Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
TR3driver
Great Pumpkin - R.I.P
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The main reason is that the higher pressure provides a higher margin before the coolant boils. Each psi of pressure raises the boiling point about 3 degrees F. But the higher pressure also improves cooling slightly, essentially by improving the contact between the water and engine/radiator.<div class="ubbcode-block"><div class="ubbcode-header">Quote:]My heater valve is closed, so does that mean the heater core doesn't see radiator pressure?[/QUOTE]Unfortunately, the other side of the heater is still connected; the valve only closed one side. So it will still see roughly the same pressure, at least under some conditions.TruCraft said:
I don't know if 10 psi would be a problem for a stock heater core in good condition; it might be OK. I just meant that it is the area that would concern me the most. Many years ago I 'helped' someone with their TR3A ... replacing the radiator cap with one that held the proper 4 psi (his old cap didn't hold pressure at all) was enough to blow the heater core apart!
But come to think of it, one of our local club members accidentally installed a long-reach cap on a short-reach neck (forgot that he had made the same modification I mentioned); and his upper radiator hose blew up first. I don't know for certain that his car had a heater, but I think it did.
