Swift Recommends Limiting Peak Or Lean Of Peak Operations With 94UL For Now


Swift Fuels is recommending pilots using its UL94 unleaded avgas adjust operation of their engines until the precise cause of dramatic valve seat recession in Piper Archers flown by the University of North Dakota is determined. Last week Lycoming, which made the engines in the Archers, said it didn’t believe the engines were at fault and instead pointed the finger at high levels of aromatics in UL94 and engine operation procedures at UND.

In response to our story on that analysis, Swift Fuels CEO Chris D’Acosta wrote a statement in the comments section below that story (copied in full below) calling Lycoming’s assessment “inconclusive” and disputing the claim about the aromatics. D’Acosta did, however, recommend adjusting operating procedures to reduce the risk of valve issues. “Swift Fuels would advise pilots to limit extended flight operations at peak EGT or lean of peak when flying UL94 unleaded avgas until the completion of a comprehensive analysis of the exhaust valve issue is documented and confirmed by industry,” D’Acosta wrote.

Meanwhile, George Braly, whose General Aviation Modifications Inc. has developed and is now making commercially available a 100-octane unleaded fuel, said he’s tested UL94 at his engine test facility and believes Lycoming’s analysis is incorrect. Braly told a news conference at Sun ‘n Fun last week that he would not make details of his analysis public until after he’s discussed it with those involved in the UND issues.

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. “Meanwhile, George Braly … believes Lycoming’s analysis is incorrect.”

    Something does seem fishy about Lycoming’s analysis. Sounds more like they’re just trying to say “it’s not our fault…oh, and BTW, lean-of-peak is bad”.

    • Who benefits? What are the potential motives?

      Lycoming’s potential motives might include trying to avoid liability, but that *should* be balanced by their desire for people to keep using their engines, and for them to appear friendly to what seems inevitable as far as the green initiative.

      OTOH, the green people have every reason to push for Lycoming to make a change.

    • Yes under low load conditions most modern vehicle engines run lean of peak, every modern diesel engine runs lean of peak. There are a few engines that run HCCI. Lycomings position is flawed and they know it. Cheaper to point fingers than fix their problem.

    • Well, most modern cars are tuned for emissions and mileage and automatically adjust mixture 30 times a second, and have individual intakes and runners and injectors.. Airplane engines are tuned to run at 65-75% continuous power output, have a manual mixture but no O2 sensor, and have a carburetor on an unequal updraft intake. It’s really hard to set exact settings for all cylinders…

  2. “Swift Fuels is recommending pilots using its UL94 unleaded avgas go easy on their engines until the precise cause …”

    Please, for the love of all that is Holy, just give us 100LL without the lead, like we’ve wanted for the last 40 years. I do not want higher costs and more decades of testing. Why is it so hard to just do cheap and easy and reliable and now?

  3. Um, I beg your pardon but running lean of peak is the very definition of “going easy on your engine”. Once in the 20°-50° LOP range, stress, power, and temperatures are all very significantly reduced. Yes, you lay with a little speed reduction, but fuel economy, range and greatly reduced wear and tear on the engine(including valves) makes it very worthwhile.

    The trick is that mixture distribution must be very good to operate in this range without roughness, but taking the time to tune injectors so that all cylinders peak at the same fuel flow will generally do the trick. I hope Mike Busch will weigh in here as well.

    • Exactly correct, Otis. I wondered why Swift would suggest running LOP is a bad thing. They didn’t qualify the recommendation with the use of appropriate power settings for peak or LOP ops, which is may further confuse people that want to run UL94.

  4. Concur with Otis Holt. While obviously it is a known possibility you can get into trouble running peak EGT at high power levels, when you properly set LOP you are actually going easy on the valve system. Or is the theory that there as some sort of “aura” around the valve seat within which an exotic process is eating away the metal regardless of temperature?

  5. I understood UL94 was essentially 100LL without the lead. So, what’s the source of these aromatics, that apparently aren’t in 100LL?

  6. Nobody at UND was running “lean of peak?… They were running AT PEAK EGT… That is the hottest combination of combustion chamber temperature and pressure, which likely contributed to the so-called micro-welding and metal transfer and abrasion of the valve seats. Running rich of peak is the recommendation from Swift, which is not uncommon, especially for planes without engine monitors. And I think Lycoming still should have a good explanation for why a single valve seat out of 8 on the Seminole that triggered the issue was so badly recessed. To me, that raises a presumption of metal spec or heat treating issue in there somewhere…

      • It doesn’t. Peak cylinder pressures are found at around 25 degrees F ROP. Leaning to peak EGT at lower power settings reduces pressure. Leaning a little further (LOP) reduces them further.

    • If they were indeed taught to run the engines at peak, that would explain things – torturing the engine like that isn’t the fault of a fuel.
      Flight schools are funny things – they can get ‘stuck’ on a particular way of doing things in the belief that it is good without actually checking it out.

      • At low power settings, peak EGT isn’t hard on the engine. It’s actually easier than 25°F rich due to lower CHTs and lower internal cylinder pressures.

  7. I have run LOP for 25 years now, on a couple of IO360 (with good results and long lived cylinders). Its not for everyone. The highest temps and pressures (they are not the same thing, but they travel together), do occur about 25-75 ROP. Yes, the manuals point us to this area and its the worst place. The reduction at peak however, is relatively small.
    The key issue being missed here, is that of the temperature slopes. The temperature area ROP is broad and relatively slow (can still be tricky with some mixture controls). The area LOP is steep. There is little room for inaccuracies. The slot between peak and engine stoppage can be narrow. Most carb engines will not run well in this area due to mixture distribution. Even fuel injection, usually requires some tweaking of injector flows to match intake path differences (sometimes we do get lucky).
    Proper instruments are required (you cannot cheap out here). The minimum, is an EGT and CHT probe on every cylinder (not on everyone’s panel). A plan for runaway temps must be in mind before you start up (needed no matter what regime you operate in). I believe Lycoming’s issue is not with LOP as such, but the ability of ALL pilots to operate there. Keep in mind, that this stuff is ultimately decided by lawyers, and the lowest common denominator is the target.

    Fly well,