Its a simple constant

[Will McGonegal]

> 1 Btu = 778.17 ft-lbf
> 1 min = 60 sec
> 1 hp = 0.70678 Btu/sec
> 1 rev = 2*pi rad

> T [ft-lbf] * N [rev/min] * [1 Btu/778.17 ft-lbf] * 
>   [1 min/60 sec] * [1 hp/{0.70678 Btu/sec}] * [2*pi rad/rev] = 
> T*N/5252 = P [hp]

British thermal unit (BTU)!?! Awkward way of calculating a simple constant: 
2 * PI / (550 [ft lbs / sec / Hp] * 60 [sec / min]). 
(= 1 / 5252.113122.........[Hp min / (rev ft lb)] )

Last year John Faubion submitted an excellent explanation of why James Watt 
chose 550 ft lbs/ sec to be one horsepower.  Do a context search for 
capstan in the diy_efi archives to get the explanation.  This conversion 
topic has been covered (beat to death) before.