[scottj]

Negative on lower flow rates (water or air).

It's a closed system. The longer coolant is in the radiator, the longer it's in the block gaining heat.

There is a reason companies like Stewart Components make electric booster pumps that install in the return hose. By increasing pressure behind the mechanical pump they can get higher flow rate without cavitation of the mechanical impeller.

Rate of heat transfer (Q) equals the mass flow rate (M) times a Constant (the specific heat of water) times the Delta T (fluid temp out minus fluid temp in):


Q = M x C x Delta T

The rate of heat transfer is directly proportional to flow rate. If you increase the flow rate, you will then increase the rate of heat transfer.