The people of Kawasaki, obviously, developed continuously their superb Hien. By the middle 1944 they’d produced the Ki-61-II(-KAI) and -III, heavier improved models powered by a 1500hp Kawasaki Ha-140 liquid-cooled engine. Sadly, their problems already endemic with the previous liquid-cooled engines only get worse so late in the war, engine factory destroyed included. So bad was the situatin, it forced Kawasaki to look for dractic solutions. In the end like the Russian with the Lavochkin, they modified a Ki-61-II-KAI airframe to carry a Mitsubishi 1500hp Ha-112-II radial engine. Converted in record time, the prototype flew in Feb. 1945 and soon demonstrated the qualities that turned this design into one of the best interceptors used by the Army during the entire war, if not the best. Powerful, rugged and maneuverable it only lacked real high-altitude performances. Almost 400 in two basic models were produced either modified from Ki-61s or built from scratch.
Such engine adaptation was certainly not easy: the Kinsei radial engine was both larger and shorter than the Ha-140. Aerodynamic and centrage issues were to be considered. To get some ideas, Takeo Doi and company studied an imported Fw 190A-5. That influence is somehow present in this lovely drawing of a “razorback” early model, specially those exhaust pipes.