Sun ‘n Fun 2024: GAMI Has Theory On UND Valve Recession Cause


General Aviation Modifications Inc. founder George Braly says he thinks he knows why the University of North Dakota’s flight training program had valve seat recession issues using Swift 94UL fuel but he won’t release the details until he’s discussed it with UND and Swift. “I have to extend them that courtesy,” he said. The school used 94UL for about four months before it found the valve problems and switched back to 100LL. At Sun ‘n Fun on Tuesday, Braly said he bought three barrels of 94UL and did his own tests in his own engine test facility and thinks he’s isolated the issue.

Meanwhile, Braly said his 100-octane unleaded fuel G100UL will be commercially available likely within the next couple of months for any fuel distributor or retailer who would like to start selling it. Refining company Vitol has made a “large quantity” of the fuel and is in the final stages of preparing it for delivery. Robert Emmett, Vitol’s project manager for the avgas project, is a Bellanca owner and pilot and said it’s something his company is enthusiastic about getting on the market. It’s likely the first retail sales of G100UL will happen in California where court settlements and the political environment are pushing a switch to unleaded aviation gasoline.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.


  1. Why not share with the rest of us? A number of us were having valve guide and valve issues and we’re collecting data. Did UND find all issues have stopped?

    • It’s not about octane or lead; it’s chemistry.
      100LL without the lead (also 94UL) works just fine.

  2. You shouldn’t need him to tell you!! The understanding of this isn’t difficult and has been around for maybe 80 years. There are Russian and western engineering and combustion science studies all published and available for free (they’ve been out for that long) that you can read for hours. No, I’m not going to provide links etc, it’s good for peoples broader self-education to go to your local university and do a literature search…. it’s all there. It’s going to happen with all of the UL fuels currently being offered IMO, and none really any less so than others the way I see it all. It’s not about the lead either, it’s about temperature and where and when the combustion heat is being released in the cycle. Any of the more thermally stressed engines are likely to suffer much more than things like training aircraft. I’d hate to see any of this stuff in something like a the ‘traditional engines’ used in a mustering operation in hot, and especially in hot and humid conditions. Go forth and learn……

  3. When automotive engines went to unleaded gasoline there were a lot of changes necessary for valves and seats. By the same token, an engine designed for no-lead should not burn 100LL.

    • Can you elaborate on why an engine that is capable of running on non-ethanol cannot run on 100LL? Lots of engines have been doing so for decades. In the automotive world, the catalytic converter is what keeps them from running leaded fuels, not the engine design.

      • I know at least for the Rotax engines, running 100LL requires much more frequent oil changes. The lead bromide deposits contaminate the oil.

  4. Can someone explain why when we had ethanol-free gas back in the day and autogas STCs this was not a problem? I flew a C-150 on gas from the local station for years and never had an issue until I had to buy 100LL and it would get sticky valves unless I used the treatment.

  5. UND was running their engines like they always did, lean of peak. When using unleaded fuels one must run rich of peak. Damage such as what happened at UND can be caused by running lean of peak. It’s just that simple. It’s not a theory, it’s been well known for years. It’s the reason people using our autogas STC’s haven’t had the same problem as UND.

  6. Petersen, you said that UND training aircraft are “run lean of peak?” That would be the first time I’ve ever heard of a trainer being run LOP. Can you provide a source for that statement?

  7. According to public reports from UND:
    “… UL94 was used exclusively at Grand Forks, however all off-station refueling was with 100LL AVGAS. The fuels are mixable and interchangeable so no changes to performance
    calculations or operating procedures were made. Leaning procedures were followed in accordance with the respective pilot’s operating handbooks (POH). For the Piper Archers and Seminoles, that is full rich for takeoff, climb and descent, while leaning for peak EGT (Best Economy) during cruise.” “Initial analysis showed that most aircraft had more than 400 hours using UL94 before seeing valve seat recession or loss of compression.”

    Reports from Swift Fuels:
    Swift Fuels has conducted research for the past 4+ years in developing a new 100-octane unleaded avgas called 100R. 100R unleaded avgas is designed to replace 100LL globally. Testing shows that 100R does not produce valve seat recession in piston aircraft. This fuel has recently completed intensive 400-hour FAA-Conforming engine endurance and durability tests. Initial 100R certification results have been reported to FAA and ASTM International.
    A briefing can be seen online at a recent FAA Safety webinar:

    To contact Swift Fuels: (765) 464-8336 or

  8. What about the aircraft engines designed for 80 Octane Avgas? They didn’t seem to have any problems running nearly lead free fuel or completely lead free. ASTM D610 specifies a maximum Pb content of 0.14 g/gallon. There is no minimum spec. Automotive fuels in the ’80s had 1.1 g/gallon which was reduced to 0.10 g/gallon in 1985. 100LL specs 0.56 g/gallon. I have run nearly 90% unleaded fuel primarily mogas for decades in an O-470R and run well past TBO without issue, leaned per Cessna/Continental book (lean until rough, enrichen till smooth). With modern instrumentation, this appears to be just lean of peak at 65% power and lower. CHTs are cool, valves, guides and compressions have run beyond TBO both Continental and Superior Cylinders. The only problem now is far fewer airports are only supplying 100LL, which is an increasing cross country planning problem.

  9. It’s nuts that any aircraft engine should have such a problem as valve seat recession which is relatively rare for most piston engine fields.

    • Why? Running LOP is easier on the engine, assuming cruise power of 70% or less. It reduces ICP and CHTs. Running ROP incorrectly is worse for the engine. See Mike Busch’s articles and videos on the topic, along with John Deakin’s Pelican Perch articles.

        • Besides drawing the throttle valve in the wrong place, what do you find wrong with Manifold Pressure Sucks article?

    • My experience is similar to raymo. My CHTs run in the low 300s, 350 or so in extended climbs at high density altitudes, at book cruise power settings of 65% and below, leaned with recommended techniques in the 30 years I’ve owned the airplane with no fuel related engine problems well past TBO and presently just passed TBO without a single issue on unleaded fuel. What problems does leaning to peak or slightly lean of peak cause that will be avoided? CGR30 which was not initially installed indicates -14 deg LOP when using mfr recommended leaning techniques and provides a much better fuel burn with little loss of airspeed. If there is a reason to change the leaning techniques, I’m interested in learning that reason. I ran the engine on 100LL when it was last majored, until break in was complete, per overhauler’s guidelines, then switched back to mogas 87 unleaded. CHTs now run 310-330 in cruise and 350-360 in cruise climb, EGTs were formerly 1200-1250, and after EA EIS installation, they dropped to mid 1100s. There’s a lot being said and has been said about leaning, fueling, and TBO over the past couple of generations without supporting data. I’m very interested in that data.

  10. George Braly: — More mystery, mystique and… what ??? —

    “Braly said he bought three barrels of 94UL and did his own tests in his own engine test facility and thinks he’s isolated the issue.”
    “but he won’t release the details until he’s discussed it with UND and Swift. “I have to extend them that courtesy,” he said.

    ‘Meanwhile, Braly said “his 100-octane unleaded fuel G100UL will be commercially available likely within the next couple of months for any fuel distributor or retailer who would like to start selling it.”

    So all of this tells us nothing –

  11. The notion that LOP operation should be avoided when running on unleaded fuel is pure nonsense.

    The valve recession problems at UND involved high-compression (8.6-to-1) engines. Those engines would NOT have been eligible for the autogas STC. As I suggested in my AOPA Pilot article on the subject, my best guess is that the recession issues UND experienced running those engines on Swift UL94 had more to do with the 94 than it did with the UL. I’m anxious for George to release his test cell data.

      • The concoction of fuel chemical blend from SWIFT, the octane, the lack of lead, and the aggressive leaning in North Dakota probably ALL conspired to do in the vales prematurely. This is why these fuels are not a “drop in” replacement.

      • Please detail how LOP ops were the cause of the problem. If a fuel cannot be used when running LOP, it is not a viable fuel. Period. I find it unlikely LOP was the root cause. If an engine is not operated *properly* and correct operating temperatures and parameters maintained, bad things happen. Unless conflicting data is provided, I will assume the school knows how to ensure proper operation.

  12. Indeed they are not eligible for the autogas STC if they’re 8.6 to 1, but perhaps eligible for 93AKI via Lycoming SI1070 – depending on the engine model number. If they’re not on SI1070, then they had no business using UL94 in the first place.

    • It’s not autogas, it’s not high compression, it’s not turbocharged.
      It’s a dorkey old normally aspirated O-360 running on labeled AvGas.
      No need for an “autogas stc” for this configuration since that’s not the situation here.

      If they wanted Mogas, then 91 Octane MoGas “should” be okay with a stock (180 HP) O-360 with 8.5:1 pistons. 94 octane is just fine for the stock setup regardless of AvGas or MoGas.

  13. I’ll rephrase that. If the specific engine model numbers used on UND’s PA-28’s are not listed as approved on SI1070 for 93AKI or UL91/94, then I don’t know how they could legally use it in the first place. Therefore, they must be on SI1070. Who knows the model numbers?

  14. I was a ASE-certified Master Automotive Technician back in the 1970s, when the switch to unleaded automotive fuel was made, and we immediately began noticing valve recession/valve burning issues in several different domestically built engines. It was determined that the valve seats were not made of sufficiently hard steel, and as a result we began seeing an upsurge of valve burning issues in those cars. The problem was ultimately corrected by using harder valve seats and valves to match, but it took a while for that to filter through all the engines that were in production at that time. Curiously, we did not see much of this issue in any foreign-built engines. You are free to draw your own conclusions in regard to that.