I have seen reports that leaded car petrol makes an engine run quite a bit hotter. So the people were putting car petrol in one wing (used for cruise) and 100LL in the other wing (for climbs). This is apparently a very old practice, especially in Africa, etc.
Never officially because the said planes (say, normal Cessna 182s) were never certified for MOGAS – whatever “MOGAS” means – and MOGAS is nowadays unleaded.
Can anyone report on 100LL versus 91UL? The combustion is obviously very different.
Just because an engine is certified for 91UL doesn’t mean the temps are the same. Especially as the max Lyco CHT is something pretty wild like 500F 
Peter wrote:
I have seen reports that leaded car petrol makes an engine run quite a bit hotter.
Did you mean UNleaded car petrol?
Peter wrote:
Can anyone report on 100LL versus 91UL?
Not quite 91UL, but close enough,
there is very little difference in CHTs between 100LL and Shell premium mogas sold over here under the brand name “V-Power Racing 100”
leaving the cawl flap open, just for 1 cm would have much greater effect on engine temps in my airplane
Combustion temps of 100LL and UL91 are rather similar with details more dependent on different engine types. Just from experience, 91UL in an carb’ed O300D will burn a tad cooler compared to 100LL, while on a TIO540 it feels as if it burns a little hotter – but still can be due to different modes of operation,
Unleaded Autofuel in the other hand usually burns hotter, but that depends vastly on the brand. While the continental ‘BP/ARAL Ultimate (102, car octane)’ is fine with i.e. the Petersen Autofuel STC, due to no additives untested in it, ‘SHELL V-Power Racing (100)’ is not, due to ingredients not tested and ‘TOTAL Excellium (98)’ is the Ethanol-free stuff sold as ‘Mogas’ in former times when fuel was easier and less Arts of Composition ;-).
But, timing is key and the trouble raises when switching fuels. Rule of thumbs of my maintenance organization was to try to have at least 10 hours (2 full tanks) of one sort of fuel before adjusting magneto timing, as adjustments are slightly different for 100LL, 91UL and Autofuel. As I get Autofuel at most places and try to avoid 100LL, I do my maintenance on Autofuel and know 100LL on tour will not be burning perfekt. Just keep in mind we fly fixed timing engines and not the fancy stuff in cars (for the younger add: before they became e-toys).
No noticeable change in EGT or CHT, in a IO540, when I use UL91 from Total or BP. That said, I have never had more than 80% UL91 in the tank, due to irregular availability (home region has expensive fuel and the typical en route fjel places have no UL – i often get it in France)
Did you mean UNleaded car petrol?
I kind of meant either. It’s a general question about fuels being “legally compatible” but perhaps not identical in performance.
No noticeable change in EGT or CHT, in a IO540, when I use UL91 from Total or BP
That’s really interesting. It suggests that TEL does nothing to normal combustion and merely rises the detonation threshold. I guess that is obvious – it doesn’t participate in combustion
I run UL 91 most of the time in an IO360 and an IO540. Then run leaded sometimes too. I’ve never spotted a difference, so it must be small.
One thing I HAVE noticed is the lack of lead fouling! No need to lean on the ground, UL91 is good as gold.
MichaLSA
„TIO540 it feels as if it burns a little hotter”
Can you use UL91 in that engine without a detonation risk?
I would be more than happy to have some alternative for my Tsio360.
Raven wrote:
MichaLSA
„TIO540 it feels as if it burns a little hotter”Can you use UL91 in that engine without a detonation risk?
I would be more than happy to have some alternative for my Tsio360.
Obviously, Cessna puts a TIO-540-AK1A in new Skylane-NAs and it is cleared for UL91.
Are you sure?
TIO540 is turbocharged. AK1A seems to need 100LL.
