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Old 03-10-2013, 09:20 AM
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doru doru is offline
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Mein Auto: 2003 530iA
Originally Posted by 540iman View Post
This is what scares me when someone goes from temperature to pressure. They are in a proportion to one another, but pressure and temperature only have a certain relationship depending on the fluid. Water at 126 degrees C will have a huge difference in pressure than oil at 126 degrees C or a 50-50 mix of coolant, or a 75-25 mix of coolant. Just keep a 1.2 bar cap for what it is. Has nothing to do with temp-only pressure. You make your own assumptions as to what fluid is in the closed system and what amount of air gap exists that the fluid may expand to fill first. Cap will just vent to atmosphere at a lower pressure and that is all. If it does vent and your coolant level decreases, you have less coolant for the radiator to use to cool. Pretend you had a .6 bar cap. You would never over-heat until most of your fluid was all along the road behind you. So, the over-heating would be CAUSED by a cap releasing fluid when the pressure was too low....having NOTHING to do with temperature other than it gettinhot enough to overcome a .6 bar relief valve. Take what you said about a 1.2 bar cap holding the coolant up to 126 degrees. TYour first sentence is already making assumptions you may not have such as absolute mixture. What if your radiator was full of oil? Far fetched sure, but 1.2 cap would hold it just fine.

Actually I don't make ANY assumptions. It's my car, and I service it. I know what coolant I have, and what proportions of coolant/water I have. Maybe you don't know what you have in your car, but I do know what I have in mine.
And just for your info: it's a 50/50 BMW coolant/water mixture.
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