Mustang Water Pulley
Jim Smart
January 8, 2016

We’ve seen this one time and time again: water pump pulley ratio, meaning crank pulley and water pump pulley sizing. The ratio in size between the pulleys on the crankshaft and water pump must be what Ford originally designed into the engine. If in doubt, check the Ford Master Parts Catalog and look at pulley size and type for your engine. If you’ve opted for aftermarket pulleys go with the same pulley diameter as stock. Normally the water pump and crank pulley ratio is 1:1, meaning they are the same diameter. You will also see a crank pulley larger than the water pump pulley in some applications.

The reason pulley ratio is so important is pump speed. If you spin the pump too fast you can wind up with pump cavitation or a blown upper radiator hose. When coolant flows too quickly through the engine’s water jackets it can’t carry heat effectively to the radiator. If the fan (which turns the same speed as the water pump, obviously) is turning too fast you run into similar issues with airspeed through the radiator’s cooling fins. Turbulence and boundary layer issues through the radiator limit heat transfer at high speeds. This is when you wind up with overheating that impossible to understand.