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So I have been thinking... I was going to use the stock water pump from my 289, and put it on my 351W when it gets swapped in. Then I realized, that my 351 is probably in the 400hp range, and that water pump was only ever meant to keep a 200hp engine cool (more hp = more heat). Will a stock (re manufactured) pump keep 400hp cool? With this in mind, the water pump merely pumps the coolant, but it is the rad that actually does the heat exchanging. On one hand, I think it should be fine because my cooling ability is limited by the size of my rad, but on the other hand, the stock pump just many not get the job done... Thoughts?

P.s, the rad I have now is 22"x20", dual core aluminum rad out of a 90's chevy pickup (don't laugh) which has plastic tanks.
 

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Stick with what you've got. If you find out it isn't enough when you start driving it, buy something better. I can't tell you at what HP you would need one, but it is probably not 400 HP.

You are probably correct. The size of the radiator matters more at this point. This is an interesting question. I'll have to do the calculation to figure out when a high volume pump matters more than a bigger radiator (or better fan).
 

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Your stock pump should work fine. The coolant needs to stay in the radiator long enough to cool off. Ford flathead V8s had two water pumps and pumped the coolant too fast. My dad changed the pulleys on his 39 Ford to slow them down. As long as you have a large enough radiator you'll keep it cool.
 

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My 351W-based 393 is pushing 400hp to the ground and more like 480 at the crank with a stock water pump, and with a small diameter (4") crank pulley to slow it down even further... Stock clutch-fan/shroud. Cools it just fine! Then again, my ride has a rather large radiator...
 

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For a given amount of heat energy, the temperature rise is inversely proportional to the volume of coolant: T = Q/CV. If your coolant enters the block cool, but exits the block too hot, you need a faster pump.

The amount time your coolant spends in the radiator is t = Radiator Volume/ Pump Rate. The amount of time needed depends on the surface area of your radiator and the heat transfer coefficient. If the coolant re-entering the block is too hot, you need a bigger radiator or lower volume pump.
 
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