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Old 10-04-2012, 10:29 PM   #57
SeanC
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Join Date: May 2007
Location: TX
Posts: 4,053
My Ride: 330i
Quote:
Originally Posted by lcoleman View Post
I will agree, but only to the point that I can't refute your logic. Just so I am clear, you believe the overall pressure in the system is somehow determined or limited by the radiator cap itself? You mention that a "2 bar cap would raise the boiling point xx degrees."
Overall pressure is determined not by the cap, it is determined only by temperature of the gas (in other words, temperature of the coolant in the ET at thermal equilibrium). Since fluids are incompressible, the gas pressure is transferred throughout the system. There will, of course, be pressure differences at different parts of the system, but the average pressure is the pressure of the gas.

Cap pressure rating important, say, when you're tracking the car. In situations where you are driving above 4k rpm in 2nd-3rd-4th gears all the time, your radiator cannot keep up with the cooling requirement of your engine alone. So temperature of your engine, hence that of the coolant increases (your gauge doesn't show this, but this is what happens). Temperature increase results in pressure increases. Gas molecules start beating on the cap more and more. But since the cap can take it, overall effect is just an increase in pressure. It is not the cap that increases the boiling point per se, but the pressure of the gas that does so.

A simple example is the following: Pressure cookers cook the food faster by allowing pressure build up inside. This increases the boiling point of the water. Result is your beans cook faster at a temperature higher than 100 degrees.
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