[68LSS1]

Atmospheric pressure changes with altitude. That's good because I have "denser" air down here at sea level which means more power. The rating of the cap is at what pressure it opens (and that's how many pounds of pressure per square inch the cap is seeing). There is a spring that opens as pressure goes up which allows coolant to go out the nipple on the radiator. There is also a check valve that will open when pressure drops down (which creates a vacuum) and "suck" the coolant back in. Didn't think radiator caps had that many parts did you?