Interesting… The R3350 turbo-compounds that were in DC7s, and Lockheed Super Gs were probably the final design to extract just about all you can out of a reciprocating engine running on gasoline. Although that being said, I believe that current top fuel dragsters are getting almost 10,000 hp out of a pushrod, V8. However those engines last about 30 seconds and have to be torn down with new heads and pistons between runs. You can make anything very powerful if you intend it to run for only 30 seconds. The radial turbo compounds were very complex affairs with bevel gears and sprague clutches on all three turbine output shafts. They must have been bear to maintain. While the B29s had the R3350s in them, I don't believe they were turbo-compound. I think they had the usual GE turbochargers instead. Turbo-compounds really came along after the war. Does anyone know of any water-cooled versions? The schematic looked like a turbo-prop engine with the piston engine acting as a gas generator.