Originally Posted by TQA
Coolant flow from a water pump with an impeller is linked to rpm in a linear manner over it's normal rpm range. IE double the rpm and you double the flow. RAW WATE PUMP
However flow from a centrifugal pump is NOT remotely linear. This type of pump is likely to be used to circulate coolant around the engine. At very low engine RPMs coolant circulation can be impaired.
I test pumps and fluid flowrates for a living. Centrifugal pumps and fans operate with direct proportional flow rate vs rpm.
Decrease a pump by 15% rpm and you get 15% less flow.