Q How much pressure should the cooling system be under?
A Cooling system operating pressures are largely determined by water pump operating pressures and we prefer to keep it under 1 pound. We all know by increasing the system pressure by 1 pound we increase the boiling point by 3 degrees, so by running a 12-pound cap our water won’t boil until it gets to 248 degrees F, and an engine that wants to run at 248 degrees F will open that cap up long before it gets that hot. To deliberately increase the operating pressure to increase cooling is redundant in my opinion and again only points out the need for more efficient heat transfer.
Keep in mind, pressures will increase in the system just after turning off the engine as the coolant absorbs existing engine heat but can’t move through the radiator to dissipate it. The resulting increase in pressure pushes coolant past the cap and hence the need for a coolant recovery system. Once the coolant in the idle engine starts to cool, a vacuum is created and another valve in the cap opens and prevents the radiator from collapsing a top tank but more importantly returns the coolant to the radiator so no outside atmosphere or air (contamination) enters the sealed system. Unfortunately most aftermarket recovery tanks are smaller than the needed capacity and that varies with cubic inches and size of the engine.
Q How is the size of radiator determined?
A There are formulas to determine appropriate radiator size based on engine heat output (operating Btu’s) and radiator heat transfer rates (also stated in Btu’s). They can be found in any engineering handbook but my recommendation to a hobbyist is to put in the most efficient radiator that fits the hole or intended application, up to a four-row copper/brass or two-row aluminum core. I think everyone knows by now that copper/brass units use 1/2-inch tubes while aluminum uses 1-inch tubes. That way the thermostat or lower limit control can maintain the lowest temperature you’ve determined best for all driving conditions.