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Pressure actually increases the boiling point of a fluid as you may know from high school physics class. The pressure literally forces the liquid to remain a liquid longer and does not allow it to transform into vapor. All modern automotive cooling systems are under pressure, completely regulated by the radiator cap. 1.1 bar is roughly 15psi, and 1.3 bar is around 18psi.



How much does the pressure raise the boiling point? Well, it’s about 2-3 degrees for every psi that we increase the pressure of the system. Therefore, by using a 1.1 bar cap we make the average boiling point of a stock cooling system somewhere closer to around 257-260 degrees.



When we change from a a 1.1 bar to 1.3 bar cap, we gain 0.2 bar or roughly 2.9psi of pressure. So, we effectively get 8.7 degrees (or around that) on top of the 257-260 degrees before we might experience boiling coolant in the system.



So if some extra pressure is good, why not a lot? Well, it may seem obvious, but the cooling system on your car is rated to a certain pressure. The radiator cap is designed to be the weak point in your cooling system so it can safely vent pressure, you don’t want to use a cap that is so resistant to venting pressure that it causes some other part of the system to become the weak point.

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