Homebuilt 70% Scale Spitfire Mk 24

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Engine failure two months after first flight.

I admire and appreciate the considerable ingenuity and effort involved. Beautiful work.
Too bad it wound up looking more like a Chipmunk than a Spitfire.
Engine failure two months after first flight.
The experience of using automobile engines in aircraft is not a happy one. Aircraft engines have to sustain high power levels for much longer time periods than do automobile power plants and things like valves tend to wear out quickly. One guy recently installed a truck engine modified like a boat engine in a Cessna 172 and got excellent results.
Thrust bearings are a real issue as auto crankshafts are loaded strangely, even with reduction gear boxes.

There have been great modifications from companies like Thielert, converting auto diesels to aircraft.

Note that Toyota/Lexus have been experimenting with specially designed aircraft engines, but also some serious composite manufacturing techniques. Also, Honda has been marketing their fully certified light executive HondaJet, designing not only the airframe but the turbine engine.
Back in the 1980's Toyota supposedly was making serious efforts to built aircraft engines based on their automobile pwoerplants. They hung a prototype engine on one wing of a twin to accomplish flight testing and also purchased a large FBO, but nothing ever came of it.
Toyota has taken the Deming methodology of management to heart, and has had worldwide impact with their Toyota Management System, developed in concert with major universities around the world, including Carnegie Mellon, the General Motors Institute, Kings College, Cambridge and Zurich Polytech. Porsche and Harley-Davidson remodeled their processes based on their input.
Toyota took over the somewhat disorganized FBO at Torrance, and using TMS made it efficient and profitable.
They made considerable investments in exotic composite component production, heretofore confined to low production quantity specialty items, like sailplanes, race cars, sporting equipment, etc. They funded research and application studies that simplified the curing and quality control methods, which were bottlenecks. This should benefit the industry as a whole, even though they determined that there was no business future in light, civil aircraft. (IMHO, pilot training, insurance and increase of regulations were major factors.)
The problem with general aviation (GA) engines is that GA as a whole is more or less moribund, and there's just very little money to spend on R&D (and certification if you ever get that far!). As a result Lycoming and Continental dominate the market with engines firmly stuck in the 1940'ies (think carburetors, magneto ignition, manual mixture control, and whatnot). They are called "Lycosaurus" for a reason.. So with very little money to spend on a clean sheet engine, repurposing automotive engines surely must look enticing. Thielert (which went bankrupt, and I think the Thielert engines are now made by Continental) and Austro are making aviation diesel engines based on a Mercedes diesel car/van engine, and have apparently been somewhat successful. That being said, I would be somewhat suspicious of DIY auto engine conversions.

On the gasoline engine side, AFAIK the only really successful alternative to Lycosaurus seems to be Rotax. And they started with making small engines for very light aircraft, which have significantly less red tape associated with them than GA proper. Rotax engines are in a way quite impressive, they cruise at close to 6000 rpm yet manage a TBO of up to 2000 hours.

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