[labcomputer]

That's part of it. Ethanol is incompatible with materials commonly found in airplane fuel systems.

Another aspect is that many piston airplane engines need high octane because they have relatively high compression ratios: WWII era engines need 130 octane to develop full horsepower (they can be operated at reduced manifold pressure on currently available 100 low-lead gas), and even many post-war civilian engines require 100 octane.

Still another aspect is that the FAA is relatively conservative and doesn't want to approve something that might lead to, for example, vapor lock or fuel freezing issues.

[mrguyorama]

>airplane engines need high octane because they have relatively high compression ratios: WWII era engines need 130 octane to develop full horsepower

I mean, high octane and compression relative to other 1960s engines. The common engines in nearly any single prop cessna have about a 9:1 compression ratio, which was massive back when it was built, but laughably bad compared to anything manufactured after the advent of Fuel injection and better piston geometry.

Modern cars regularly have over 12-1 compression ratios on 87 octane.

1960s era carbs and top ends were just abysmal and did a terrible job of mixing the fuel and air charge and controlling the flame front, because we just didn't have the kinds of computer controls and fluid dynamic simulations we have now, to dynamically prevent knock.

Rotax engines are modern and can reach identical performance figures or better, with the same weight or lighter, simply by using modern techniques like fuel injection or a small turbocharger. They do this while running on 91 octane

[lazide]

Except none of that can be used until someone goes through the millions of dollars to do all the design work, safety testing, and mountains of FAA paperwork. Oh, and take on the liability of a mistake killing a bunch of people.

In the mean time, rebuilds have to meet the existing design. Which has specific standards.

It's not like Cessnas themselves can't be wildly improved on in general!

As an old Boeing engineer friend of mine used to say 'when the weight of the paperwork exceeds the gross weight of the plane, it will fly.'.

[jabl]

Rotax 900 series also have a maximum rpm of 5800, substantially higher than old school aero engines, which also helps improving power/weight.

[mchid]

Low lead is a misnomer, you might as well call it LL.