from the great wikipedia..
The most-common definition of horsepower for engines is the one originally proposed by James Watt in 1782. Under this system, one horsepower is defined as:
1 hp = 33,000 ft·lbf·min−1 = exactly 745.69987158227022 W
Metric horsepower, as a rule, is defined as 735.49875 W, or roughly 98.6% of mechanical horsepower. This was a minor issue in the days when measurement systems varied widely and engines produced less power, but has become a major sticking point today. Exotic cars from Europe like the McLaren F1 and Bugatti Veyron are often quoted using the wrong definition, and their power output is sometimes even converted twice due to confusion over whether the original "horsepower" number was metric or mechanical.