Big Pistons Forever:
The Alison engine in the CV580 and L 188 indicate torque in horsepower, which to me seems the most sensible way.
Yes, it's a nicety made possible by the fact it's a single-shaft engine running a single constant RPM. But if RPM varies, the HP varies proportionately. The equation for SHP goes like this, if torque Q is expressed in lb-ft:
SHP = Q x RPM / 5252
Also please note the torque is NOT ft/lb nor in/lb - torque is the PRODUCT of force and radius, not a fraction of the two, regardless of how the gage is marked.
But some torque sensors are hydraulic devices (particularly on large recips, and some helo gearboxes), and the torque is calibrated as some fluid pressure. In that case the gage will read psi or kPa or whatever, and the conversion to true torque will have some constant factor in the equation.