As far as I can tell, aviation piston engine power output is typically measured in HP (horsepower).
Automotive engine power output is typically measured by running the host vehicle on a rolling road, or dyno or dynamometer. This directly produces a torque curve, which may be integrated with respect to RPM to provide a power curve.
Obviously it wouldn't make much sense to put a piston-engined aircraft on such a device. Instead, how is horsepower measured for aviation piston engines?