HP is a derived number. RPM and torque are measured, and then HP is calculated.
The equation is HP= Torque (lb/ft) X RPM Divided by 5252
As RPM increass Hp goes up dramatically. This is why I believe that the old and slow engines compare so well. High rotational enertia, as the parts are huge, and very high torque ratings at low RPM's. The advantage in the day was extremely long life from low RPM's and surprising effeincy as well due to low speed, extracting more heat energy from the air fuel charge. Gramp's was no dummy, I think....
My Hart Parr 18/36 makes about 45 HP at the belt, but at only 900 RPM. ( they weere actually rate very conservatively, and Nebraska tests point this out) If we could only sqeeze 2,500 PRM out of it you would be surprised at the HP a 500 cubic inch 2 cylinder could make. Alas at 2,500 rpm it would a big pile of scap with connecting rods poking out the bottom end...
Regards, Kirk