Further to the above, the traditional term "horse power" was in fact an amount based on an equation. Taking into account things like number of cylinders, compression ratio etc etc. It was Loosely based on some measure of actual "horse" power. Brake Horse Power is a direct force measurement based on the amount of energy required to "brake" the engine, an instrument called a dynomometer measures the amount of power output at the wheels and as such is considered an accurate measure. When the term "horse power" is used these days, it is usually Brake horse power that they are referring to.