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They are simply conditions of the cooling systems capability, the ambient air temperature, and air flow. Remember, none of the above running temperatures have anything to do with the cap rating. The loss of coolant would eventually fill the radiator with air and the temperature would rise even higher. Now the temperature starts to rise to 240 and without a pressurized system the coolant would boil and spew all over the place. This is not a problem either, but then traffic stops, and you can’t move. Then on a hot day the coolant temperature goes to 200-210. For example, let’s say your classic car normally runs at 180-190 and boiling is not a concern. The real purpose of pressurizing your cooling system is to give the vehicle a higher operating zone in case of extreme conditions.

A cooling system using a 15 psi radiator cap will add 45 degrees to the boiling point for a final boiling point of 268 degrees. A standard coolant mixture of 50 percent water and 50 percent ethylene glycol (such as our own DeWitts High Performance Coolant has a boiling point of 223 degrees. Now let’s talk the same pressure advantage but in an automotive application. We have now increased the boiling point to 257 degrees instead of 212 degrees. If the pressure goes up to 15 psi, then we have added 45 degrees to the water’s boiling point. The boiling point of the water increases three degrees for every 1 psi the pressure goes up. The water will expand and create pressure inside the cooker. Now let's take a pressure cooker and do the same thing. The water heats up and boils at 212 degrees Fahrenheit. Let’s take a pan of water and heat it to a boil.

But before we go any further let’s discuss boiling points and what it means for your cooling system. We highly recommend updating your vehicle to use an expansion tank of some sort if it currently just vents to the ground, such as our universal expansion tank available in several sizes to fit most any engine compartment.

This is the cap’s relief point, at which the pressure exceeds the cap’s designated pressure rating and vents the pressure to atmosphere, either through a drain hose right to the ground (often seen in classic cars) or to a recovery/expansion tank. Depending upon the original application you will see original radiator caps rated for anywhere from 13 psi up to 18 psi. The cap’s pressure rating has nothing to do with regulating your engine’s coolant temperature, nor will it make your engine run cooler. We are asked this question daily and most people misunderstand what the pressure cap does.