Originally Posted by
stratoduck
In general, air cooled engines need higher octane gas because of higher cylinder heat temperatures. Follow the STC. There was plenty of testing done, and engines, airframes and octane limits were the results of the testing. Trying something different will probably not put you into unproven territory. It will probably put you into disproven territory.
Ethanol is bad news. It does attack older rubber. It also causes the engine to run leaner. It also reduces power output. And the worst problem is that it holds water. How much water is temperature dependant. A saturated mix will shed water as the fuel cools as you climb, potentially flooding the carburetor with water and causing an engine failure.
Octane requirements and ethanol prohibition were far from flippant decisions.
Yet, millions of cars make the trek up and down the hills of this great country with 87 octane ethanol-laden gasoline from Joe bob's underground undipped gas station tank, and nothing happens to their 9:1 compression engines.
Same goes for the ethanol. Water absorption? Of course. Bad enough to quit an aircraft engine? At what altitude and temperature? I guess if it's bad enough to quit a car engine, yet you don't see that happening very much at all. It's all about positive pressure. Heck, back in the 70s you saw chevy novas quiting. Was it because of ethanol induced water? heck no, the things were vapor locking cause of bad carburetors and an engine driven mechanical pump (suction pressure) that was hot enough to fry an omelet on and sat higher than the gas tank. Same goes for your run of the mill certificated spam can. Eliminate that fuel system anachronism, and no more vapor lock. 100LL got nothing to do with it, neither does 87 mogas for that matter.
Which is why, even by simply putting mogas on a high wing contraption, that very above-the-engine gravity provides enough positive pressure to keep a cessna 172 from vapor locking on crapola octane, *ethanol laden in theory wink wink*, climbing all the way to 10'K at 90degF on a Texas summer. Hmmmm.... So petersen and eaa say one thing but from my anecdotal reference point (i did go to school for eight years and managed to learn how to count with my fingers) octane, and lead content for that matter, got jack to do with it. I think it's legalese and political pressure to keep the floodgates of technology from collapsing the GA propulsion racket. 100LL is on its way out though, and that will make the industry finally admit the facts behind unleaded automotive fuel in recreational aviation use. Instead of defending the merits of 100LL (which many of these engines were not even designed for either!) we should be defending the merits of killing 100LL. The acceptance of both unleaded fuels and yes, even ethanol-laden fuels, in GA should be a point worth pushing forth.
I will contact petersen and get their take on the 91 vs 87. They may have indeed found empirical data to back up the clear cut line deliniation between 87 and 91 use, but by their own literature online it really shows as a weak case. Regarding ethanol I concede the erosive properties on older rubber, but not to the fatal scales that the pro-100LL crowd seems to argue to. What it does say about Lyco and ConTy is that they should giddy up to the late 20th century and make alcohol friendly gaskets, like the rest of the industrialized world. Likewise they could make non-TEL-requiring valve seats, like every car since the 81' oldsmobile cutlass and the invention of the Roman aqueduct. But then the 100LL crowd wouldn't have a leg to keep pushing for the survival of tetra-ethyl lead. Disproven territory? It looks to me like government sanctioned misinformation.