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Old 10-04-2012, 03:13 PM   #47
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Join Date: Mar 2009
Location: Walnut Creek, CA
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My Ride: 325Ci, 325Cic, C300
Originally Posted by bryce-o View Post
There's more to it though. The n54 electric water pump is variable speed and runs after the engine is shut off.

Point was the n54 cooling system operates a more traditional 1 bar cooling system.
I agree there's more to it and I doubt we'd ever get to the bottom of the mystery of the M54's 2bar cap. The discussion is healthy though and I thank you for bringing it up.

On the above quote, my point wasn't that the N54's pump doesn't vary in speed, but rather that at full rev the M54 water pump has no choice but to rev with the motor. At a 5.5" crank and a 5.185" stock pump pulley, the motor's 7000 rpm translates to around 7400 at the pump. I suspect BMW upped the pressure to alleviate some of the cavitation similar to an outboard when you give it too much revs.

The N54 water pump may have a variable-speed pump, but I doubt it gets to those high revolutions. As "proof" I submit our own daily drivers. Our water pumps cool our cars just fine at a paltry 750 rpm even in a 120-degree ambient, heat-soaked engine.

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