Reasons to Pressurize the Cooling System.
The coolant mixture is mostly water, and glycol. Mixed together, they have a boiling point below that of water. The normal engine operating temperature is near the un-pressurized boiling point. A slight increase in engine temperature could lead to boiling coolant. This creates a problem as a boiling fluid has a much lower ability to transport heat. It generates vapor bubbles that rise and create hot spots. Thereby the cooling system is unable to remove excess heat. This narrow margin is very apparent with pressure loss. There are a few ways to mitigate this issue.
The engineers could increase the volume of coolant. Combined with an oversized radiator, the thermal capacity would be harder to exceed. Another would be to lower the normal operating temperature. Both of these have serious drawbacks such as weight and inefficient combustion.
A better method is a pressurized cooling system. A fluid under pressure will remain in the liquid form far beyond its normal boiling point. This is an optimal solution but isn’t fail safe. The moment the system loses pressure, if the fluid is above it’s boiling point, it will immediately boil.
As such, the maximum cooling system temperature depends on the pressure. The radiator cap has a pressure release mechanism. It will hold pressure up to a certain amount. If the coolant boils before that temperature, the system is not holding pressure.
Stay Away from the Safety Margin
One way to lower coolant temperature is to use a summer mixture with better thermal characteristics. Water is the best heat conductors. So it is advantageous to favor its use. A summer mixture of 70% water to 30% glycol will also have a higher boiling point. But more importantly, it will transfer the heat away from the source more efficiently.
Once in the radiator, it’s important to dissipate heat as much as possible. This can be achieved with a more aggressive use of the cooling fan. Setting the fan controller to activate the fan at lower temperatures can prevent temperature spikes. The purpose is to provide the engine with colder coolant. The larger the temperature difference, the less residual heat builds up in the engine block.