When an automobile engine can run at 75-90% for several thousand hours efficiently, without fail.... I'll put one in my airplane. Oh and it cant weigh more than the IO-360 I have in there now. The homebuilt crowd has been having this argument for decades and attempt after attempt has been made, with no real success for a reason. It dosent work. Only recently has electronic ignition and adjustable timing started to make it's way into the scene reliably. Mazda rotary engines are probably the closest thing to an answer, but no one has been able to crack the code on a reliable reduction drive unit. And running mogas above 8.5:1 is just fine as long as you run very very rich. Start leaning and your detonation margin is almost nil. Never mind the ethanol debacle.

If there is an answer, it's diesel. Someday.