I think that is just the reality of non-mass manufacturing. In the 50's those engines were going into production cars in the tens of thousands, or hundreds of thousands. Now they are produced in numbers of hundreds, maybe thousands, and as such they are hand-built specialty engines, even as primitive as they are.

This is what most people miss. Yes the FAA is slow, certification is costly, and liability messed things up in the 80s -- but you can't get the huge cost reductions we see in the automotive sector without mass-production. Aircraft are essentially hand-made in a labor intensive processes, using individually crafted components.

It's not hard to see what certification costs, take an EAB kit, use all non-certified avionics, and even take an automotive engine and convert it for aircraft use if you want. If you spec that out equivalently* to a certified aircraft, taking labor costs into account (you don't get to count the 1500 hours in your garage as free), it's less than comparable certified aircraft, but not orders of magnitude less.

(By equivalent, I mean safety and redundancy -- ifr capable. Not talking about making my own AP with cheap servos and a raspberrypi and navigating with a handheld gps)

None of the common air-cooled aviation engines designed in the 50's were used in production cars.

I think the mooney bravo had a porsche engine at some point? But otherwise you are correct. It wouldn't make any sense to use engines in both cars and planes because they are such massively different requirements: One needs extreme reliability at basically one RPM and power output and has things like double spark plugs for robustness, the other wants to keep costs down and reach higher RPMs

Price gauging. Same for Rotax 912, the most popular small engine on planes that don't need certification (LSA/ULM) and they are 20,000 Euro for a 1.3 liter engine with an 100 HP output.

9k for a rebuilt Toyota 2f designed in to 50s & meant for a measly ground vehicle. 30k seems about right for an airplane. What do you do for a living? Why don’t you do it for less?

That's the whole point, Toyota automotive engines are mass-produced by machines that literally work for less.

The Toyota 2F is an out of production engine that fulfills a niche market, it costs 3x as much to rebuild one as a new Toyota engine. The parts don’t cost much more and there are fewer of them. They cost more to rebuild because that is the going rate. Should the shops doing 2F rebuilds charge less? As others have pointed out in this thread, car engines are not well suited for air use, so the market will remain small and made up of people willing and able to pay 30k.

+ if the Toyota engine fails in a car you tow it to a shop -- if the 172 engine fails you emergency land in the best case (or crash in the worst). There is just a whole lot more onus to work on plane engines and that should and does come with a price tag...

Their mod of a car engine is claimed to work with 4 failed cylinders out of 8, has redundant ignition and fuel injection.

The very short answer is certification and what goes into the overall environment of certified aircraft.

(The slightly longer answer is it's a cottage industry. Small shops, small quantities. Car buyers are spoiled, general aviation is more like buying a built motor from a tuning shop.)

Haven't these engines been certified many many decades ago, so that cost should already have been paid?

Yes but!

There are still many other things in certificated aircraft that are changing regularly based on ADs. This means “we found something in this decades old design that shows up when the fleet is in its 30s.”

Aircraft have VERY long lifetimes in comparison to most things. I own a 1962 Cessna - it is on its second engine, the first having been overhauled once. Some Cessnas from the 70s are still running on their originally installed engine.

Defects and issues are still addressed ongoing, and maintaining the certification for things like modifications for improved power generation still means significant work. It’s definitely not a one-and-done thing - part of the value of certification is in the continuous future improvement that comes from NTSB findings or inspection reports sent to the FAA.

It’s like an engine fault light spread across the whole fleet. The certification keeps the light working.

The market is not determined by covering costs.

Ultimately, how many new engines are getting certified to put the current ones out of business through lower prices?

Even if the design is certified, I’d imagine the shops that produce them would still have ongoing certification costs to validate the equipment, tooling, etc. On going costs that would have the distributed across a limited number of engines.