The radiator caps purpose is to maintain a certain pressure in the coolant system. When the cap pressure is exceeded, the cap releases the pressure into the expansion tank. When the coolant cools down forming a negative pressure, the cap allows the flow back into the system.
High pressure caps
The original caps are usually rated to 0.7 - 0.9 bar. Mishimoto caps are rated at 1.3 bar.
The boiling point of water raises by pressure:
0.0 bar: 100,0 C (atmospheric pressure)
0,7 bar: 115,4 C
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