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Old 03-10-2013, 02:34 PM
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gary@germanautosolutions gary@germanautosolutions is offline
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Location: Grand Rapids, Michigan
Join Date: Jul 2010
Posts: 311
Mein Auto: 528-530 Hybrid, Duc. 748
Gary, we agree on everything, but your statement, as written in #5 is simply not true UNLESS you also include your assumptions! Really not trying to nitpick, I swear! You can't just make a statement like that..It is probably an oversight, but is really misleading as written. You must state what material you are heating to 126 Degrees C to equal 1.2 bar and how much free air space existed. If you want to say that the E39, when filled to the correct COLD level and containing a perfect 50-50 mix of anti-freeze will be at 1.2 bars of pressure if it reaches 126 degrees C then you have a possibly valid statement. I trust your math, but you can't make a statement without specifying the liquid involved and the air gap. Like you pointed-out earlier in a way-air can compress, a liquid can't. Water can only compress if it changes state; like from a liquid to a solid.
Yes, I was making the assumption of a 50/50 mix of water and OEM BMW antifreeze, filled to the correct cold level, and as measured in an m52 or m54 engine. It was a bullet point list and I didn't see the need to get much into the engineering math behind the measurements. But, you are absolutely correct.

The single most compelling reason in my mind for the lower pressure cap is for those that over-fill their reservoir on a regular basis. Then you stress the entire system to 2 bar to let the excess coolant out so the system can return to under 2 bar. If you fill the system full, the pressure in the system will exceed 2 bar and release ONLY enough coolant to get the pressure back to under 2 bar...say 1.9 bar for arguments sake. The next time you start and run your car, the coolant level will still be too high and the pressure will likely not reach 2 bar a second time, but may reach 1.9 bar quickly and stay there, in theory, forever, or until you somehow lose enough coolant to create the proper air space.
Thank you for pointing this out as I forgot to mention it. If you over fill the expansion tank, the system can run at excessive pressures for many drive cycles, not just one.

It is only an advantage for those who A) like the look and B) over-fill their systems regularly.
I disagree with this statement. Due to any number of reasons your vehicle can overheat. Just a few examples would be broken fan belt, bad clutch fan, bad electric fan, clogged radiator fins due to leaves or dirt, etc. In any of these situations you have ample warning via the dash display and temp gauge that something has gone wrong. I would rather the cap limit the max pressure to 1.2 bar than to have something in my coolant system explode, leave me stranded and cost me much more money.

98-104 degrees C iirc
Correct, the m54s run a little hotter at times due to the ECM controlled thermostat which allows hotter temps at part throttle cruise conditions.

Thanks for that! Now if someone would do the same for the I6.
That is what all the data from my post was taken from.

"I'm a little rascal on my Little Rascal"
- unknown
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