[QUOTE="rv6ejguy, post: 480663 But to say coolant can flow too fast to take in or give up its heat in an engine or rad is simply false and not supported by the physics. Feelings don't make facts.[/QUOTE] So you are saying that if water is flowing through a heat exchanger tube that has cooler air flowing around it, and it is not satisfactorily dissipating enough heat for the system to function properly......that if I keep the water in the heat exchanger twice as long it will not be cooler when it exits the heat exchanger ? Heat exchange is a result of both time of interaction and area of interaction. If the cooling surface area used is minimal....as in many aircraft installations, the time the water is in contact with that cooling surface may need to be maximized. It's basically a matter of surface area vs time of contact. A builder has to have either more surface area or more time in contact with the existing smaller surface area. In both instances I will have applied twice as much cooling air to the water before it returns to the engine. Now that is considering the flow in the heat exchanger independently from the flow in the engine. In real life they are not independent. It simply cannot be said that the amount of time that it takes water to pass thru a radiator has no effect on the amount of heat that will be removed from the water. Edit: Had another thought that might put this in a better context. As long as the contact area of heat transfer has an excess capacity of the heat absorbing medium surrounding it, the speed of the medium (water) is basically inconsequential. If you have marginal or less than sufficient heat transfer ability, the amount of time that heat needs in order to dissipate (generally air) would need to be increased.Since water is a more effective medium for heat transfer than air, it usually has excess capacity and the air side is generally where transferring the heat struggles to keep up. Anyway, at this point I have given my opinion and explained why it makes sense (to me). Don't want to belabor the thread any further with my opinion (right or wrong), so I'll step back and enjoy what others have to say.