What is the formula to calculate the horsepower of a car's engine? And how do you measure it?

Answer 1

http://www.dsm.org/tools/calchp.htm
http://library.thinkquest.org/4116/Science/horse.htm

The power of an engine may be measured or estimated at several points in the transmission of the power from its generation to its application. A number of names are used for the power developed at various stages in this process, but none is a clear indicator of both the measurement system and definition used.

In general:

Indicated or gross horsepower (theoretical capability of the engine) minus frictional losses within the engine (bearings, rods, etc), equals

Brake or net horsepower (power delivered directly by the engine) minus frictional losses in the transmission (bearings, gears, etc.), equals

Shaft horsepower (power delivered to the driveshaft) minus shaft losses (friction, slip, cavitation, etc), equals

Effective or wheel horsepower