I mean, brand new single-engine prop planes with modern fuel-injection systems are also running on leaded gas, so the fact that really old planes also are is kind of obvious/irrelevant. With a handful of exceptions, single-engine prop planes are only certified to run on 100-octane leaded gasoline. For many (maybe even most, but I don't know the exact numbers) certified aircraft there is no legal path to get it to run on any other type of fuel. Certain planes have supplemental type certificates to allow an expensive conversion but it's far from the norm.

> With a handful of exceptions, single-engine prop planes are only certified to run on 100-octane leaded gasoline.

This was true up until last year, when G100UL was approved as a drop-in replacement for 100LL. It does require a STC and the associated paperwork, but this is apparently pro forma and quite cheap, and doesn't require any modification to the aircraft apart from some placards. The problem, of course, is that G100UL isn't really available anywhere yet, while 100LL is still ubiquitous.

There are 100 RON fuels without lead.

Sure there are, but the 100 in 100 octane avgas refers to MON, not RON. 100 MON fuel is a bit trickier to be done without lead, although we now have G100UL which does it.

> "For many (maybe even most, but I don't know the exact numbers) certified aircraft there is no legal path to get it to run on any other type of fuel."

Until recently this was true. But since September 2022, G100UL is legal and certified for "every spark ignition piston engine and every airframe using a spark ignition piston engine in the FAA’s Type Certificate database.”

Yes, I understand using leaded gas in older designs. But what was the manufacturer's too keep designing engines that require leaded gas even on fuel-injected, ECU equipped newer designs?

Not a rhetorical question, I am genuinely curious.

New and old designs have been tested and can work with unleaded fuel. (Edit: there are surely some exceptions here, so I don’t mean this to be all encompassing)

New designs must work with leaded fuel since that is the fuel which is available - until avgas contains no lead this will continue to be the case.

Side note, most new planes are sold without an ecu.

You're right, some GA engines use fuel injection, also a 1950s technology. I used to fly a Cardinal with Lycoming IO-360. We got the fancy GAMIjectors and it made a huge improvement. Yes, the big exciting 2000s technology was "the holes in the injector are sized to provide calibrated fuel flow".

While I'm rambling, the dual spark in a GA engine is really interesting.