simply by definition, Robin

OK. Remember Watt? He said that 33,000 foot pounds of work per minute was equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of that weight into 33,000 foot pounds, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 foot pounds per minute of work, and is the equivalent of one horsepower. If we only move that weight at the rate of 2626 rpm, it's the equivalent of 1/2 horsepower (16,500 foot pounds per minute), and so on. Therefore, the following formula applies for calculating horsepower from a torque measurement:




Horsepower =

Torque * RPM/ 5252

Nore:
Visualize that one pound weight we mentioned, one foot from the fulcrum on its weightless bar. If we rotate that weight for one full revolution against a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two foot circle), and, incidently, we have done 6.2832 foot pounds of work.


Pippin


1966 F100, 410, 625 CFM Carter AFB, XE262, 3-on the tree, 3.50 open.
1968 Torino GT Fastbask, 390 GT, 750 CFM edl, C6, 3.70 open.
1968 Fairlane 500 Convertible, 302 2v autolite, C-4, 2.79 open.