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!