We are asked this question every day, and it seems that most people misunderstand the role of the upper limit of pressure. The top cover pressure level is independent of the engine coolant temperature and does not cause the engine to run cooler.
Let's heat it with a pot of water. The water is heated to boiling 212 degrees. Now let's take the same thing with a pressure cooker. Water will swell and create pressure. The boiling point of water increases by 3 degrees for every pound of pressure. If the pressure rises by 15 psi, we increase it by 45 degrees. We now increase the boiling point to 257 instead of 212 degrees, and we can cook faster.
The coolant mixture of 50% water and 50% ethylene glycol has a boiling point of 223 degrees. A system with a 15 psi lid will increase 45 degrees with a final boiling point of 268 degrees. The real purpose of pressurization is to provide the driver with a higher operating area under extreme conditions. For example, suppose your classic car usually runs at 180-190 and boiling is not a problem. Then on a hot day, the temperature becomes 200-210. This is not a problem, but the traffic stops and you can't move. Now that the temperature starts to rise to 240, there is no pressurized system and the coolant will boil and spray everywhere. The loss of coolant will eventually fill the radiator with air and the temperature will rise even higher. Keep in mind that the above operating temperatures are independent of the upper limit rating. They are only the conditions for cooling system capacity, ambient air temperature and air flow.
Typically, customers report that their system is boiling at 220 or lower, as described above, which is not possible. What happened was that the system was simply overfilled. When the coolant swells, it has no place to leave and people confuse it with boiling. After the coolant is discharged into the lane and the engine is cooled, some of the coolant will refill the system and continue to circulate over and over again. The simple solution is to stop adding liquids. Once the system introduces excess liquid, it will naturally find the right level.