Quote:
Originally Posted by Huskymaniac
I thought the expansion causes the pressure when the cap is in place to constrain the expansion. Without the cap I would think the coolant would just freely expand.
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this isn't a simple explanation, as long as your not reving the engine which causes the waterpump to push harder the coolant does not overflow, so at idle with the cap off even up to "normal" operating temp you won't see boiling, which remember happens at 212F and the temp of the thermostat is 180 even on cars that use a 195 temp t-stat if your idling your not going to boil the water. The cap, and pressure is actually in place to increase the normal boiling point, so as mentioned normal boil is 212F when you enclose the system and let the pressure build (builds because as the water gets hotter the air starts separating out) the pressure forces the water not to boil, at 15 psi, typical oem coolant system the temp inside the engine can get to be 225 and still not boil the water. they also make high pressure systems for race cars, 30 to 50 psi F1 cars run at 265F all day long
here is a website that has some information.
http://automotivemileposts.com/garage/v2n15.html