicates the 80 octane aviation fuel that these engines were designed to run on...both in octane rating and lead content.

To my surprise I've found that my O470 continues to run smoother and smoother the more MoGas I run.

As an example, I made a 300NM round trip X/C last week. Here were the parameters:

Cruising at 8500' MSL
63dF OAT
20" MP
2300 RPM
50dF Carb Temp.
Big pull 'till rough.
Enrichen 'till smooth.

And here's what I see:
attachment.php


This is typical, I've been seeing between 15 and 25dF spreads lately. And we're talking a carbureted engine here! Heck, I think the Gami boys would be happy with 15dF!

Before, even when burning 50% MoGas, I was lucky to get the spread below 60 to 75dF.

Another nice benefit...I've picked up 3kts at cruise since all pistons are performing more equally.
 
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timwinters said:
I've burned about a 50/50 mix of MoGas/100LL in both of the airplanes I've owned. (as an aside, I refuse to burn straight MoGas in an airplane for reasons I won't go into here).
Click to expand...

Hmmm - I wonder how my 520s would do on that stuff.
 
2256Q said:
Hmmm - I wonder how my 520s would do on that stuff.
Click to expand...

Watch the octane rating. 520s need higher ratings than most mogas offers. You might get destructive detonation. Petersen Aviation can inform you. A low-wing aircraft is also less likely to be found with mogas STCs available, since it is more volatile and vapor-locks much more easily. Gravity-feed systems (high-wing) are not prone to vapor-lock.
We used to run a 75/25 mogas/100LL mix in our Continentals and Lycs (flight training airplanes) thinking that we were getting the same lead levels as 80, but the Continentals didn't like it. The exhaust valve guides wore faster and we had problems with valves burning once they started hitting the seat off-center. Lycomings also had some issues, but they've since gone to a better guide material. We eventually went to straight 100LL when the price differential got so small as to be little or no savings, once the valve hassles were figured into it. We have to put up with plug fouling.
Unleaded mogas works for modern auto engines that have advanced materials designed to operate without the lead. We don't have that advantage.
Unleaded mogas, as I understand (maybe someone else can tell us more accurately) is made using different methods than leaded fuels. IIRC, the petroleum isn't just cracked and recombined into a fuel that is then doped with tetraethyl lead, as 100LL is. Mogas is made from petroleum broken down to the molecular level and reconstructed entirely, making it more synthetic and more expensive to produce. We've been hearing promises for a long time now about 82UL, an 82 octane unleaded aviation fuel that is designed for our older-tech engines and their fussy valves and guides. Haven't seen it yet and I bet it won't be cheap, either, since it'll use the same complex processes that mogas does now.
I wish Dan Horton was still around. He used to lurk on the RAP group, and seemed to be a chemical engineer that had worked with this stuff.

Dan
 
Dan Thomas said:
Watch the octane rating. 520s need higher ratings than most mogas offers. You might get destructive detonation. Petersen Aviation can inform you. A low-wing aircraft is also less likely to be found with mogas STCs available, since it is more volatile and vapor-locks much more easily. Gravity-feed systems (high-wing) are not prone to vapor-lock.
We used to run a 75/25 mogas/100LL mix in our Continentals and Lycs (flight training airplanes) thinking that we were getting the same lead levels as 80, but the Continentals didn't like it. The exhaust valve guides wore faster and we had problems with valves burning once they started hitting the seat off-center. Lycomings also had some issues, but they've since gone to a better guide material. We eventually went to straight 100LL when the price differential got so small as to be little or no savings, once the valve hassles were figured into it. We have to put up with plug fouling.
Unleaded mogas works for modern auto engines that have advanced materials designed to operate without the lead. We don't have that advantage.
Unleaded mogas, as I understand (maybe someone else can tell us more accurately) is made using different methods than leaded fuels. IIRC, the petroleum isn't just cracked and recombined into a fuel that is then doped with tetraethyl lead, as 100LL is. Mogas is made from petroleum broken down to the molecular level and reconstructed entirely, making it more synthetic and more expensive to produce. We've been hearing promises for a long time now about 82UL, an 82 octane unleaded aviation fuel that is designed for our older-tech engines and their fussy valves and guides. Haven't seen it yet and I bet it won't be cheap, either, since it'll use the same complex processes that mogas does now.
I wish Dan Horton was still around. He used to lurk on the RAP group, and seemed to be a chemical engineer that had worked with this stuff.

Dan
Click to expand...

I'm flying gitso (GTSIO) 520 Cs on a Cessna 411. Seems like they were originally designed for the old green gas, 130 octane. The 100 LL works okay, I guess, but they've probably retuned everything to accommodate the lower octane rating.
 
Dan Thomas said:
We used to run a 75/25 mogas/100LL mix in our Continentals and Lycs (flight training airplanes)
Click to expand...

For some reason I thought that the MoGas STC was only authorized for private, not-for-hire, aircraft.

Don't recall where I heard or read that. Maybe it was verbal and an OWT.

I did A LOT of research before burning the first drop of MoGas in an airplane because of all the misinformation out there. This research included speaking with two "formulists" in the oil industry. One in Bartlesville, the other in Ponca City. Both were very eager to talk (too eager, actually...I envisioned geeks with pocket protectors on the other end of the phone who never got to see daylight or talk with anyone. Both conversations lasted 30 min +) and both assured me (off the record) that auto gas is not an issue in aircraft if:

1. Low compression engine
2. Flown at moderately low altitudes (since I don't suck oxygen, I'm okay)
3. Flown regularly. This is important for two reasons. First MoGas "sours" much more quickly than AvGas and, second, because of the MoGas seasonal blends.

This was 6 years ago and our discussions at the time were based on 100% MoGas. I can't bring myself to do this mostly because of the Reid vapor pressure differences. My theory (supported by research) is that any downsides of MoGas are mitigated by blending with 25% to 33% AvGas.

As an aside, I'm confident my low EGT spreads are a direct result of MoGas's propensity to vaporize and remain vaporized in the induction system (lower RVP than AvGas). O-470s are notorious for their poorly designed induction systems and many flying these engines struggle to get spreads of less than 100dF

Of course, in today's world, the biggest challenge is ensuring that MoGas is ethanol free. It's a simple test but ethanol free fuel is getting harder to find.
 
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Hm. I oughtta try that, since I don't even need an STC to run mogas in my O-200. (The POH spends a page on reprinting Teledyne Continental Motors' dire warnings about mogas, and then the very first words on the next page are "Use of unleaded automobile gasoline in this aircraft is permitted.")
 
timwinters said:
For some reason I thought that the MoGas STC was only authorized for private, not-for-hire, aircraft.

Don't recall where I heard or read that. Maybe it was verbal and and OWT.

I did A LOT of research before burning the first drop of MoGas in an airplane because of all the misinformation out there. This research included speaking with two "formulists" in the oil industry. One in Bartlesville, the other in Ponca City. Both were very eager to talk (too eager, actually...I envisioned geeks with pocket protectors on the other end of the phone who never got to see daylight or talk with anyone. Both conversations lasted 30 min +) and both assured me (off the record) that auto gas is not an issue in aircraft if:

1. Low compression engine
2. Flown at moderately low altitudes (since I don't suck oxygen, I'm okay)
3. Flown regularly. This is important for two reasons. First MoGas "sours" much more quickly than AvGas and, second, because of the MoGas seasonal blends.

This was 6 years ago and our discussions at the time were based on 100% MoGas. I can't bring myself to do this mostly because of the Reid vapor pressure differences. My theory is that any downsides of MoGas are mitigated by blending with 25% to 33% AvGas.

As an aside, I'm confident my low EGT spreads are a direct result of MoGas's propensity to vaporize and remain vaporized in the induction system (lower RVP than AvGas). O-470s are notorious for their poorly designed induction systems and many flying these engines struggle to get spreads of less than 100dF

Of course, in today's world, the biggest challenge is ensuring that MoGas is ethanol free. It's a simple test but ethanol free fuel is getting harder to find.
Click to expand...

Tim,
Where in Missouri are you finding MoGas? I think Diana is having hers trucked in from Kansas.
 
Dean said:
Tim,
Where in Missouri are you finding MoGas? I think Diana is having hers trucked in from Kansas.
Click to expand...

The law mandating 10% ethanol went into effect last January. This law only mandates ethanol in regular. Some stations still have premium w/o ethanol. There is one (Rapid Roberts) right around the corner from my house that doesn't put ethanol in their premium.

Most all mid-grades will have at least 5% ethanol in missouri since most stations just have two tanks and when you pump mid-grade you're (presumably) getting 50% from the regular tank and 50% from the premium.

Also, the first gallon or two of gas you purchase at single hose pumps will be whatever the person before you was pumping. So, when I'm getting gas for the plane, the first two gallons go into the car/truck.
 
timwinters said:
For some reason I thought that the MoGas STC was only authorized for private, not-for-hire, aircraft.
Click to expand...

If you're an American, it's sort of true.

We bought ours from Petersen Aviation:

http://www.autofuelstc.com/autofuelstc/pa/PetersenAviation.html

A couple of excerpts from their "Limitations" page:

"The FAA does not allow the use of mogas while carrying paying passengers under Part 135. The airplane may have a mogas STC on it and may use mogas otherwise, but not when a "paying passenger" is riding in the airplane. Some governments may impose other restrictions."

"In stark contrast to the U.S. FAA, Transport Canada does not differentiate between paying and non-paying passengers. Auto fuel can therefore be used in Canada while carrying paying passengers."

So in the US Mogas could be used in a Commercial airplane but not while carrying paying pax. Presumably, freight would be OK, as would cropspraying or aerial photography or banner towing, maybe glider towing.

Here in Canada, they don't care. Maybe our colder climate reduces the hazards of vapor lock enough that the risks are lower. Or maybe they just think the FAA is too fussy.

Dan
 
timwinters said:
For some reason I thought that the MoGas STC was only authorized for private, not-for-hire, aircraft.
Click to expand...

The last flying club I was a member of fueled 90% of their fleet with MoGas, unless you wanted to pay out of pocket for 100LL. :dunno:

The only exceptions were a 172 on lease-back (owner didn't want anything but 100LL used in it) and a Beech Travel Air that I never flew anyway.
 
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