Craig
I remember reading somewhere that with too much flow, the coolant does not have enough time in the rad to cool down. I think there is a thread on this in forbbodies only.
Having too much flow and not enough radiator to support it will cause the engine to overheat due to the fact that the radiator can't do it's job of absorbing the heat from the coolant to expel into the air. That's why taking your thermostat out completely will actually make your engine run hotter. Standard volume pumps are more than sufficient enough. You just want to be sure your getting the right pump for your application. There are 6 blade and 8 blade pumps. Depending upon if you have A/C or not will decide the blade count for your application. If there is an option for Heavy Duty (HD) then go for that. The HD pumps should not be mistaken for high volume, but instead, offer a heavier front bearing for longer life. Good Luck!
