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?
Those power rating explanations in other answers are incorrect. The Helicopter engines are rated at a lower horsepower at a higher RPM because they cannot accept the SAE "5% less than rated is acceptable" rule. The power rating must be correct, and is set at an altitude above sea level, for example, 1500' ASL. This is easily understood when you look at the engine operator's manual. I have an IVO-360A1A from a Brantley B2B Helicopter modified to fit in my RV-8. It's a parallel valve engine, 8.5:1 compression ratio, rated at 180 HP @ 2900 RPM. It has a TBO of 1000 hours. It had a huge oil cooler and it also has oil lines that extend from the oil galleries on the engine block to the exhaust valve guides, where oil flows into the head, around the outside of the exhaust valve guide and drains into the valve chest to cool the head. That is because helicopter engines work very hard and run very hot. Internally they are nearly identical to aircraft engines, and use the same parts.
While engines of the same size may have different ratings for a host of reasons, the FAA rules (14CFR33) for rating the power of a helicopter engine are much more demanding than those for rating an airplane engine, and it has nothing to do with SAE.
Specifically, the endurance test for certifying rated power is more stringent. Both sets of rules require 150 hours of sustained running (servicing allowed) and alternate between periods (mostly 1.5-2.5hr) of rated speed/power and reduced speed/power.
The average reduced power for helicopters is 71%, for airplanes it is 64%
The average reduced speed for helicopters is 71%, for airplanes it is 84%
The speed range for helicopters is 30% to 110%, for airplanes it is 79.5% to 100%
The helicopter rules require a sustained 15hr run at 105% of continuous rated power and speed, and has 12.5hrs at 110% speed. There is no similar requirement for airplane engines.
An airplane engine spends half of its time at reduced speed, a helicopter engine spends 15hrs more at rated power than reduced power.
Helicopters must produce more power under a wider variety of more difficult test conditions: 71% power at 71% speed is 24% harder on the engine than 64% power at 84% speed. Consequently, a helicopter engine that is internally exactly the same as an airplane engine will be rated 5-10% lower in power at 5-10% higher in speed. The helicopter version of similarly rated engines will be beefier than the airplane version.
The Lycoming HIO-360 is a horizontally-mounted version of the IO-360. (H for Horizontal). It reaches its power band peak and has a lower peak at different RPM for that reason. This is often the case when you work against gravity / work with gravity.
Some helicopter engines are designed with thinner cylinder walls for lighter weight. (IO-360-J2A for the Robinson R22 for example). These are further derated because of the weaker cylinder walls. Note that there is no H designation here because a)H is for horizontal and this is not one, and b)H is NOT for helicopter :)
Reciprocating light helicopter engines are derated to allow for out of ground effect (OGE) hover at an altitude above sea level. To the best of my recollection the R22 is limited to a manifold pressure of 25 inches. This allows for rated power to be achievable at roughly 5000 ft. altitude.