Peter wrote:
91UL is just 100LL without the lead. It’s not MOGAS, or just car petrol which contains all kinds of rubbish nowadays.
Who will be the first to test it to FL200?
I flew FL200 (and > FL150 several times) with Premium 95 Octane from a car station without having any issues, however with a Rotax.
AVGAS 91/96 UL is approved to be mixed with AVGAS 100 LL
96UL is however not the same as 91UL. 96UL is a different formulation, which has been approved for many more engines than 91UL has been (or ever will be).
I flew FL200 (and > FL150 several times) with Premium 95 Octane from a car station without having any issues, however with a Rotax.
I know a pilot (not on here) who flew with car petrol, in a turbocharged 540 engine, high altitude, and reported substantially higher CHTs. A number of people do this, and reportedly a lot of them put 100LL in one wing and the other stuff in the other wing, and use 100LL for the takeoff and climb.
However car petrol is not 91UL, which should work the same as 100LL provided detonation is not actually taking place – AIUI.
Peter wrote:
IMHO it [AVGAS UL91] should work OK in a TB20 because you are down to 75% power at FL080, and down to maybe 50% at FL130, so the chances of detonation (the real risk with unleaded) is just about zero.
But Goaround wrote:
Lycoming had released the unleaded Fuel UL91 for the Lycoming IO540 C-Series
( typical engine TB20), published on Lycomings website.
So the engine is approved for AVGAS UL91 then it will run well on AVGAS UL91 regardless of altitude.
Peter wrote:
96UL is however not the same as 91UL. 96UL is a different formulation, which has been approved for many more engines than 91UL has been (or ever will be).
Hjelmco 91/96UL is 100LL with the lead removed. UL91 is something else, but the octane rating is the same, 96/91 so for all practical purposes it’s the same and they are both aviation fuel.
Just the other day, Air BP started the worlds first (for Air BP) UL91 distribution at Kjeller. At our club at ENVA, we will fix fuel ourselves with a separate facility. It will be UL91 and pay by card, so that everybody visiting can fill. Ordering bulk we will pay 20-30% less than 100LL from Shell, and this includes having to have our own tank facility..
Hjelmco 91/96UL is 100LL with the lead removed
They can’t both be 100LL with the lead removed 
No, Hjelmco is 100LL spec, only the lead is removed (not added in the first place). UL91 is specified in ASTM D7547 (it’s not unleaded 100LL, but something else). Who cares, the octane rating is the same, and both are aviation spec. It’s the same thing for all practical purposes.
Anders wrote:
So the engine is approved for AVGAS UL91 then it will run well on AVGAS UL91 regardless of altitude
If I remember the regs. an EASA aircraft can use the fuel once the engine manufacturer approved it while N reg aircraft can’t because according to the FAA the aircraft manufacturer has to approve the fuel and I am not aware of any maker approving the UL fuels.
In this March 2016 Lyco PDF ( local copy ) there are engines which are approved for Hjemco 91UL and not for other 91UL.
Peter wrote:
there are engines which are approved for Hjemco 91UL and not for other 91UL.
And also the other way around.
LeSving wrote:
And also the other way around.
Which is pretty weird. How come one of the fuels is not strictly “better” than the other?
