Unleaded Avgas - Thoughts?

[bertschb]

Like many of you, I'm following the transition to unleaded Avgas with interest. I've found some lively discussion around the internet but very little here on VAF. I find that odd so I thought I'd start a new thread here.

Just to get some discussion going, the unleaded fuels currently being considered have shown some potential drawbacks - both real and perceived. Here are my concerns in order:

1- Fuel tank sealant compatibility
As stressful as it was to build my fuel tanks, if there is even a 1% chance that a candidate fuel will cause tank sealant to fail resulting in fuel tanks leaks, I won't use it. Instead, I'd probably sell my airplane and buy something with fuel bladders or something that isn't damaged by the fuel. No way I'm removing the tanks, removing the existing sealant and re-applying something else.

2- Paint damage
Paint damage caused by spilled/leaking fuel is almost as serious a concern to me. I will be paying a good chunk of change to get my airplane painted next year and if a new unleaded fuel ruins my paint job, I will not be a happy camper.

3- Engine timing changes
Any changes that might be required to ignition timing don't concern me too much. I think we have some leeway as experimental builders to adjust timing. I know that will reduce performance but I'm naively thinking I'll have some "overhead" to work with (215HP). I know the certified folks are saying this is a deal breaker.

4- O-ring deterioration
O-rings - little concern. Swapping o-rings doesn't seem like a big deal but again, I'm naive on this one as well.

5- Miscibility (mixing fuels)
This could be a problem initially but I'm not worried about it. During the transition, I'll just avoid mixing fuels if that ends up being an issue. Inconvenient? Yeah. Deal breaker? Not for me.

 

[Steve Iacoviello]

I look forward to unleaded aviation fuel becoming widely available. I trust the issues you point out will be solved as fuels are test marketed. If early adopters see paint damage, fuel leaks, and seal swelling most others will not use that fuel. I do not believe that the FAA/EPA will require us to damage our aircraft. Timing adjustments may only be needed for turbo charged engines, not normally aspirated.

 

[torquen]

Out here in California they put GAMI 100 at a few airport in the Bay Area. I ran into a guy just a couple of days after he filled up with it. His plane was sitting behind mine and was leaking like crazy and the paint was destroyed.
Several other planes have been completely totaled out from using the GAMI fuel. Aircraft and Engine manufacturers have already placed a voided warranty if you use the fuel.

There are some videos on YouTube my Mike Lavara where he does several tests while the camera is rolling the whole time.

It’s definitely not a fuel I will be using. The airports stopped selling it because of the results.

 

[airguy]

I fail to see the big fuss. I've been running unleaded for 10 years now, 2000 hours in my airplane. No paint problems and only 1 leak event due to an O-ring I failed to change out.

It can be done, and without all the pearl clutching and hand-wringing. Many folks, myself included, have written about it and shown the way.

You want a true drop-in replacement with zero engine or fuel system mods? Yeah, I don't think that's gonna happen. Those engines and fuel delivery systems were designed around a particular fuel - change the fuel, and you need to change the design.

 

[bertschb]

I've been running unleaded for 10 years now.

Lycoming doesn't allow mogas in my 390.

 

[Toobuilder]

Lycoming doesn't allow mogas in my 390.

I suspect that Lycoming does not “approve” mogas. Big difference.

 

[Dan 57]

I fail to see the big fuss. I've been running unleaded for 10 years now, 2000 hours in my airplane

+1, and same experience.

As already shared a few times here, my airplane was built, and modified to use Mogas (automobile gasoline, including some ethanol which I experience in northern countries around here). These mods, used on 2 carbed O-360s, include a T downline of the EDP with a return line to the selected tank (dual control fuel valve), replacement of all pressurised 90° fittings with 45°s, heat shields on the EDP and protecting the hoses.

Looking at the freedoms the E/AB category offers, I'm surprised how few take advantage of running cheaper and cleaner fuel... old wives tales, lemmings instinct, fear of new, or just ear-say speculations?
The design of the Lyco flat four is short of 100 year old, and still owes more to the agricultural equipment of the time than a modern racing machine...

 

[ve0kog]

I noticed that unleaded fuel is available at numerous airports, usually cheaper than 100LL, widely used in aviation, and has no known issues. The Jet-A. All Lycoming needs to do is procuce the engine for it. They already have something the back pocket - DEL-120 but it's a big secret.

 

[bertschb]

To you guys currently running mogas-
Would you use it in a new 390?

 

[12vaitor]

Like many of you, I'm following the transition to unleaded Avgas with interest. I've found some lively discussion around the internet but very little here on VAF. I find that odd so I thought I'd start a new thread here.

Just to get some discussion going, the unleaded fuels currently being considered have shown some potential drawbacks - both real and perceived. Here are my concerns in order:

1- Fuel tank sealant compatibility
As stressful as it was to build my fuel tanks, if there is even a 1% chance that a candidate fuel will cause tank sealant to fail resulting in fuel tanks leaks, I won't use it. Instead, I'd probably sell my airplane and buy something with fuel bladders or something that isn't damaged by the fuel. No way I'm removing the tanks, removing the existing sealant and re-applying something else.

2- Paint damage
Paint damage caused by spilled/leaking fuel is almost as serious a concern to me. I will be paying a good chunk of change to get my airplane painted next year and if a new unleaded fuel ruins my paint job, I will not be a happy camper.

3- Engine timing changes
Any changes that might be required to ignition timing don't concern me too much. I think we have some leeway as experimental builders to adjust timing. I know that will reduce performance but I'm naively thinking I'll have some "overhead" to work with (215HP). I know the certified folks are saying this is a deal breaker.

4- O-ring deterioration
O-rings - little concern. Swapping o-rings doesn't seem like a big deal but again, I'm naive on this one as well.

5- Miscibility (mixing fuels)
This could be a problem initially but I'm not worried about it. During the transition, I'll just avoid mixing fuels if that ends up being an issue. Inconvenient? Yeah. Deal breaker? Not for me.

Perhaps it would be more useful to direct your concerns to the FAA in submitting public comments to the Draft Transition Plan to Unleaded Aviation Gasoline. If I read it correctly, the plan basically lets the Type Certificate holder and engine manufacturers off the hook for any "approvals" and places the burden on the aircraft owner to figure out changes like material compatibility, retarding the timing and the associated performance impacts. For most GA/EAB use miscibility should be the number 1 issue.

If you want to have some impact on the discussion and outcome, submit your comments by 13 March 2026.

https://www.faa.gov/aircraft/draft_docs/draft_unleaded_avgas_transition_plan

 

[rv8ch]

To you guys currently running mogas-
Would you use it in a new 390?

What compression ratio? What ignition and timing? I'd probably sneak up on it, ensuring that I had good detonation margin by carefully looking at my engine monitor data, then adding a bit of unleaded in one tank, take it up to altitude and lower power settings, and confirm nothing crazy happens. Slowly increase percentage of unleaded, and increase power. As others have said, this is experimental aviation, so you can make it work if you are willing to invest the time. I'd also ensure that your fuel system is ok with a bit of ethanol (mainly o-rings), ensure that your timing is not too advanced, make sure your fuel flow is adequate (not too lean), and start experimenting. Obviously YMMV, so if you are not comfortable, just don't do it - 100LL will be around for a few more decades, it seems.

 

[bertschb]

What compression ratio? What ignition and timing?

The standard compression for the 390 is 8.9:1. I'll be using dual P-Mags.

 

[Carl Froehlich]

In my area the best option for pump gas is 93E10 from Costco (a top tier fuel). Ethanol free unleaded is only offered at 87 octane.

In a perfect world, we would have Lycoming engine test stand data to answer the hard questions, Such as:
- For a parrallel valve, 8.5 compression engine do I need to establish any limitations on using 93E10 pump gas. If so, what limitations?
- If running 93E10 do I need to preclude using the winter blend (to avoid vapor lock)?
- For an angle head IO-360/390 type engine with 8.9 pistons, do I need to retard timing to ensure acceptable detonation margin? If so, what should I set for the new base timing?

I find it hard to believe Lycoming has not explored this type of test stand data. Every time I was able to corner a Lycoming rep asking such quesions I get the automatic response to reference their published engine fuel requirements.

I can only assume liability concerns drive this knowledge void.

We are left with antidotal information of what other people have tried. Not a great way to make decisions on machines that leave the ground.

Carl

 

[Steve Iacoviello]

Perhaps it would be more useful to direct your concerns to the FAA in submitting public comments to the Draft Transition Plan to Unleaded Aviation Gasoline. If I read it correctly, the plan basically lets the Type Certificate holder and engine manufacturers off the hook for any "approvals" and places the burden on the aircraft owner to figure out changes like material compatibility, retarding the timing and the associated performance impacts. For most GA/EAB use miscibility should be the number 1 issue.

If you want to have some impact on the discussion and outcome, submit your comments by 13 March 2026.

https://www.faa.gov/aircraft/draft_docs/draft_unleaded_avgas_transition_plan

Engine and airframe OEM’s are involved in some of the testing.

From Lyondell’s press release “
These milestones include material compatibility testing by several
aircraft OEMs including Van’s Aircraft Inc., Piper Aircraft, Textron Aviation Inc., and Cirrus;
engine testing by Lycoming Engines and Continental Aerospace technologies and by the
FAA at the William J. Hughes Technical Center for Advanced Aerospace in Atlantic City, NJ.”

 

[rv8ch]

In my area the best option for pump gas is 93E10 from Costco (a top tier fuel). Ethanol free unleaded is only offered at 87 octane.

In a perfect world, we would have Lycoming engine test stand data to answer the hard questions, Such as:
- For a parrallel valve, 8.5 compression engine do I need to establish any limitations on using 93E10 pump gas. If so, what limitations?
- If running 93E10 do I need to preclude using the winter blend (to avoid vapor lock)?
- For an angle head IO-360/390 type engine with 8.9 pistons, do I need to retard timing to ensure acceptable detonation margin? If so, what should I set for the new base timing?

I find it hard to believe Lycoming has not explored this type of test stand data. Every time I was able to corner a Lycoming rep asking such quesions I get the automatic response to reference their published engine fuel requirements.

I can only assume liability concerns drive this knowledge void.

We are left with antidotal information of what other people have tried. Not a great way to make decisions on machines that leave the ground.

Carl

Of course you are right Carl, we should not have to do the experimentation ourselves, but we are "experimental" aviation - so we kind of signed up for this.

I don't know if there is a central repository of experience running unleaded fuel, and I think it would be a great idea to create one. Anyone?

My experience is that I've run some pretty low octane mogas of dubious vintage when traveling. The engine ran fine, and I could not see any difference in the engine monitor numbers or graphs, but I didn't do an extremely detailed analysis. I have viton o-rings everywhere, and when I bought my engine from Mattituck with an AFP FM-200 - they said that everything is ready for mogas with a little bit of ethanol - I assume that they used viton, but I can't be sure. I don't change my timing (PMAGS) or fuel flow when running 100ll or mogas, and I have an 8.5:1 PV IO-360. I have seen vapor issues from time to time on the ground, and running the boost pump for a while clears it up. It shows up as very high fuel flow rates, and less smooth idling. Never had an issue in the air.

If you ask your favorite LLM a question like "For Lycoming aircraft engines that normally run on 100LL, what would be a thorough technical checklist of items to inspect or modify - fuel system, materials compatibility, operating procedures, and maintenance - before and after switching to automotive gasoline (mogas) containing up to 10% ethanol, especially in experimental or non‑STC contexts?” you will get a pretty good list of things to consider.

The stability of 100LL seems to be excellent, and it doesn't deposit any crud in the fuel system - the same can't be said for mogas, particularly mogas with ethanol.

 

[airguy]

Lycoming doesn't allow mogas in my 390.

They don't allow it in my IO360M1B either. You'll be surprised what you "can do" when you stop listening to the people saying "you can't do that".

To you guys currently running mogas-
Would you use it in a new 390?

Depends on the compression. 8.7 or less, you bet.

 

[Toobuilder]

The standard compression for the 390 is 8.9:1. I'll be using dual P-Mags.

The number one thing to do to run mogas in an angle valve is tailor the ignition map. With that consideration, Pmags are a step or two in the wrong direction. Pmags are marginal on an angle valve even WITH 100LL.

 

[airguy]

I have viton o-rings everywhere, and when I bought my engine from Mattituck with an AFP FM-200 - they said that everything is ready for mogas with a little bit of ethanol - I assume that they used viton, but I can't be sure.

More likely fluorosilicone elastomers - viton is common for O-rings, but anywhere you use a bellows-type diaphragm, a large flat piece of elastomer that has to move like in a pressure regulator or fuel flow controller (present in some form on all Bendix or AFP injection systems, and carburetors), the material of choice since 1987 (if memory serves) to replace standard rubber has been a flourosilicone variant. They don't care about ethanol or higher volatility fuels. I know that AFP is/was using fluorosilicone for fuel controller rebuilds, as I had them overhaul my (used) Bendix injection system on 2015 as I was assembling my FWF and that's what went into it.

These elastomers became the de-facto standard in the automotive world after the problems with natural rubbers became more apparent following the mandate for unleaded gasoline in the late 70's. The unleaded fuel has to be a slightly different mix of hydrocarbons to get the same anti-knock rating without the lead, and that mix was a little less friendly to natural rubber. The same problem is encountered today with ethanol mixes. I doubt you could even find a rebuild kit anymore with natural rubber diaphragms for anything larger than a lawn mower - and that, by the way, is precisely why small engines don't like fuel with ethanol.

And yes, I am that old. I lived through that, and remember it well.

 

[Amadeus]

In my area the best option for pump gas is 93E10 from Costco (a top tier fuel). Ethanol free unleaded is only offered at 87 octane.

E10 typically will test out to be between 3% and 5% ethanol. I've yet to test and find "up to 10% ethanol" to have 6% or higher levels of ethanol. I no longer test.

 

[Toobuilder]

…I don't change my timing (PMAGS) or fuel flow when running 100ll or mogas, and I have an 8.5:1 PV IO-360…

Wondering if you have done an in flight switch of mogas in one wing and 100LL in the other? I have, and the EGT and CHT behavior of the two fuels is very noticeable. Disclaimer - my experiment was with California summer blend auto fuel from the corner gas station.

It does run, but clearly the two fuels I used need their own distinct ignition and mixture maps to run “right”.

 

[plehrke]

I don't know if there is a central repository of experience running unleaded fuel, and I think it would be a great idea to create one. Anyone?

Has anyone heard if Savvy has started collecting any/all unleaded data? They were involved in the AOPA twin testing of one side with leaded and other side with unleaded.
Savvy caters more to certified aircraft but they do collect a lot of data.

 

[bertschb]

My primary concern using lower octane mogas with an 8.9:1 engine is detonation. Not too excited about detonating my new $80,000 engine.

I found a KitPlanes article that said for a Superior XP-360 engine, HP dropped from 185 to 180 when dropping the compression ratio from 9.0:1 to 8.5:1. So 5hp for a 0.5 drop in compression ratio. That's not bad when you start with 215 for the 390EXP119.

The last time I checked, Lycoming offered 7.3, 8.9 and 10.0 compression ratios for the 390EXP119 Thunderbolt. I sent an email to Lycoming asking if they could do a 8.0:1. If the KitPlanes article is accurate, I'm guessing I'd lose maybe 10-15hp with this lower compression ratio so I'd be down to 200-205hp maybe? Just winging it here for argument sake.

I can get 92 (or is it 93?) octane non-ethanol fuel pretty easily around here. My ER tanks hold 68 gallons so if I can get an 8.0 compression ratio engine, this might work for 95% of my flying. The only time I wouldn't have access to 92/93 octane non-ethanol would be on a loooong cross country flight.

 

[rv8ch]

I sent an email to Lycoming asking if they could do a 8.0:1. If the KitPlanes article is accurate, I'm guessing I'd lose maybe 10-15hp with this lower compression ratio so I'd be down to 200-205hp maybe? Just winging it here for argument sake.

I'd stay with the 8.9:1 and find a way to make it work with timing and FF.

 

[RV-9erA]

Our airport manager at Paine Field (KPAE) will not accept any new aviation UL fuel that isn't ASTM approved. Currently, Swift 100R is the only fuel that has ASTM approval and has also proven to meet all of the OP requirements, and also an additive to prevent valve seat recession. GAMI G100UL is not planning on getting ASTM approval and can't meet the OP's list of concerns. And after extensive factory testing, Cirrus Aircraft won't approve G100UL in their aircraft. Yes -- Swift 100R contains an Ether additive, but the base fuel is exactly the same as in 100LL. So far, Swift 100R is looking really good as a 100LL replacement from what I'm seeing!

 

[rv8ch]

Wondering if you have done an in flight switch of mogas in one wing and 100LL in the other? I have, and the EGT and CHT behavior of the two fuels is very noticeable. Disclaimer - my experiment was with California summer blend auto fuel from the corner gas station.

It does run, but clearly the two fuels I used need their own distinct ignition and mixture maps to run “right”.

I have not tried this, but will. I'm sure that there are other folks running one tank 100LL and one tank MOGAS who might also be able to chime in.

 

[plehrke]

I have not tried this, but will. I'm sure that there are other folks running one tank 100LL and one tank MOGAS who might also be able to chime in.

I think a few have one side 100LL and one side Mogas and use the Mogas only in cruise. 100LL is used during all taxi, take-offs and landings.
My biggest concern is not cruise flight. What needs to get tested, by more than a few individuals, is all the off design points. That is where you need industry cooperation and it is most likely the odd occurrence that will cause the biggest issues.

 

[Dan 57]

@bertschb aka Brian, I wonder, why are you starting this thread in the first place? As in what is your motivation for the use of non-leaded fuel? Is it only financial, or is there something on top of that?

I do fly between 300-400+ hours a year, and savings ain't part of the equation. Flying is expensive, and even given no choice as in having to buy 100LL would not change a thing to my flying habits. Call me lucky in that I can at least afford my flying habits, leaded or not.
Then why do I highly prioritize my use of lead-free? Simple answer my friends, cleanliness.
As in a cleaner engine... and a cleaner environment. This air, loaded by lead (sic) burning aviators, is the same stupid air as being breathed in by all of us, present and future generations.
One more personal factor, is serious doubts against the whole info we are being fed by the industry, engine manufacturers and petroleum companies...
Last reason around here is in commodity, as in availability. Mogas being pump gas, is pretty much available everywhere (but on most US fields where 100LL only has been forced in) around here, and all those Rotax powered high and low end UL ships also prefer to quench their thirst sipping non-leaded.

Wanna abide to the manufacturer's recommendations? Wanna save a couple of $ flying 20 hours/year? Don't care about the environment? Never question anything you're being told? Assembled an "experimental" category craft but don't want to experiment? Afraid of loosing that precious and expensive new engine warranty?
Any doubt, then stay in the safe realms of history, and burn lead.

 

[Toobuilder]

I have not tried this, but will. I'm sure that there are other folks running one tank 100LL and one tank MOGAS who might also be able to chime in.

Ok, but be ready to switch back to 100LL and land or richen the mixture and/or change timing somehow. My optimized “LOP” setting is both mixture and timing with my SDS EFI and it’s great for 100LL, but CHT and EGT went nuts as soon as the auto gas made it to the injectors (About 2 seconds after the fuel selector move). To get it back to something resembling normal, I had to retard timing and then sneak back up on the LOP mixture with the manual knob. Ran fine after that and the off nominal behavior was not “bad” but the engine was clearly not happy.

Not meant to be a scare tactic and I can and do run auto fuel in my airplane, but the state of tune between the two fuels is different in my experience. Be cautious if you do not have the level of in flight adjustability I have.

 

[airguy]

@bertschb aka Brian, I wonder, why are you starting this thread in the first place? As in what is your motivation for the use of non-leaded fuel? Is it only financial, or is there something on top of that?

I do fly between 300-400+ hours a year, and savings ain't part of the equation. Flying is expensive, and even given no choice as in having to buy 100LL would not change a thing to my flying habits. Call me lucky in that I can at least afford my flying habits, leaded or not.
Then why do I highly prioritize my use of lead-free? Simple answer my friends, cleanliness.
As in a cleaner engine... and a cleaner environment. This air, loaded by lead (sic) burning aviators, is the same stupid air as being breathed in by all of us, present and future generations.
One more personal factor, is serious doubts against the whole info we are being fed by the industry, engine manufacturers and petroleum companies...
Last reason around here is in commodity, as in availability. Mogas being pump gas, is pretty much available everywhere (but on most US fields where 100LL only has been forced in) around here, and all those Rotax powered high and low end UL ships also prefer to quench their thirst sipping non-leaded.

Wanna abide to the manufacturer's recommendations? Wanna save a couple of $ flying 20 hours/year? Don't care about the environment? Never question anything you're being told? Assembled an "experimental" category craft but don't want to experiment? Afraid of loosing that precious and expensive new engine warranty?
Any doubt, then stay in the safe realms of history, and burn lead.

Preach, brother.

The only thing I would add to that is, I'm an experimenter and gearhead at heart, and an engineer by trade. I know there is a better way, and I follow it. Others can too - but first they must give up the shackles of being told what they cannot do.

 

[bertschb]

@bertschb aka Brian, I wonder, why are you starting this thread in the first place? As in what is your motivation for the use of non-leaded fuel? Is it only financial, or is there something on top of that?

Why did I start the thread?
As stated in my OP, to try to get some discussion started on this topic so we can all learn more.

My motivation for the use of non-leaded fuel?
1- 100LL is going away in four years so I'd like to learn more about the three UL fuels under consideration and how they may affect my engine/airplane.
2- I would like a cleaner engine and unleaded fuel should help in that regard.

I couldn't care less how much aviation fuel costs. Rounding error for me in the grand scheme of things.

<<Wanna abide to the manufacturer's recommendations? Wanna save a couple of $ flying 20 hours/year? Don't care about the environment? Never question anything you're being told? Assembled an "experimental" category craft but don't want to experiment? Afraid of loosing that precious and expensive new engine warranty?
Any doubt, then stay in the safe realms of history, and burn lead.>>

 

[RV-9erA]

@bertschb aka Brian, I wonder, why are you starting this thread in the first place? As in what is your motivation for the use of non-leaded fuel? Is it only financial, or is there something on top of that?

I do fly between 300-400+ hours a year, and savings ain't part of the equation. Flying is expensive, and even given no choice as in having to buy 100LL would not change a thing to my flying habits. Call me lucky in that I can at least afford my flying habits, leaded or not.
Then why do I highly prioritize my use of lead-free? Simple answer my friends, cleanliness.
As in a cleaner engine... and a cleaner environment. This air, loaded by lead (sic) burning aviators, is the same stupid air as being breathed in by all of us, present and future generations.
One more personal factor, is serious doubts against the whole info we are being fed by the industry, engine manufacturers and petroleum companies...
Last reason around here is in commodity, as in availability. Mogas being pump gas, is pretty much available everywhere (but on most US fields where 100LL only has been forced in) around here, and all those Rotax powered high and low end UL ships also prefer to quench their thirst sipping non-leaded.

Wanna abide to the manufacturer's recommendations? Wanna save a couple of $ flying 20 hours/year? Don't care about the environment? Never question anything you're being told? Assembled an "experimental" category craft but don't want to experiment? Afraid of loosing that precious and expensive new engine warranty?
Any doubt, then stay in the safe realms of history, and burn lead.

Last summer, our airport performed an extensive third party lead test in and around the airport (within a 5-nm radius), but only to find zero detectable lead. This was one of several high-traffic GA airports tested in WA, CO and CA, and with the same zero detectable lead results. The likely reason is the anti-fouling additives in 100LL, which are Tricresyl Phosphate (TCP) and Decalin RunUp. These two scavenger additives convert solid lead oxide deposits into a highly volatile compound (lead Bromide) which mostly burn-off, vaporize and get expelled through the exhaust. And, without these additives in 100LL, valves and sparkplug would foul-up very quickly.

 

[airguy]

<<Wanna abide to the manufacturer's recommendations? Wanna save a couple of $ flying 20 hours/year? Don't care about the environment? Never question anything you're being told? Assembled an "experimental" category craft but don't want to experiment? Afraid of loosing that precious and expensive new engine warranty?
Any doubt, then stay in the safe realms of history, and burn lead.>>

Unfortunately, a lot of truth there. Experimental aviation is about "education and entertainment", remember? If you want to venture off the beaten path, some education is required. There is no glossy print book available on Amazon that tells you what to do (at least not yet) - but every bit of knowledge you would need is available on this forum if you search back in history.

 

[airguy]

Ok, but be ready to switch back to 100LL and land or richen the mixture and/or change timing somehow. My optimized “LOP” setting is both mixture and timing with my SDS EFI and it’s great for 100LL, but CHT and EGT went nuts as soon as the auto gas made it to the injectors (About 2 seconds after the fuel selector move). To get it back to something resembling normal, I had to retard timing and then sneak back up on the LOP mixture with the manual knob. Ran fine after that and the off nominal behavior was not “bad” but the engine was clearly not happy.

Not meant to be a scare tactic and I can and do run auto fuel in my airplane, but the state of tune between the two fuels is different in my experience. Be cautious if you do not have the level of in flight adjustability I have.

Point of order here - if you'll configure the LOP settings for autofuel, it will tolerate 100L or any mixture of the two just fine. The autofuel is the more restrictive fuel with regard to detonation - set up for that, and live with it everywhere on the map.

 

[Toobuilder]

Point of order here - if you'll configure the LOP settings for autofuel, it will tolerate 100L or any mixture of the two just fine. The autofuel is the more restrictive fuel with regard to detonation - set up for that, and live with it everywhere on the map.

“Tolerate” is an important word. In my case, my ignition timing LOP is well researched and anything off nominal from my best number is a decrease in cruise speed. When I did the in flight switch describe earlier, this required backing off the timing a few degrees to make the engine happy on the auto gas. While I have not explored “optimum” timing and mixture on auto gas, this reduction in timing seems to indicate I’d be leaving some performance on the table if my tuning map is optimized for the auto gas. Not the end of the world and probably a fine trade if 100LL goes away, but people need to be aware that there is a difference in the chemical behavior of the two fuels at the edges of the envelope.

 

[BoydBirchler]

I have been running un-leaded non-ethanol 91 octane auto fuel for over 800 hours in my RV7. I eliminated the Gascolator and use a inline fliter and drain any system water from the fuel tank quick drains. I also have a heat cooling shroud on the fuel pump. I retarded the timing to 23 degrees on my 8.5 to one PV IO-360, although a few friends have had good results leaving the timing at 25 degrees.

I do feel it is necessary to occasionally run a bit of 100LL to keep valve seat recession at bay. I have had no issues with fuel vapor locking. Oil stays very clean and spark plugs have no deposits.

 

[gasman]

Unfortunately, a lot of truth there. Experimental aviation is about "education and entertainment", remember? If you want to venture off the beaten path, some education is required. There is no glossy print book available on Amazon that tells you what to do (at least not yet) - but every bit of knowledge you would need is available on this forum if you search back in history.

Doug should write a book....... THE BEST OF VAF.... $$$!!

 

[bertschb]

...but every bit of knowledge you would need is available on this forum if you search back in history.

I would agree with you when it comes to building and flying RV's. Just about every topic has been covered - with the exception of the unleaded fuels currently under development. Not much interest in discussing it here on VAF for some reason even though it's a very hot topic elsewhere.

 

[Ironflight]

I would agree with you when it comes to building and flying RV's. Just about every topic has been covered - with the exception of the unleaded fuels currently under development. Not much interest in discussing it here on VAF for some reason even though it's a very hot topic elsewhere.

It probably hasn’t gotten “over the top” discussion here BECAUSE it is getting discussed everywhere else, and the Lycomings we use don’t much know if they are installed in a Piper or an RV. If it’s RV-specific, then that’s a great topic for VAF IMHO - but if it’s generic and has been beaten to earth elsewhere, then ……

 

[Toobuilder]

…Just about every topic has been covered - with the exception of the unleaded fuels currently under development.

If you are talking about the drop in replacement candidates, there’s not much to talk about beyond the fact that one of them seems to be an effective paint stripper. Still very much in a “wait and see” phase there. If you are talking mogas or auto fuel, there are people who run it quite a bit, and there’s plenty of info on this forum on how to set the airplane up to use it.

I understand that the 94UL that’s been used a bunch in Europe is essentially 100LL with the lead removed. If that’s the case, I’d run that in a heartbeat today. Sure, the big bore turbo twin guys can’t run it, but every airplane except the L-39 I’ve owned in 30 years would eat it fine (and the L-39, being Soviet era, probably WOULD run on it, but I digress…).

 

[bertschb]

If you are talking about the drop in replacement candidates, there’s not much to talk about beyond the fact that one of them seems to be an effective paint stripper. Still very much in a “wait and see” phase there.

The reason I started this thread was to try to get some discussion going about the three new unleaded avgas candidates. For those who are interested, there is actually quite a bit to talk about based both on formal and informal test results as well as real world experience from some pilots. I thought folks would be interested to learn more from the discussion. My mistake! Nothing to see here...move along...

 

[RV-9erA]

The reason I started this thread was to try to get some discussion going about the three new unleaded avgas candidates. For those who are interested, there is actually quite a bit to talk about based both on formal and informal test results as well as real world experience from some pilots. I thought folks would be interested to learn more from the discussion. My mistake! Nothing to see here...move along...

Then maybe you didn't read my comments and input above?

 

[bertschb]

Then maybe you didn't read my comments and input above?

Yes, I read your comments and appreciate your thoughts. Thank you for sharing them.

 

[Steve Iacoviello]

Our airport manager at Paine Field (KPAE) will not accept any new aviation UL fuel that isn't ASTM approved. Currently, Swift 100R is the only fuel that has ASTM approval and has also proven to meet all of the OP requirements, and also an additive to prevent valve seat recession. GAMI G100UL is not planning on getting ASTM approval and can't meet the OP's list of concerns. And after extensive factory testing, Cirrus Aircraft won't approve G100UL in their aircraft. Yes -- Swift 100R contains an Ether additive, but the base fuel is exactly the same as in 100LL. So far, Swift 100R is looking really good as a 100LL replacement from what I'm seeing!

Both Swift and Lyondell candidate fuels contain ETBE. Both will have ASTM specs. ETBE is a well known oxygenate used extensively in Europe and Japan in motor fuel. It is used instead of ethanol. ETBE is produced in large quantities the US and loaded on vessels for export. Swift is working for approval via STC with Lyondell working toward fleet authorization. I expect both fuels to work fine.

 

[ve0kog]

I would agree with you when it comes to building and flying RV's. Just about every topic has been covered - with the exception of the unleaded fuels currently under development.

i think you may be overlooking something in the search results. unleaded gas has been discussed here quite a bit including the high school chemistry masterpiece which dissolves gaskets, sealants, and paint it comes in contact with

 

[bertschb]

i think you may be overlooking something in the search results. unleaded gas has been discussed here quite a bit including the high school chemistry masterpiece which dissolves gaskets, sealants, and paint it comes in contact with

Unleaded automobile gas? Yes, absolutely. Lots and lots of posts on that going back many years.

G100UL, UL100E and 100R? Not so much. Yes, a few posts - including the one that covered the "unofficial" testing of G100UL which was very interesting.

 

[RV-9erA]

i think you may be overlooking something in the search results. unleaded gas has been discussed here quite a bit including the high school chemistry masterpiece which dissolves gaskets, sealants, and paint it comes in contact with

However, both Swift 100R and LyondellBasell UL100E have been shown to not have these issues. Been impressed, especially with the Swift 100R fuel, which also has an additive to prevent valve seat recession, which was seen with UL94 (base 100LL fuel) in the University of North Dakota (UND) Cessna 172 fleet.

 

[bertschb]

My only real concern with both UL100E and 100R is what, if any effect we may see with potential timing changes that may be required. I don't know enough about engines and ignitions to know if this is a big deal or a minor detail. I do know that if timing changes are required, we are way better off than the certified world.

 

[bertschb]

However, both Swift 100R and LyondellBasell UL100E have been shown to not have these issues.

Yes, this is very encouraging!

 

[RV-9erA]

Both Swift and Lyondell candidate fuels contain ETBE. Both will have ASTM specs. ETBE is a well known oxygenate used extensively in Europe and Japan in motor fuel. It is used instead of ethanol. ETBE is produced in large quantities the US and loaded on vessels for export. Swift is working for approval via STC with Lyondell working toward fleet authorization. I expect both fuels to work fine.

So, Ethyl Tertiary Butyl Ether (ETBE) based fuel doesn't absorb water and separate from gasoline in the same way that ethanol does. Unlike ethanol, which is hygroscopic (absorbs moisture from the air) and causes "phase separation" in gasoline-water mixtures, ETBE is stable in gasoline and maintains blend integrity without needing extra handling precautions, making it much better than ethanol in that way. Also, ETBE based fuel has low vapor pressure and blends predictably with gasoline, which are both what you're looking for in aviation fuel.

 

[RV-9erA]

My only real concern with both UL100E and 100R is what, if any effect we may see with potential timing changes that may be required. I don't know enough about engines and ignitions to know if this is a big deal or a minor detail. I do know that if timing changes are required, we are way better off than the certified world.

I was always under the impression that by using the same octane rating (100 here), the timing advance should remain the same regardless of fuel type used. But, if the octane is lowered, then the timing should be retarded as was done in the 70's when unleaded fuel made the scene, and the compression on most new engines was also lowered. Being a "muscle car" guy back then with a Chevy 350 V8, I really didn't like those days!