Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
The first Kawasaki built DB 601A, designated the Ha-40, was available from July 1941 and by November it had passed all ground running tests. Kawasaki, however, struggled with the responsibility of developing both the engine and the airframe....The Ha-40 inverted-vee engine... was slightly lighter than the DB 601, but more powerful - it was rated at 1175 hp. Kawasaki had been unsuccessful in gaining license manufacturing rights to the DB 601's all-important Bosch fuel injection system, so it had to incorporate a Mitsubishi-designed alternative. (page 8 )
The very high pressure of the Mitsubishi fuel injection system led to fuel leakage and fuel line ruptures. The ignition system was troublesome, with poor quality spark plugs causing pre-ignition or constant fouling. Leaks in the coolant system due to poor castings caused overheating and the quality of the bearings caused many problems....
In New Guinea there were ongoing problems with insufficient numbers of mechanics and inadequate maintenance facilities....the Japanese Army had little experience with liquid-cooled engines, and most of the maintenance crews had no expertise with this type of engine.(page 16)
Spent a while on google and English language info on Japanese and Italian engines is frustratingly sparse. I am only going on fragments but it seems the Japanese models had problems with crankshafts, crank bearings and lubrication.
All the information I have found on the Aichi Atsuta V12 could be written on the back of a stamp with a marker pen.
As far as i know all the series 5 italian fighters were tested and produced with their Db 605s at 1,3 ata and italian propellers which were of inferior performance than the german propellers. Were the Fiat G55, Re 2005, and C205 ever cleared for 1,42 ata? i dont know it. But the Bf109 was cleared for 1.42 ata in octomber 43. By that time italy had changed sides.I find it unlikely that in the chaos of those days any improvement was introduced.
Even with 1.3 ata all three series 5 fighters look overall better than the mid war Bf109, except the production cost and man hours