This formula should work only if torque limiter was flat.
My Mondeo has 330 Nm @ 1900 rpm (360 Nm with overboost) and 131 HP at 3800 rpm. Should have 176 HP if calculated, and I spent lots of time just to pass the mark 180 HP.. :?
In stock configuration it injects 62 mg/str at 1800 rpm WOT (330 Nm) and 50,5 mg/str at 3800 rpm (131 HP). So torque at 3800 rpm is a bit less than (330/62*50,5) 269 Nm, let's say 260 Nm because of decreased efficiency at high rpm (more friction, higher pumping losses etc). Then we get 260*3800/7120 = 138,8 HP.
Anyway, the constant 7120 is just a shortcut, modified to fit one's needs. It comes from that:
power [W] = torque [Nm] * angular velocity [radian/s]
That is a correct formula, well known from physics lessons. :thumbup: