How come US and Japanese power curve for Ha-45-21 differs?
1990hp at SL with WEP
1625hp at 6100m with Military Power
US on the other hand state 1970hp at SL with WEP
1675hp at 6100m with Military Power
Probably because each country has its own testing standards and processes.
Identical standards/processes should produce the same results.
Different standards/processes will almost certainly produce different results.
Also Japan used metric measurements and the USA used exclusively inferial during ww2 so conversion from metric to inferial would provide further errors in rounding and the US was almost certainly testing an engine repaired in the US with US fuel instead of as built with Japanese fuel
I would trust the Japanese figures more than any other