I measured my crank pulley which is half an inch smaller than the water pump pulley. Crank pulley 5 inches, water pump pulley 5 3/8inches. This being the case it means the crank has to turn more to turn the pump which could cause a water flow problem. I have been told the water pump pulley should be 10 per cent smaller (not larger) than the crank pulley making the pump turn faster than the crank, but I could be wrong.

 

Underdrives slow down the water pump, which is better at higher engine speed since it allows more time for the coolant to absorb and transfer heat while in the engine/radiator, and can also reduce the risk of cavitation(where the pump churns the coolant and aerates it, creating foam). A high volume may help, or it may not. If the radiator/fan are adequate for the engine, then a HV pump with the underdrives may solve the problem, since it will compensate by flowing more volume at any given speed than a standard volume.

 

Can anyone tell me the width of a stock pulley on the water pump and on a crank pulley.