There are a couple of types of impellers in water pumps and one is better that the other. One type looks like a fan that is made of stamped steel much like an alternater cooling fan blades. This design has alot of open area allowing a bit of inefficienty of the pumping action. The next type is an impellar that is cast with a closed back and vanes that radiate either in a curve or perpendicular design. An example can be found on this site: http://www.evanscooling.com/main1.htm then click on coolant pumps (under: Competition & High Performance Applications). This last design forces all the coolant on it's merry way, were as the open design I'm sure you've all seen them, cannot pressure the coolant as efficently. I've descussed the merits with my napa jobber and they agree. The open impellar type is just cheaper to make. I think it's just one thing to consider when fighting the subject of our beloved getting hot.