When they talk about horse power, you see it as HP. What is BHP?

That's Brake Horse Power. I'm not an engineer by trade but I'm in the process of looking this up now...

Whats the difference?

There's also cBhp. Corrected brake horse power. Horsepower is not a measured number anyway. Only torque is measured and horsepower is calculated.

Horse" pow`er (?).

1. The power which a horse exerts.

2. (Mach.) A unit of power, used in stating the power required to drive machinery, and in estimating the capabilities of animals or steam engines and other prime movers for doing work. It is the power required for the performance of work at the rate of 33,000 English units of work per minute; hence, it is the power that must be exerted in lifting 33,000 pounds at the rate of one foot per minute, or 550 pounds at the rate of one foot per second, or 55 pounds at the rate of ten feet per second, etc. &hand; The power of a draught horse, of average strength, working eight hours per day, is about four fifths of a standard horse power. Brake horse power , the net effective power of a prime mover, as a steam engine, water wheel, etc., in horse powers, as shown by a friction brake. See Friction brake, under Friction. -- Indicated horse power, the power exerted in the cylinder of an engine, stated in horse powers, estimated from the diameter and speed of the piston, and the mean effective pressure upon it as shown by an indicator. See Indicator. -- Nominal horse power (Steam Engine), a term still sometimes used in England to express certain proportions of cylinder, but having no value as a standard of measurement.

Horsepower figures are usually measured at the crank. Brake horsepower takes into account any parasitic losses incurred in the "system". Quick example with an automobile engine. The quoted horsepower is usually taken with the motor on a test stand, without the transmission and accessories (A.C., water pump, smog pump, mufflers, etc.) installed. Brake horsepower would be measured at the drive wheels, taking into account all the losses in power incurred by drag. Most cars loose around 30% of the engines power because of parasitic losses. Hope this helps. See ya.

HP can be calculated if enough is known about the variables going on inside an engine but in practise it's done by putting a variable load (Brake) while it's running at full throttle. This brake can be anything from some sort of friction device to spinning a very large water pump or even (in our case) a propellor. All the brake does is to load the engine and then the torque reaction is measured. The torque is then converted to BHP by a simple calculation.

But the point is that when you read that a certain engine has 1.5HP at whatever revs then it really means 1.5BHP.

Like Downunder sez,

The "brake" on our planes is the prop.