You need to get the whole airframe certified to change the engine. If the airplane hasn't been in production for 40 years+ who will pay for that? Does the engineering data needed to start that process even exist anymore?

Interestingly, I found that the airplane in question here, the Cessna 172 was actually sold with the Thielert / Continental diesel, but ended up cancelling it because they were so much more expensive than the gasoline models that nobody bought them:

https://en.wikipedia.org/wiki/Cessna_172#Special_versions

I think this is the $100000 conversion alluded to in the article. Not sure what makes it cost that much. Maybe the same thing that seems to have caused GA to stagnate, very small volumes combined with high certification costs.

(There's also a company called Austro engines that makes aero diesel engines, AFAIU even based on the same MB car engines Continental/Thielert uses. I think Austro is a subsidiary or spinoff of Diamond, so that's where you see these engines used.)

I would guess that whatever makes it cost so much would equally apply to this solution once it is certified and not experimental. The gas V8 will be much less fuel efficient than a small turbo diesel, especially at altitude where the turbo offers huge advantages.

Those Mercedes engines are found in small cars and delivery vans around the world, and are widely available and pretty cheap. For example, the Centurion 3.0 uses a Mercedes-Benz OM642 which is found in Freightliner/Sprinter vans, Jeeps, and tons of other very common cars. You can buy these engines straight from Mercedes for like $8k.

In this case, the author mentions using an aluminum marine GMC V8, but not which one exactly. This is most likely based on the LS V8s, which are fairly expensive motors, and a pretty ancient design (e.g. pushrods).

> I would guess that whatever makes it cost so much would equally apply to this solution once it is certified and not experimental.

Bingo! Yeah, I was wondering the same while reading the article.

> a pretty ancient design (e.g. pushrods)

Does it matter? The engine is significantly derated vs. the original car engine, so max rpm is going to be much lower, thus no problem with valve float.

Modern direct injection diesels still use OHC, even though they run at really low RPM, so valve float can't be the only reason virtually all modern engines use it. I think this is because it allows for larger and better positioned valves, or even more valves (e.g. DOHC/4 valves per cylinder), making the engine breath better. I imagine the advantages of that would be even greater at altitude.

I'm not saying OHC aren't better, they certainly are (in most respects). Just saying that I wouldn't consider pushrods a showstopper either.

(As an aside, when aviation engines started going with 4V designs in the 30'ies, it wasn't only about breathing, but better cooling of the valves was an equally big motivation.)