The efficiency difference isn't as much as you might think.
Car engines today are much more efficient than they were 50-70 years ago, but that's because the difference between maximum power and cruise power for a car ranges from about 5x to 20x. Much of the efficiency gains come from devising ways to deliver short bursts of power from a small engine, such as variable valve lift and turbocharging.
General aviation aircraft cruise at about 75% power. There's much less to be gained by optimizing the engine to run efficiently at 10%, and running a small engine at high RPM and high boost continuously is a recipe for frequent service intervals.
That's ignoring improvements to transmissions, tires, and aerodynamics in cars that aren't relevant to planes. There's certainly room for improvement, but not to the dramatic degree we've seen with cars.
> The efficiency difference isn't as much as you might think.
True, the BSFC numbers of these old school aero engines aren't that bad, actually.
> General aviation aircraft cruise at about 75% power. There's much less to be gained by optimizing the engine to run efficiently at 10%
I think the real advantage isn't a small efficiency gain in optimal conditions, but rather that with a modern FADEC there's less pilot workload, with a single lever for selecting the thrust the pilot wants, and the electronics takes care of selecting the optimal parameters (throttle position, amount of fuel, ignition timing etc.) for that thrust setting, taking into account input from a number of sensors (temperatures, pressure, etc.). And all those sensors provide data that can be logged and help with planning maintenance before something goes pear-shaped up in the air.
> running a small engine at high RPM and high boost continuously is a recipe for frequent service intervals.
The Rotax aero engines rev to a max of almost 6000 rpm, cruise is ~5000 rpm, and they have a normal(?) TBO of 2000 hours, including the turbocharged 914. (The turbocharged 915iS has a TBO of 1200 hours, but I suspect it's due to the engine being relatively new, probably the interval will be extended when more experience has been gained?). Of course, these engines are designed for that from the ground up, not saying you could take a random car engine off the street and run it at 5000 rpm for 2000 hours.
Car engines today are much more efficient than they were 50-70 years ago, but that's because the difference between maximum power and cruise power for a car ranges from about 5x to 20x. Much of the efficiency gains come from devising ways to deliver short bursts of power from a small engine, such as variable valve lift and turbocharging.
General aviation aircraft cruise at about 75% power. There's much less to be gained by optimizing the engine to run efficiently at 10%, and running a small engine at high RPM and high boost continuously is a recipe for frequent service intervals.
That's ignoring improvements to transmissions, tires, and aerodynamics in cars that aren't relevant to planes. There's certainly room for improvement, but not to the dramatic degree we've seen with cars.