The radiator manufacturer will have a maximum number based on how stiff the flat sided tubes are. Too much pressure rounds those tubes and then pulls them loose from the fins when pressure is removed. Also the rounded tubes slow airflow through the radiator.

 

Racing radiators tend to have higher pressure ratings than auto radiators.

 

Most racers run 16 to 22 pound caps. Higher pressure means higher boiling temp for the coolant.  Also more stress on the connections. With a 2" hose and 22 pounds you have just about 70 pounds trying to pull the hose off of the fitting. With silicone hose and safety wired clamps it would not be a factor. That does not include Chinese replica clamps. Just real American made name brand clamps. You can also use real aircraft quality clamps already drilled for safety wire.

 

Many modern cars use no pressure cap. Just a sealing filler cap on a make up bottle.

An air space above the coolant level provides a volume of air to compress to control the pressure. If the moron at the gas station fills that bottle all the way up while reading the message on the bottle not to fill above this line,

the end tank will blow off of the radiator just like on my wife's car.  

 

The 22 pound cap is middle ground, a bit more insurance, not likely to hurt the radiator.

 

I use 22 pounds. Distilled water with 10% glycol and either Redline Water Wetter or 1/2 teaspoon full of dish washing detergent. It is gentle on the hands. Never a problem since 1980.

 

Lynn E. Hanover