Coolant pump impeller R&R

The GMC came with a cast 4.25" water pump impeller. This can be swapped to a replacement water pump. Replacements mostly have a stamped open back impeller of a smaller diameter.

       

 Original 4.25" cast impeller compared to the generic 3.875" impeller

I set the clearance .015" minimum. Using a pump affinity calculator like-

https://en.wikipedia.org/wiki/Affinity_laws


Changing only the impeller diameter makes around a 12% difference in pressure.

              Jacket water pressure at road speed with OEM 4.25" impeller