Steve said:
I've always been told that pumps make flow, and that a restriction is needed to generate pressure. The Davies Craig pumps are what has been suggested for my application (Mercedes Diesel). If they can flow enough for a stout Duramax, they should be enough for this MB engine.
You are right "kinda" most if not all water pumps for engines are of the centrifugal type, they make pressure by accelerating a mass of water to a high speed and this provides a high flow rate. When a restriction is encountered with a centrifugal pump it does cause pressure but because the impeller on the pump has to have clearance between itself and the housing, any increase in pressure means that amount of slippage between the water and the impeller increases and the actual amount of water that flows out of the pump drops. If you can get the flow vs. pressure curves you will see that increased pressure means reduced flow. Engine driven pumps typically turn at much higher speeds 4-5000 rpm, which means the water leaving the impeller is at a higher speed has more inertia and can sustain higher outlet pressure with out as much slippage (loss of flow) I think that most electric pumps probably don't spin much over 2000 rpm, (correct me if I am wrong here) you might try to double the input voltage which would double the speed, in theory, and maybe sustain higher pressure.
The high flow rate is needed to insure that the water flow through the engine and the radiator is turbulent, the more the better. When Yamaha brought out they killer 750 two stroke road racer in the 70s it had a cooling problem. The fix was Yamaha sent everyone a new set of drive gears for the water pump to increase its' speed and flow rate. Problem solved. High internal water pressure from both the radiator cap pressure and the flow resistance to the water from the pump increase the water boiling temp and the high flow rate reduces or eliminates hot spots around the exhaust valve, r.e. Sparky's experience with his big block. If you use the Davis Craig pumps you need to see what the actual flow rate is when it is connected to your engine and cooling system. I will bet a cup of coffee (retired that's all I can afford!) that is is way under what they rate it for.
Rex
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