This may also help you understand that BHP from a RR can oly ever be a calculated not measured figure and why the torque/power curves cross at 5250rpm on the graph ( given that equal axis are used for bhp torque )
We often hear the term horsepower as a measurement of power. Power is the rate at which work is done. Work is a force applied over a distance. Work lets us do things like lifting loads, moving objects and in general, just making things operate. You can do the same amount of work quickly or slowly. Whether you lift 50 pounds of sand 6 feet high a shovel-full at a time or you grab the entire 50 pound sack and lift it in 2 seconds to the 6 foot height, you are still doing exactly 300 foot-pounds of work. Since power is the rate at which work is done, the more time it takes to do the work the less power is being applied.
When the steam engine began to do the work of horses in the mines during the early 1800's, the mine owners began to ask how many horses an engine would replace. James Watt, who made steam engines, figured out a mathematical way to equate horses to engine power. So, the term horsepower was invented. Watt measured the capability of a big horse to pull a load and found it could pull at 150-pounds while walking at 2.5 miles per hour. This works out to 33,000 foot-pounds per minute, or 550 foot-pounds per second.
A device was then invented to measure an engine's horsepower. It is called a Prony brake. It is attached with a pulley block system and spring balance to the rotating shaft of the engine to measure the output of the engine. The speed of the engine is also recorded and with this information we can calculate horsepower using this equation:
Horsepower = (Force * 2pi * Radius * RPM) / 33,000
This equation can be simplified by dividing 2p into both terms, which will give you:
Horsepower = ( Force * Radius * RPM) / 5,250
Where
Force = the scale reading from the spring balance attached via the pulley block spring balance system to the engine shaft.
Radius = the distance from the center of the engine's shaft to the spring balance arm. It is called the radius or torque arm.
RPM = the engine speed in revolutions per minute.