The Power of Pressure: How Does a New Radiator Cap Keep Your Coolant Cooler?

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Dear ATVC:

I’m unclear on how putting a higher-pressure radiator cap on my ATV can keep the coolant inside from boiling over. Can you please elaborate on how that’s possible?

Absolutely. To begin you must realize that what we consider the boiling point of a fluid is not a constant but rather the temperature at which a liquid begins to boil at a specific atmospheric pressure. This is because boiling point corresponds to the temperature at which the vapor pressure of the liquid equals the surrounding environmental pressure.

Think about that for a moment- if you can increase environmental pressure, you are simultaneously increasing the temperature it takes for the vapor pressure to equalize. In plain English: increase the pressure, you increase the boiling point.

Since your ATV’s (or car’s) coolant system is sealed, the radiator cap is a very simple way to control how many pounds per square inch (PSI) are exerted into the coolant within. This is accomplished by sandwiching a spring between rubber seals. When we screw the cap on then torque down on the outer-lever (connected to the spring), we apply a set amount of pressure into the system. Generally speaking for every pound of pressure exerted upon the water in your radiator/ cooling system, the static boiling point of that water will increase by 3 degrees Fahrenheit.

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