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Old 10-04-2012, 03:19 PM   #48
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Originally Posted by SeanC View Post
Local pressures is not the issue here. The pressure can drop locally below 1 atm no problem, but it's integral over the entire system can never drop below 1 atm (in a closed system of course, due to conservation). It if drops below 1 atm by x amount across the impeller, it should increase by the same x amount somewhere else.

Regardless, we are not talking about the pressure at the impeller or how much it can drop during operation. Question is why would a 1 bar cap would allow a higher pressure gradient across the impeller than a 2-bar cap? If the cavitation is a concern with a 1-bar cap, it could very well be a concern with a 2-bar cap. In fact, more of a concern with the 2-bar cap than the 1-bar cap. 2-bar cap has a greater tolerance than a 1-bar cap, hence by conservation, could allow even lower pressures to be attained across the impeller.
Edit: I was thinking about it (I misse Sean's earlier post explaining the same realization)- and there's no way the temperatures increase enough to bring the system pressure to 2 bar (let alone 3 bar) if the volume stays constant. In a closed loop (no venting), going from 25C to 95C would increase the pressure by like .2 atm (from 1 atm). So the 2 bar cap must be there mostly to alleviate pressure in the instance that the coolant level rises too high (say when the heater core closes). So I think a 1 bar cap would just make it even safer.

Agreed, if something drops by 1atm at one spot, then it should increase by 1atm somewhere else. But there are properties dependent on absolute pressure. If you have localized regions where the pressure is lower than the vapor pressure of the coolant at a given temperature, you will run into cavitation. At higher system pressures (integrated pressure being overall 3atm or whatever) it is unlikely there would be even localized regions where the pressure drops low enough, since the difference stays constant.

Hypothetically speaking, let's say that at one point in the loop, pressure drops by 0.8atm. If the system pressure overall is 1atm, then that one point will be at 0.2 - quite possibly a coolant mixture would form bubbles at that pressure. If the system pressure was 3atm, then that one point will be at 2.2 - still well above atmospheric pressure.

So the question isn't about a pressure gradient - the cap will not effect the difference in pressures for any given points. But it will effect absolute pressure throughout the whole loop, and that is worth considering.

We know the boiling point isn't an issue. Even running straight water, the bp should be high enough at 2atm to not be an issue at normal operating temperatures. Cars like the 335 are proof, especially since they run similar coolant temperatures. So then we must consider other reasons BMW elected to use a 2 bar cap instead of a 1 or even 1.4 bar cap. Cavitation is a good theory - at red line and a high enough operating temperature, the difference in vapor pressure might just be too close to the pressure where the coolant is flowing the fastest (around the water pump's impeller)

Originally Posted by bryce-o View Post
167 degree coolant temp today with 78 degree ambient
I think your thermostat is broken

Last edited by TerraPhantm; 10-04-2012 at 04:12 PM.
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