Hey folks,

Researching automotive gas usage on aviation piston dinosaur engines and came up with a little spec that puzzled me about Lyco's O-320 in 150HP and 160 HP configuration. Anybody have some fidelity on why there is a requirement for 87 min octane use on the 7:1 compression models and 91 min octane use on the 8.5:1 compression ratio?

There's nothing magical that happens between 7:1 and 8.5:1, so why the mandate on these mogas STCs to stick with "premium" mogas above 7:1 ?. My 2000 jeep cherokee, along with most of our cars driving every day out there, have engine compression of 9+:1 and the manuals all allow and even recommend the use of 87 octane. My cherokee doesn't even have variable valve timing or anti-knocking sensors, and it runs fine. Something doesn't add up. There's no way 87 octane gas is all of a sudden going to start pinging at a paltry 1.5:1 compression ratio increase.

The only possible variable I can see is in the fuel delivery system, where most of these aircraft have poor positive pressure delivery systems (sans the gravity fed fuel systems on high wing a/c of course), many with just an engine driven fuel pump instead of positive pressure electric fuel pumps at the tanks, and open themselves up for vapor locking when the higher vaporization rating of mogas is introduced to a hot engine/mechanical pump climate in a poorly insulated/designed carburated aircraft fuel lines system. But even then the difference in octane between 91 and 87 is not significant to compensate for these fuel system shortcomings, i.e. a vapor lock scenario in a poor fuel delivery system is just as likely to occur at 87 octane as it would running on 91. Ergo, electric fuel pump installation, positive pressure delivery versus suction delivery, and fuel line insulation (in other words, 20th century fuel injection system components) seem to have more to do with the avoidance of vapor locking than 91 vs 87.

Any thoughts on this? The engineer in me just refuses to pay a premium on the operation of an aircraft based on old wives tales (which is why I don't run 100LL) that are not cemented on empirical physics and chemistry. Lots of misinformation out there propagated by rent-seekers dead set on keeping a market cornered (and people grounded, without access to affordable flying) with archaic propulsion technology.

Thanks a bunch!

 

The "low compression" A and E models were built for 80/87 avgas. The "high compression" B and D models were built for 91/96 avgas, which I don't even remember from the 70's, or 100/130, which was green gas. Not sure that helps with your question, but I think the STC's for autogas followed the "low compression"/"high compression" line of thought when they got approval from the FAA. Will my 160hp Apache run okay on low test autogas? I suppose, but I'd be breaking an FAA rule and running afoul of my insurance company. The real question is, where do you find autogas without ethanol in it anymore? I have pretty much given up on autogas (sadly), because of this issue.

Outside that, I'm a BIG believer in autogas and have used it in the past without any trouble at all.

 

I still have access to non-ethanol autogas on base. As to the legalese, the beauty of experimental setups is that there is NO STCs for experimental aircraft, and I know the engine doesn't care about FAA mandates when considering the physics and chemistry of octane ratings and compression ratios.

Going back to the certificated game, I seem to agree with you that the STC makers did do a blanket cursory "low compression/ high compression" crossover of avgas ratings to automotive use equivalent octane ratings (different octane calculations, granted). Petersen goes into great detail to discuss the "evils of ethanol", but they do not challenge or provide test data to support the blanket assertion that these 8.5:1 ratio engines "require" 91 octane. I'm inclined to say that provided a suitable positive pressure fuel system (electric pumps at the tanks and insulated lines firewall forward) 87 octane would provide safe and identical performance at a substantial discount when amortized over years of operation. The experimental category is necessary to even make and support this claim of course.

As to ethanol, I think this is another example of Lyco not giving up the gig. There's nothing cosmic about fitting alcohol resistant gaskets and seals, and even then, the claim that a paltry 10% ethanol by volume "eats the thing up" and makes the economy of that fuel use in aircraft engines a "false economy", greatly exagerated. It's not like we're going to be cruising at FL400 at -40F and that emulsified cintilla of water is going to freeze your lines to a fiery crash...

 

I wish somebody would come up with an STC to allow 10% ethanol in certified aircraft using autogas. I'd pay money for that. If it was easy, I'd imagine Petersen would have done it already. I didn't realize you were experimental. Of course that changes everything. I've never owned an experimental, but I think I'm slowly moving towards newer sport plane technology as opposed to a 58 Apache.

 

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.

 

Update:

Spoke with the folks at Petersen. Like I suspected, they tested the "high compression" O-320s with 89 octane, no detonation, no pinging, zip, nada, zilch. They brought the science to the FAA and got shot down. "91 or else" said the FAA, as always more interested in CYA than the truth. So they had to budge if they wanted the STCs approved. From the horse's mouth.

Completely unrelated but now I feel much more comfortable about my healthy skepticism for government numbers.

 

Does Petersen have data on running 87 octane in a high compression Lycoming 0-320, data gained from long term testing?
I thought that is what you were wondering about, not 89 octane.

Interesting thread, by the way.

 

No, they said they never could justify the effort of benchmarking the 87 octane data on the 160HP models because when they went to the FAA with the 89 octane data, the FAA stiffarmed them and said "91 or else". They effectively cut their losses on the 8.5:1 Lycos after the 89 octane push was not well received. It's a shame really.

My "anectdotal" evidence with 87 is congruent with Petersen's experience on 89 octane. Fuel delivery system construction and positive pressure delivery is a bigger player with these fuels than merely octane ratings. Which is why they directed the re-plumbing of the 160HP Warrior and Warrior IIs, even while "mandating" 91 octane (again, per the horse's mouth). It has always been a matter of old fuel system design than the "quality" or viability of modern car gas. In essence you can put magic rabbits and easter eggs in the nova, but the sucker is still gonna vapor lock..because it has a Nova fuel system. Fix the fuel delivery, you fix the "vapor lock". Octane at that level of discussion has no bearing, and serves more as a scapegoat to justify an unnecessary premium than supporting safety and performance. Which is par for the course as far as the FAA and Lyco is concerned.

Of course all this talk was academic in the context of certified aviation (since the FAA holds the hammer over us little people jousting at supposed windmills). However, it is not just academic in the realm of experimental aviation, which will be the only outlet to take advantage of these strides long before certified aviation gets slapped with the eventual phasing out of 100LL (God willing).

We also discussed ethanol and Mr. Petersen merely left it at acknowledging the real threat he documented about alcohol-based additives damaging the seals and rubber components of these old airplanes and engine technology. He did not challenge the assertion that a simple reconstruction of the target parts to be ethanol friendly (like the rest of the automotive world) would suffice to appease the scared masses. Instead he defaulted to pointing the finger at what he sees as the fundamental differences between getting such change mandated by the FAA versus the freedom car manufactures have to effect a similar change, and I quote: "simply with a press release". I really do not disagree at all with his sentiments about differences in behavior between the automotive propulsion industry and the aviation propulsion industry from a regulatory standpoint.

I think the days of 100LL are counted, and people can cling to their "guns and bibles" (100LL) and accept big brother's assertions about safety as gospel, or they can go to the people who provided the unbiased numbers (i.e. the people who probably got stiffarmed by the FAA) and use it to push forth the agenda of re-capitalizing aircraft engine technology and fuel use. Energy costs are only going to go up, and if we wish to keep American GA alive the way our parents saw it, as opposed to the prohibitive landscape the British have, we need to push forth these viability and affordability enhancing measures. Or we can just accept only the rich and the non-professionals can afford 100LL and expensive overhauls on de facto civil war steam engines, and let them enjoy aviation while your kid gets to watch them soar while he wonders why daddy and he can't do the same as y'all sit on the airpark parking lot. Not very American in spirit IMO.

 

You guys should really be careful about that STC... just because the engine is STC'd doesn't mean the fuel tanks are. Some Piper outboard tanks (i.e. Cherokee 6)... will start to bubble and distort and otherwise fall apart.