How your cooling system pressure is managed is a critical component to the overall design” says Cochran, “Open or closed recovery systems change the pressures inside the main cooling system.” The first closed-loop cooling systems for automobiles came out in the late 1930s, but they were not standard until the late 1980s. Even GM has used both systems in the same model with different packages. For example, a 1968 Buick GS had a standard closed system, but that would be an optional accessory on a Skylark. There are different pressure rates for various engines and cooling system types. The most common are 15 PSI for closed systems, and 7-15 PSI for open systems. Some race cars use 20- to 30-lb. caps, but that is not for the street.
An open system vents to a bottle, can, or to the ground (not very environmentally sound!) and uses a single-seal cap. When the coolant pressure exceeds the cap rating, the valve opens, sending coolant into the overflow tank. Closed systems are much more efficient. These use a double-acting cap which purges extra pressure and draws reserve coolant into the system when it is cold. The tank on a closed system is an expansion tank, serving to maintain the coolant level in the system when hot and cold.
