There are numerous reference books on this subject. One of the better ones I have read is Engine Cooling Systems by Ray T. Bohacz . He covers theory, design, and performance.
A pressurised system allows the coolant an additional 3 degrees Fahrenheit for each pound of pressure above the boiling point of 212 degrees Fahrenheit ( sorry for the lack of conversion today...long work day already ) . A nonpressurised system will not necessarily run hotter than a pressurised system ; however a nonpressurised system will overheat faster than a pressurised coolant system. Especially when in high altitude driving, as the pressure cap compensates for the reduction of atmospheric pressure. Also the pressurised system at the coolant pump inlet reduces the possibility of cavitation damage to the water pump.
As per reference to the radiator. The radiator has no concept of pressurised or nonpressurised coolant. Its only function is to remove heat from the coolant. Though you are correct in the fact that a pressurised system allows higher temperatures so the radiator will transfer the heat to the air quicker because of the temperature differences. ( which rely upon the First and Second Laws of Thermodynamics ) The design size of the radiator has more to do with the engineered engine power, anticipated road speeds, loads, and additional tasks ( whether it be near an aircon condenser emitting its own heat or has transmission or oil cooling capacity ) rather than the pressure. Which does plays a minor roll in the size of the radiator but the increased pressure forces the design to have to be stronger.
A bit off subject however, in reference to allowing less expensive manufacturing, in the 1970's the Germans and the French began to use an aluminum core radiator with plastic header and collector tanks. This allowed lower weight less expensive materials, and allowed more complex designs to fit in the ever changing face of the automobile.
Bit of automotive trivia. The first pressurised radiator cap was used on a 1939 Buick by John Karmazin from GM's Harrison Radiator Division.
I happened to do quite a bit of research on this subject because when my Esprit is back I will be using Evans NPG+ coolant in both the engine coolant system and the chargecooler system.
Hope this helps...probably will create a large discussion though
Edited by cjtpb13, 09 December 2011 - 02:25 PM.