The pump's job is to rotate the fluid as fast as possible around your system. If the fluid is not rotated, your radiators will be unable to remove the heat from the fluid, resulting in astronomical CPU temps, which in turn lead to melt down.

When selecting a pump you are trying to purchase one that will generate the most flow around your system. This is because lower C/W will be the result of higher flow rates.

However at the same time your trying to selecting a pump that doesn't dump large amounts of heat into the loop. This added heat counters any gains from reduced C/W due to higher flow rates.

There are 3 main factors defining the performance of a pump. They are:

1. Pressure
2. Flow rate
3. Heat generated

Pressure

The maximum pressure quoted by the manufacturer is normally measured in cmh2o or PSI. This is a measure of how high a pump can push water vertically at a flow rate of 0. Higher the pressure the more water a pump will beable to pump around a more restrictive water cooling system.

Flow rate.

The maximum flow rate quoted by the manufacturer is normally measured in l/hr or Gpm . This is a measurement of the maximum amount of water a pump can move at a height of 0. The greater the flow, the better.

A pump's performance is then defined by these two factors. Different pumps will display a different combination of these characteristics. Some pumps may generate high flow rates but low pressures. While others may generate low flow rates but high pressures. Which is more desirable will depend on the total back pressure of your system. Here is an example of two pumps and the back pressures of two systems. As you can see the best pump for the job depends on the system it will be used with.

Heat

In the process of generating the flow a large amount of the electricity used by the pump is converted into heat. In most centrifugal PC pumps just about all of this heat generated is lost back into the loop. It is difficult to be exact when estimating the amount of heat being lost into the loop. However I would expect at least 60% of the input wattage to be lost as heat to the loop.

This heat dumped into the loop works against any gains that will be made from reduced C/W's created by the increased flow rates.

This C/W v Heat generated to increase the flow rates gets to a point where increasing flow rates actually increases CPU temps because of this added heat load.