If your engine overheats three things are gonna blow under the pressure of the water, either the water hoses or the tubes in the radiator will swell or the cap will blow off.
If the cap blows off, or the hoses break, you'll lose the water and have no hope of getting the thing cooled down to get water in it. You'll have to wait it out. Takes hours.
In racing, I use a 30 pound cap because I know the steel braided hoses won't break and the cap won't blow off. I don't mind throwing out a rad. $250 is cheaper than my engine. $250 is cheaper than my entry fee's.
If the tubes swell it's still gonna give me time to get the thing cooled down with a water hose. On the street, I would run a 20 pound cap because I know the hoses won't break at that pressure and the rad tubes won't swell. It give you an extra margin for losing the water. if you blow a head gasket and the pressure goes sky high, you'll at least get a small amount of time to get the engine colled down before the cap blow off.
When you lose the water, the metal temps spike outa control and the hot spot will expand the metal too fast and cause your cracks.
I'd rather have an engine run 300' with a full radiator then a engine at 230 without water.
