Cooling system pressure crept up over the years as manufacturers figured out they could install less-efficient radiators if they ran more pressure. Every pound of pressure on a cooling system raises the boiling point 3 degrees. Modern engines are built with alloys that allow for tighter clearances and higher normal temps, which requires pressurized coolant. Stovebolts operate well at temps from 160-200, so with a big enough radiator, no pressure at all would be needed. Practically, 7 PSI is a good rule of thumb for radiators capable of tolerating a little pressure. That raises the boiling point to 233 degrees at sea level, and more like 215-220 in Denver (5,000 feet elevation) and about 204 on top of Sonora Pass at 10K feet.
The tiny aluminum radiators on newer cars need 15-17 PSI, and normal operating temps of those engines are 220-240 degrees.
Jerry
