Using Fluid mechanics to rate power output of an engine.

I trying to come up with a way to measure the power output of an engine using a water pump, And I wanted some one to go over my math and check for flaws in my thoughts. My idea is to measure the flow rate and the pressure of the output of the water pump.

Using this math to solve for power
Power = Fv
Pressure = F/A
R = Av

Pressure*A = F

Power = Pressure*A*v = Pressure*R