radiator caps, cooling system pressure

"Ashton Crusher" > wrote in message
news
>I was thinking of putting a higher pressure cap on one of my cars to
> increase the factor of safety against boiling. Looking thru the web
> for info on the likelihood of changing from 7 psi to 13 psi causing
> leaks I found little on that issue but did find a couple references to
> the pressures created by the water pump. One site boasts of a 19 PSI,
> $25 cap to get you thru your "hard driving".
> http://www.mishimoto.com/mishimoto-h...ap-13-bar.html
> Thought I'd see if anyone else has heard of this. The claim was that
> the water pump could create over 30 PSI of pressure. Since that is
> double the normal operating pressure of most modern cars I find it
> hard to believe. If the system was at full 15 psi of pressure while
> the car is idling and then your floored it and ran it up to near
> redline and created another 30psi of additional pump pressure, or
> even 10 psi of additional pressure downstream at the radiator cap, you
> would immediately cause the system to have to vent to the overflow to
> relieve this higher pressure. I've never seen a car vent due to me
> revving the engine up while I'm working on it. Thoughts?????

Lots of comments--my two cents: First of all, the water pump is a
centrifugal pump and as such the pressure rise across the pump is a function
of it's speed. The flow output of the pump is a function of the system
characteristics-- Pressure drop vs Flow So changing pressure caps and
thermostats will not change the operating behavior of the pump. The purpose
of the pressure cap is to raise the boiling point of the fluid. This allows
you to run the coolant at higher temperatures (greater than 212F) without
causing it to boil. Just because you have a 180 deg thermostat doesn't mean
that the coolant at running at 180 deg-----because of the cap, typically,
the coolant boiling point is closer to 234 deg F (or higher). That's why
they say never to remove the cap until the coolant cools down--if you remove
the cap too soon, the system pressure becomes 15 psia (atmospheric). the
boiling point becomes 212F and you get a great big flash of steam as the
coolant immediately changes from a liquid to a vapor. As the coolant
temperature increases, it's density (Specific Gravity) decreases and because
it's weight remains constant, the only other variable is it's volume which
increases. Because the cooling system volume is fixed the coolant pressure
will increase (no room to expand) from atmospheric to whatever the cap is
set for and then it is vented to the over-flow bottle.
The downside of increasing the pressure cap setting over what's specified is
two fold---At the higher cap setting, all the system components will be
subjected to pressures beyond design intent--not a good idea. The same
applies to the operating temperature. In summer like days the coolant
temperature will go beyond normal design intent. In both cases, you're
moving in a direction to accelerate component failures.
MLD