We get asked this question daily and it seems most people misunderstand what the pressure cap does. The cap pressure rating has nothing to do with regulating your engine coolant temperature, nor will it make your engine run cooler.
Let’s take a pan of water and heat it up. The water heats up and boils and 212 degrees. Now let's take a pressure cooker and do the same thing. The water will expand and create pressure. The boiling point of the water increases three degrees for every 1 psi the pressure goes up. If the pressure goes up 15 psi we add 45 degrees. We have now increased the boiling point to 257 instead of 212 degrees and we can cook much faster.
A coolant mixture of 50% water and 50% ethylene glycol has a boiling point of 223 degrees. A system with 15 psi cap will add 45 degrees for a final boiling point of 268 degrees. The real purpose of pressurizing is to give drivers a higher operating zone in case of extreme conditions. For example, let’s say your classic car normally runs at 180-190 and boiling is not a concern. Then on a hot day the temperature goes to 200-210. This is not a problem either but then traffic stops and you can’t move. Now the temperature starts to rise to 240 and without a pressurized system the coolant would boil and spew all over the place. The loss of coolant would eventually fill the radiator with air and the temperature would rise even higher. Remember, none of the above running temperatures have anything to do with the cap rating. They are simply conditions of the cooling systems capability, the ambient air temperature, and air flow.
Often customers will report their system boiling over at temperatures of 220 or lower and as explained above this isn’t possible. What is happening is the system is simply overfilled. When the coolant expands it has no place to go but exit and people confuse this with boiling. After the coolant has exhausted to the driveway and the engine cools down, some will re-fill the system and continue the cycle over and over. The simple solution is to stop adding fluid. Once the system pushes out the excess fluid it will naturally find the proper level.