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Old 10-05-2012, 03:33 PM   #93
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Join Date: Sep 2012
Location: Old Greg's Cavern
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Originally Posted by jfoj View Post
So taking into the fact that for approximately every 1 PSI in pressure increase the boiling point of water is increased by approximately 3 degrees and the fact that most coolant increases the boiling point of the cooling system.

Why in gods name, even at altitude would you need to pressurize the cooling system to close to 30 PSI and increasing the boiling point of the coolant approximately 90F!

The E46 cooling system really operates on a cushion of air inside the expansion tank anyway.

Something just does not make much sense regarding a 2 Bar coolant cap.

I sure would not want my cooling system with all the plastic and rubber parts operating at 30 PSI and over 200F!

Do all the math you want, put a pressure gauge on any E46 and drive it during the Summer months and I would bet you would never be much above 10-12 PSI at its hottest operation point.
I trust my math more than the intuition here. The math says that the system shouldn't experience any more than about 1.4 bar absolute pressure. That means the ET will experience a 0.4 bar maximum of pressure exerted on its inner surface. That is 1.6 bar below the cap's crack pressure. This never approaches 10-12 psi. Unless we are severely miscalculating something, I don't see why we have the 2 bar caps.

The only thing we haven't taken into account with the pressure calculation is water vapor in the ET air and the coolant expansion. The water expands by 3.8% (about 1/3 of a quart) when heated from 20C to 100C, so that could conceivably almost fill the ET depending on its size. It could be a very major factor. I need its dimensions, specifically air volume at min and max. We could find this, by topping off the ET with distilled water, then removing it into a measuring device until the max level, take a reading, then continue to the min. That is the easiest and most accurate way. If it so happens that the air volume is cut in half by the expansion of water, then we will find the system actually runs at around 2 bar. If this is the case, then I would recommend not filling the ET to maximum to reduce system pressure and extend ET life.

The most complex part of all this is the mixtures that are present. We have 50/50 water and antifreeze for our fluid mixture and a mix of water vapor/air/antifreeze vapor in the ET buffer volume.
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