i thought military rating was a 15 minute output rating, while WEP or take off power was usually considered a 5 minute rating.
The WEP rating typically gives a lower FTH.
It also had me thinking more of heat, something very relevant to the 5 minute rating.
The V engine has a water cooling system, which for overheating is going to give it a bit more time in the red.
The engine might get hot and overheat but nothing cooks until the water boils off, ie the engine becomes a pressure cooker, and usually you seea head gasket go or hose blow open before any other damage occurs. You can also warp heads and other parts by running hot.
After the water goes, then the oil cooks and the viscosity breaks down to where engine parts start grinding.
In a radial, there is no water to boil off, so the break down of oil viscosity happens much faster and cylinder heads are much more sensitive to damage. Also engine oil is a more of a mixture that burns with the fuel than something that cycles through, if i understand it correctly.
Holding your engine in the red would not be advisable. The advantage is that the engine will also cool off relatively sooner.
In a water cooled engine you'd have to wait for the coolant to reach a "normal" temperature setting which is dependant on radiator efficiency.
If overheating blows a hose, or gasket, you're screwed. A radial just needs to level out and increase airspeed.
Its not really about output, its about heat, and a combat climb is usually the most trying for radials because the output is high but the airspeed is low.
A climbing comparison might actually let the V engine climb longer at higher output but requiring a longer break to cool down, where the radial engine might not climb as long at higher outputs before it needs a short break to cool down some.
The rate of climb has more to do with the aircraft the engine goes on.
Aside from comparing horses, also compare propeller efficiency and design height.
Some planes just perform better at certain heights despite output.
It might fly faster at X,000 ft, but accelerates, zooms, and turns better at X,000 ft.
Consider drag, wing design, and acceleration and don't forget the octane rating.
Their ratings probably has more to do with octane and ram air efficiency something that was not well understood with either engine when they were first put into service.
If it says anything, the Hawk 75 or Mohawk /P-36 was said to climb with.. and had just a slightly wider turning radius than the Zero up to about 8000ft.
While the P-40 could not climb with the Zero or P-36 it out accelerated them (from 200IAS) in level flight up to 15,000ft.