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Old 08-02-2013, 03:04 AM   #9
BimmerBum
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Quote:
Originally Posted by spidertri View Post
1st version - plastic impeller, shattered from fatigue. 2nd version - sheet metal impeller, 3rd version - composite (brown color) impeller. Composite is still OE today and was the final fix. Composite is also about 0.5lb lighter than the sheet (and cast) metal which helps extend the bearing life.

Mainly using my eyes to test flow, lol. Cast impeller has rough, smaller sized ports. Unless you are running an overdrive pulley the pump isn't spinning any faster so how could it have the same flow as an OE pump?

I don't understand the immediate thinking that metal MUST be better. Yes, they screwed up on the plastic one but BMW also never made an OE cast version.

You'd probably have to post-machine the casting to get even and smooth ports. It was easier to do the sheet metal as a quick solid fix when all the plastics were exploding. Then back to a lighter, more durable, and most likely less expensive composite version.
You are probably right about the new OE pumps with the composite impeller being better than the 1st generation plastic impeller pumps. That said, both the plastic and composite pumps will likely fail the same way (without warning and by distributing pieces of the impeller into the cooling system while not cooling the engine at all) I still pass on anything but a metal impeller pump.

Between the metal impeller pumps I prefer the cast impeller because the sheet metal impeller looks like it is very low quality. I used to get sheet metal impeller pumps in with bent impellers all the time.

As for flow... have you seen how small the channels on the impeller of a Stewart pump are? I agree the cast pump could probably benefit from some post casting machine work I believe it is more than sufficient and I am not convinced (without real data) that it flows any less than the sheet metal impeller pump.

Bearing life... Maybe the plastic/impeller pump could benefit from a heavier impeller in the hopes that the bearing will fail before the impeller self destructs.

It's all speculation anyway... No matter what pump you have, if it has 50k or more on it you may want to consider ordering yourself another now
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