Technical Water boiling

X-cpe · Dec 6, 2025 at 7:37 AM

Bandit Billy said: ↑

So, a 4-pound cap makes the system 12 degrees cooler compared to running at ambient pressure (or a cap that has worn out seals).
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Allows it to run 12 degrees hotter without boiling.

firstinsteele · Dec 6, 2025 at 7:42 AM

Bandit Billy said: ↑

Lots of reasons.
Wrong cap is one. A lot of older cars require a deeper cap. Below is a comparison of a new vs older model. If you use the new cap on an old radiator, it will fit but inside the seal won't hit the throat in the radiator, so it won't build pressure. For every pound of pressure (over atmospheric pressure), the boiling point is increased by 3 degrees. So, a 4-pound cap makes the system 12 degrees cooler compared to running at ambient pressure (or a cap that has worn out seals). New cars have 17 pound caps or more, but they're radiators/heater cores/hoses and the like are designed for that kind of pressure. If you put a 17 pound cap on a flathead (for example), you will be replacing the radiator, the heater core, and likely the water pumps very soon.

BB, my understand is the 4 lb pressure system will allow the system to run 12 degrees hotter before boiling. How does the pressure make it cooler?

Ben

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Technical Water boiling

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