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OK, if radiator-cap pressure is say 20-pounds as opposed to 18-pounds, won't the overall impact on the engine and cooling-system be more heat/pressure stress???

Sure, the boiling-point will be raised with the higher pressure cap....but so will the pressure in the cooling-system. Also, the engine and cooling-system will be exposed to higher temperatures, with the higher pressure cap, even if that coolant is not boiling......that is still heat the engine and cooling-system have to "tolerate"?????

Doesn't the action of the radiator-cap opening, to release hot-coolant, act as a way to not just get rid of pressure, but also some heat......and isn't this earlier "relief-point" easier on your engine and cooling-system?????

Someone help me here!!!!!!!!!!!!!!

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First, you shouldn't put a higher pressure cap on your cooling system than what it's designed for, that way there'll be no issue in the first place.

That said, yes, there will be slightly more stress on the components. The part which will most likely take the brunt are the rubber parts because they are the most flexible. As long as they can take it, there won't be an issue (if all we are talking about is a couple pounds of pressure).

There is something you really aren't taking into account, though, and this has to do with heat. Remember, it isn't the coolant which dictates the heat, the thermostat does. It will be unaffected by more pressure in the system. It will still open when the temperature gets to a certain level and close when it comes back down. More pressure does not necessarily equate to more heat.

The other thing is, under lower pressure, pockets of steam can still form in certain hot spots. With a higher pressure, these pockets will be less likely to form or the pockets will be smaller. This means more coolant is touching more of the block/heads, which means the coolant can do a better job of cooling.

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    +1 most important part (I think) being "use the cap the system is designed for."
    – kyle_engineer
    Aug 1, 2017 at 0:22
  • Interesting, thanks for the clear answer. I thought the pressure in any sealed-systems rises with an increase in temperature, and vice-versa......the two are almost always linked.
    – tickyul
    Aug 1, 2017 at 0:39
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    Paulster is exactly right. The purpose of the pressurized system is to ensure that you always have liquid in the radiator and not steam. The radiator won't give off enough heat is you have steam in there. Normal systems are set to 13psi, but I've seen some vehicles up to 17 psi or so. Note: higher pressures will adversely affect the life expectancy of the aluminum heater core / radiator core. Not a good thing. Use the vehicle the way it was designed. Ensure nothing is blocking the radiator / condenser, all seals are in place to control air flow thru the radiator, and that the fan works.
    – zipzit
    Aug 1, 2017 at 6:34

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