The Kawasaki Ki-61 Hien was Japan’s only fighter powered by an inline water-cooled engine, the Kawasaki Ha-40 and later the Ha-140. Both engines were derived from German engines that powered the famous Messerschmitt Bf-109, the Daimler-Benz DB-601 and later DB-605. In 1945 Kawasaki lost its engine factory to earthquake and a bombing raid. The Army ordered Kawasaki to fit the Mitsubishi Kinsei air-cooled radial engine to the unpowered airframes.
The results was surprisingly successful: the Ki-100. The fighter, minus the heavy liquid cooling system, was was 330kg lighter, which reduced its wing loading and made it much more maneuverable. It was better than the Ki-61, and even outperformed the Ki-84 Hayate, the Army’s best fighter.
The archetype of the Hien was the Bf-109 fighter, the mainstay of the Luftwaffe. The Germans kept using it alongside the Fw-190, upgrading the engines and armaments until the end, creating many variants from A to K. The Battle of Britain was fought with the Bf-109 E “Emil”. The Emil and a later variant the 109 F “Friedrich” were considered the most nimble versions of the 109, indeed the most agile fighters of the European theatre. The 109 G “Gustav”, introduced in 1943 was powered by the more powerful DB-605 engine and given heavier weapons. It was faster, more powerful, but was now rather heavy and less agile.
It makes me wonder if the 109 might not benefit from a radial refit. Shedding some 360 kilograms might restore the agility Willy Messerschmitt had intended for his brainchild. On the other hand, the Kinsei lacked a supercharger resulting in the loss of high-altitude performance.
The Germans were certainly not averse to unlikely fighter/engine pairings. The Focke-Wulf Fw-190A was powered by an air-cooled BMW 801 radial engine. The fighter, very capable at low to medium altitude, lost much power at high altitude because the engine had no supercharger; it had been designed to power medium altitude transport aircraft. Intercepting high altitude bombers was left to the Bf-109.
The Germans became aware in 1943 of American plans for the B-29 “Superfortress”, a fast high-altitude heavy bomber, and realized the Bf-109 lacked sufficiently heavy weapons to bring it down. The Fw-190A had the weapons but could not fly that high. The DB605 engine of the Messerchmitt fighter was not powerful enough for the Fw-190, which was 50% heavier than the Bf-109.Around 1944 Kurt Tank of Focke Wulf fitted a huge water-cooled Jum0 213 to the Fw-190, creating the Fw-190 D “Dora”. The Jumo 213 had a supercharger, vital for high-altitude performance, compressing the thin air before it went into the cylinders. Some have said the Jumo was intended to be a bomber engine. As it was, the Jumo 213 was much longer and heavier, forcing Focke Wulf to lengthen the tail and add a lead weight. The Luftwaffe pilots loved it. It was considered one of the best German fighters of the late war period.