playmo:No, it works fo any engine anytime. It is simliffing of the original formula which says that power equals torque multiplied by angular velocity of the crankshaft.

So the original is P (W) = T (Nm) * omega (1/s) = T * 2 * pí * rpm / 60

And for recomputing Watts to Hp it needs to be further devided by 1000 and multiplied by 1,36. The result is the same.