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Old 03-09-2013, 01:08 PM
Mikes530 Mikes530 is offline
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Location: Toronto
Join Date: Mar 2011
Posts: 286
Mein Auto: 530
Originally Posted by 540iman View Post
Finally, a ray of light. BMW owners seem (I do too at times) to jump at any perceived superior device than what BMW gave us. The pressure that the cap will hold will make absolutely no difference in how your car performs, how you will survive say a massive radiator leak while stuck in traffic, or in normal driving. You could put a 3 bar cap on with no difference in how the car performs, the temperature the car attains nor will it possibly save you in a failure. Where it may make a difference is if you accidentally over-fill the system to the point where the coolant has no place to expand to. The cap should (if the coolant reaches an excessive pressure) release the "extra" coolant. The tests performed here on this post demonstrate that this will exceed 1.2 bar or 1.4 bar and the longer you can keep your coolant in the closed system the better. Remember the cap is rated in pressure and no relation to temperature. A cooling system full of just water will increase pressure at a lower temperature than one filled with coolant. Cap Has no effect on temperature, only on pressure.
No disagreement (in theory) to any of this. There are questions though:
- why is the industry standard 1.1 - 1.2 bar when the materials and system designs are fundamentally the same as our cars?
- why did BMW specify a 2 bar cap for the e30 and subsequently reduce it to 1.4 bar? I assume that there was a reason and it wasn't done on a whim.

Two possibilities (based more on speculation than anything):
- people could grossly overfill their cooling systems, either because they didn't know any better or because their indicator rod was stuck (seems to happen quite often). This will result in a higher system pressure and a 2 bar cap will prevent venting. Venting is bad for environmental and/or owner perception reasons. If you don't think that owner perception is huge at BMW refer to the (useless) buffered temp gauge and "lifetime" tranny fluid.
- if someone was to check their coolant level in very cold ambient temperatures, the coolant level would appear low. Topping up would again result in an overfill situation compared with the level in a more normal ambient temp.

The problem with preventing venting when it might be appropriate is that something's gotta give.....
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