I just found this. The tangent force at a 2 foot diameter pulley or the lbs of "pull" at 1 foot radius (or ft-lb) = 5254 (hp)/ RPM... So a 10 hp old engine operating at 150 rpm produces a steady torque of 350 ft lb. Of course, the operating characteristics of engines vary, as you load them, the rpm drops and the torque may keep up to maintain the rpm. This formula is only approximate, based upon pure mathematics.
