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Old 10-04-2012, 09:53 PM   #60
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Join Date: Mar 2007
Location: the barnyard
Posts: 2,458
My Ride: '79 Massey Ferguson
For the record, I was referring to 1bar caps being a waste of time. I'm enjoying this discussion.

Originally Posted by SeanC View Post
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.
I agree with this.
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.
I follow what you are saying, but I disagree with your reasoning. The radiator can keep up with the cooling requirement, or all track cars would eventually overheat (thermal runaway or whatever the proper term is). The engine heats up, thus heating the coolant. Then, then thermostat opens more to regulate the temperature at a fixed setting by varying the flow through the radiator. The cap is basically just a safety valve, to prevent parts from blowing apart or failing altogether in the event of an over-pressure situation (overheated engine). If you were for some reason in a situation where the radiator couldn't keep up and the pressure continued to rise, sure a better cap would help a little, but that's pretty much just a band-aid. By your logic, wouldn't a system with no vent would have the highest boiling point, and as such cool the "best?"

I do not think that the M54 operates below 1 bar in all running conditions, and I still don't understand how putting on a lower-pressure cap would help anything. All it will do is cause a coolant leak from cap venting when there is no need for the venting in the first place.
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