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Old 07-11-2011, 11:07 AM
440Magnum 440Magnum is offline
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I think its mostly about cost (plastic is cheaper) but also about flow. Plastic is generally very smooth and so the impeller doesn't have roughness that can trip the flow turbulent and create cavitation.

Also, the impeller really shouldn't break (yes, I know you have direct evidence to the contrary). Its never subjected to any forces that should break plastic- my wife put >250,000 miles on her old 1993 3.5L v6 with a plastic water pump impeller and we never had that failure mode. That engine has a timing belt driven water pump, so the water pump got replaced every 90,000 miles along with the timing belt just as a precaution, though. In fact the one waterpump "failure" we had was the first aftermarket pump with a steel impeller- the drive pulley sheared off the shaft. After that I went back to plastic impeller pumps and had no worries. I'm wondering if some piece of casting debris went through yours and caused the plastic to break that way.
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