Buddy, This is from C&R as they explain it better that I could. Also this is their video on how to set up a pressurized system
http://www.youtube.com/watch?v=jSG37vjU3S4 Its all about optimization -Tony
About Pressurized Water SystemsPressurized water systems are relatively new to the short track and stock car world. This technology has been around in F-1 and Indy Cars for many years. C&R has been involved with the pressurized cooling systems through our Indy Car customers since it’s inception. It’s more efficient and more failsafe than the customary closed cooling system that has been around for the last 80 years. This is the future for cooling systems that will make it’s way into the stock car and short track arenas over the next few years.
Pressure in a cooling system is vital for keeping water in contact with the metal surfaces of the cylinder heads and block. Pressure keeps the air compressed and maintains the water to metal contact that is vital to prevent localized boiling or steam pockets in the combustion chamber areas of the cylinder heads. When a situation occurs that causes temperatures to rise such as lean fuel mixture, too much ignition advance, or a clogged radiator, the air in the water will expand and form small steam pockets. This will start in the combustion chamber area (hottest spot) and the steam pocket will be attached to the metal surface. These spots get very hot and since it’s in the combustion chamber area, it will create a detonation problem. This is why engines lose power when overheating occurs. Once steam starts in the cooling system, the problem will magnify and continue to get worse.
By keeping adequate pressure in the system, the boiling point will be high enough that this overheating situation can be prevented. The higher the pressure the higher the boiling point. For instance, at sea level with a 30 lb. cap the boiling point would be around 265 degrees. That’s why we always recommend a 30 lb cap for racing. With our pressurized system we use an adjustable pressure relief valve instead of a radiator cap. We do this because it will go higher than 30 psi. The other components in this system are an accumulator and a tool to set the pressure in the system. The accumulator is a can, similar to a header tank, that the air bleeds into from the cooling system and this can also has a controlled air space that acts as an air spring. This air spring will compress under temperature expansion and keep water from going out the overflow. It keeps the pressure in the system and is the place where the system pressure is set.
What are the advantages of a pressurized system?With a pressurized system the engine can actually operate at higher temperatures safely. In the case of an Indy Car or F-1 car it means that the air ducts to the radiators can be smaller and thus more aerodynamic. On a Winston Cup car more tape can be applied to the grill opening making it more aerodynamic. There is a bigger safety margin for error such as clogging the radiator or running to lean on fuel mix. You can run leaner for power and not be in the danger zone of overheating. As an example, most F-1 teams will run cooling system pressures as high as 50psi and will race all day with temps at 265 degrees.
