peterh337 - hopefully nobody would get a nasty shock by being threatened with a (big) bill. All I was advocating was for people to check with their CAMO (or your LAA Inspector if your aircraft is on that system) before switching to UL91: it prevents people getting it wrong - which does happen. And sadly there are some unscrupulous people out there who might be slightly less pragmatic about the actions to be taken if someone accidentally used UL91 in an unapproved engine. It's a good idea to double check, is all I'm saying.

As to different oils: I concur with others - there are plenty of alternatives and no, I don't work for Shell!

AIUI, 91UL is just avgas without the lead, so the only risk is engine detonation, and tests done by GAMI suggests that in real life you need to work an engine awfully hard (with CHTs ~ 550F) for this to happen.

There is no risk to fuel system components.

The Swedish 96UL is more useful stuff and almost all Europe based GA pistons can run on it, but it isn't sold outside Sweden AFAIK.

The various "100UL" propositions involve additives which any refinery can put in but they bring varying risks to fuel system components. And since most planes currently flying are orphans in terms of manufacturer support, this is a tricky area.

At the Aero show in Friedrichshafen, I talked to an engineer from Lycoming who used to travel the world for engine failure investigation. I questioned him on detonation and he said that they have only seen it when several things went wrong at the same time. One contributor is very high CHTs as peterh337 mentioned and apparently even worse is incorrect magneto timing.

I tried to find out how large the detonation margin is and while trying to be not specific (lawyers...), I understood that even for the non/not yet certified engines, it is virtually impossible to experience detonation with UL91.

His recipe was simple: make sure the magneto timing is right and do not lean above 75% HP. Below 75%, lean as you like.

Having no kerosene/diesel story, it is not a surprise that Lycoming is actively promoting UL91 and the like.

Based on all that I've read, the above does not suprise me at all.

In any normal ops, one gets nowhere near the CHTs required for detonation.

I can however imagine there are people out there who do manage it, due to stupid operating practices (like climbing at Vx all the way to cruise level) combined with having no engine instruments.

The highest CHT I have ever seen on mine was 430F and that was departing from Turkey at +35C, after a long wait on the ground with the engine running, and having to climb fairly steeply as well. And the TB20 does suffer from less than great engine cooling. Normally one doesn't exceed 400F; ~360F in cruise.

The strange thing is that TOTAL seem to be charging almost the same for 91UL as for 100LL. I expect they are hoping to push down 100LL volumes and once enough airfields stop stocking it, they can then screw the market.

At the Total booth, they told me that there are currently two companies in Europe producing AVGAS: Total in France and Shell in Holland (they meet once a month for breakfast to agree on the price). Production in the UK stopped some time ago. In Total's terminology, 91UL is AVGAS. The number 91 is MON (motor octane number) versus the RON (researched octane number) common for car fuel which tends to be higher. The 100 octane fuels that Shell and others try to sell across Europe only have a MON of 88.

So 91UL is similar to 100LL and therefore about as costly/inexpensive to produce as 100LL.

In that case, why don't Lyco just approve 91UL for all their engines?

The Lycoming guy said it requires individual testing. They are planning to add more, at least all engines that are certified for the Hjelmco AVGAS. However, we will probably not see the turbo/high-compression engines. I did not understand the specific reasons for that.

Wouldn't it be funny if it turned out that actually none of the Lyco/Conti engines needed 100LL after all?

GAMI tested the worst of them all (some TSIO-550) and had a lot of problems making it detonate, being able to do it only with CHTs of 550F+. They have written this up somewhere...