The ratio of any two drive accessories can be determined by dividing the diameter of the smaller pulley into the larger one. For example, if a 6-inch water-pump pulley is used in conjunction with a 7-inch crank pulley, the pulley ratio is 1.16:1.

So for every 1,000 rpm the crank pulley spins, the water pump would rotate 1,116 times. Since the water pump spins at a faster rate than the crank, it's said to be overdriven. Flop things around, and the water pump would spin slower than the crank and be under driven. Underdrive pulley sets are often used to reduce the parasitic drag on the crankshaft to increase horsepower; however, altering the speed of the accessories can have adverse affects on the performance of the related systems, especially if they are already overburdened. (courtesy of Motortrend magazine)

So should I over drive my pulley or underdrive my pulley with a hi flow FlowKooler waterpump?

Our product is designed to increase flow rate so we are not sure why people would want to underdrive the pump to slow coolant unless they stills subscribe to the fallacy of slow coolant for cooling.

If you believe in science and want higher flow you might be tempted to spin the pump faster you will still reduce the cycle time of the fluid through your system and you will build system pressure thereby reducing hot spots and vapor barriers from forming.  No problem in those things however when you take it to the extreme your risk cavitation, hose collapse and will likely shorten the life of your  bearing and/or seal. 

We design the Flowkooler pump impeller to reduce the onset of cavitation but cavitation cannot be eliminated at the highest rpms.  Due to manufacturing advances, hoses collapse far less frequently then when we started the company but we still caution people to be mindful of this problem that is typically observed at higher rpms.  Bearings and seals are mechanical part that have a life; like people; work them harder, they live shorter.

We do not make a recommendation beyond the stock set-up found in your car manual and we cannot predict the myriad of results you may see when you mix and match pulleys or speed up or slow down the rpms of said pulleys.  It is just too many variables and we are not equipped to field such inquiries when you go into "tinker-land."  As such, we do not make a recommendation and we do not diagnose problems you experience.