radiator caps, cooling system pressure

"David L. Martel" > wrote in message
...
> Ashton,
>
>
>> No, the car is not overheating. It's never overheated. What happened
>> is that I installed an AC unit in this 60 Dodge. That has made it run
>> about 10 degrees hotter then it used to, mainly from just having the
>> condenser there in front of the radiator. So I was thinking about
>> whether it would be a good idea to go to a 13 pound cap instead of the
>> 7 pound one on it.
>
> I don't follow this. Your radiator is running about 10 deg. F (?) hotter
> with the AC, so what? Why is that a problem? You aren't anywhere near the
> boiling point of your coolant, are you? Assuming you have a 50:50 mixture
> and 7 psi your boiling point is about 255 deg. Changing to a 13 psi cap
> would improve things by raising the boiling point to about 270 deg.
> Unless you are getting close to 250 I don't understand why you want to
> do this. Be sure to check my math. These figures are quick guesstimates.
> Sorry, sounds like a waste of money. Keep an eye on the temp gauge this
> Summer though till you're sure.
>
> Dave M.
>
There is noting wrong with your logic. The coolant is already being ported
into the overflow tank with the OEM pressure cap. Putting in a cap that
cracks at a higher pressure probably won't even change anything with respect
to when the coolant starts to go into the overflow tank. In a closed system
(completely full and cap shut) the coolant pressure increases rapidly as the
fluid starts expanding (without the cap opening the pressure could easily
get up to a 1000 psi). Going from 7 to 15 psi would have a relatively
insignificant change in the way things work. It doesn't change the
operating temperature, just raises the boiling point of the coolant. And
operating at a higher pressure than OEM is moving in a trouble direction.
BTW, one can calculate pressure vs temperature in a closed system--just look
up "Bulk Modulus" and fluid compressibility.
MLD