I have to correct some false information about the Ki-84. The performance you've given is calculated by US Intelligence. No example was ever flown to those speeds. Those numbers show up in Mar 1945 TAIC supplement, before any examples were ever flown ( in June, I believe ). I have the T-2 Frank I manual and the fuel specified is 91/96 octane with the use of 50/50 water/alcohol ( the alcohol being 50/50 ethanol and methanol ). There was no use of any 140 octane fuel, as published by Aero Detail. There were no higher boost pressures used to test the engine than those specified by the Japanese. The unpainted version that was flown in the Philippines never used more than +250mm boost pressure and 2900 rpm because the engine had only run 8 hours. It never reached ts maximum speed due to problems with the propeller operation. Even so, they figured the top speed would be about 400 mph (644 km/h ) at 20000 ft and 330 mph at sea level with the model 21 engine ( less with the model 12 engine ), so the lack of paint is also not an explanation. A Japanese trial showed 634 km/h @ 6650 m using rated power ( +350 mm and 3000 rpm ). My own calculations for +500mm boost pressure gives 648 km/h @ 5700 m, which is in pretty close agreement with what intelligence figured in the Philippines.