E46Fanatics - View Single Post - [Cooling System Upgrade] 1-BAR Radiator Cap
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Old 10-07-2012, 01:40 PM   #118
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Originally Posted by SeanC View Post
Great, looks like people in the community has too much "street" knowledge, but when it comes down to actually calculating numbers, there is only WDE46, Terra and myself who are doing the work. I will look forward to your proof.

In my calculation of the pressure (without taking into account expansion), I used ideal gas law (PV=nRT), along with the knowledge of maximum operating temperature of 96 degrees Celcius. This gives a pressure of 1.3614 bars as the operating pressure (i.e. 0.3614 cap should suffice). For details and justification, read my posts above.

Now that we have an interval for the expansion (235-325 mL, we still don't know the absolute fill, hence we don't know the absolute expansion), we can start thinking about how this will effect the pressure that was calculated to be 1.3614 bars without taking into account the expansion. I still don't think it will effect it much, unless you're a clueless BMW owner (as most are), and top off your coolant as you would do on a domestic car.

So at this point, it seems to me that BMW is trying to minimize a possible premature ET failure by using a 2 bar cap. However, that's a good reason only for those clueless owners who tend to top things off, and I fail to see the advantages for the rest of us.
lol, your posts are much longer than they need be.

So for your ideal gas law, you are only acknowledging the air within the expansion tank, and your variables are:

P=variable you're solving for
T=coolant temperature

I think we are all in agreement that if you completely top off the expansion tank with coolant, you will get the 2bar relief pressure on the expansion tank cap?

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