radiator cap pressure

[mydadsvette]

I'm having trouble understanding the relationship between the radiator cap pressure and it's effect on the boiling point of the coolant. Looking over a chart here, Pressure and Boiling Points of Water , the boiling point of water at 14.69psi is 100degC.. I've seen a 7psi cap - got one somewhere I use for getting the air out.. the chart says 80.4degC for that value. I was under the impression that water in a boiler on a stove boils at 100degC at atmospheric pressure - that this is what the Celsius scale is based on, right? http://didyouknow.org/celsius/
Nevertheless, we all have these 15/16lb caps so there must be something about that chart I'm not getting..
Do you really have to have a 15lb cap just to get your coolant to behave like normal water (neverminding the antifreeze to keep it simple)? What's going on here?


EDIT: I see now that atmospheric pressure at sea level is 14.69psi, so I get it now. :)

[c4c5specialist]

HI there,

When you add the coolant, you change the effective boiling point beyond what water is.

A 15 lb cap will prevent boiling over to around 250, considering a 50/50 mix of water.

Allthebest, c4c5

[markw8]

In a closed heated system, the higher the pressure that can be maintained, the higher the liquid temperature can be without boiling.

Mark

[ChuckG]

Isn't the OEM C4 cap 18-19lbs?

[mydadsvette]

Originally Posted by ChuckG

Isn't the OEM C4 cap 18-19lbs?

I dunno, but last summer dad put a 16lb replacement cap on.. I had to put a radiator in it, and shortly after, a heater core due to leaks. So you have to be prepared for that any time you replace a cap.