Where was the hp measured from on WW2 a/c engines?

1. prop shaft
2. crankshaft

Logically, it should be measured at the engine output, which may or may not be the prop shaft or crankshaft, since the major engines in use had built in gearboxes. Some, like the P-39, for instance, had remote gearboxes at the propeller, so the engine output would not be at the prop shaft, while in the P-40, engine output and propeller shaft would be the same. The other question between engines of different nationalities, would be whether & what accessories were used and what the other testing protocols would be.

Unfortunately, a very brief look in a couple of my references didn't turn up a description of the test protocols, but some digging online can probably turn up some USAF reports that describe the test setups.

Douglas

Wouldn't it be measured independent of the aircraft, at the crankshaft?

Yes. But the engine model is often specific to the aircraft so the integral gearbox on the engine could change, which would affect gear losses. Similarly, different aircraft used different supercharger ratios and impellers, which will also affect power output. The dyno brake doesn't necessarily get connected to directly to the crankshaft, it gets connected to the engine output. Which depends on what dash number the engine is.

Douglas

Any difference would only be the power lost in the reduction gear. Unlike cars where power is lost in the transmission (more in automatics). the differential (changing direction 90 degrees loses power) and friction in the axle shafts.

The different supercharger supercharger ratios and impellors would show up in the brake HP ratings of the different engine models.