Â…Unless, like me, you're running waterless coolant, in which case the cap needn't hold any pressure at all because the stuff won't boil at any temperature your engine can actually reach. See here, here, and here for discussion and here for the company's website. It is costly but beneficial -- not only in terms of engine-cooling performance, but when there's no water in the cooling system there's no corrosion, and when there's no pressure in the cooling system all the hoses, solder joints, seals, and gaskets are under a great deal less stress. Works well fer me.