Why do the Lycoming HIO-360 A1A and IO-360 engines produce different power? I have just acquired an aerobatic aircraft fitted with a Lycoming HIO-360 A1A. Reading the specs on the engine I see it has the higher (8.7:1) compression of the 200hp IO-360 engines, which seem mainly to make that power at 2 700 rpm, but this one actually makes the same power (180 hp) of the 8.5:1 carburetter engines, but at higher rpm (2 900) than they turn. This was a helicopter engine originally which I think accounts for the higher rpm rating, but it seems to me wholly counter-intuitive that an engine with the same swept volume, fuel injection for greater efficiency AND more revs would make - relatively - so little power. Can anyone explain this?
Helicopter engines have different operating parameters than aircraft engines. They operate in a narrower power band and the difference in horsepower would be due to  camshaft lift and duration optimized for that power band.