Depends on what you are running. The higher the pressure cap the higher the boiling point. Some radiators won't take much pressure. Use what the manufacturer recommends.
you use the one that the engine is desigh for like my 50 Merc has a 63 Chevy in it the Rad Cap is a 63 Chevy Cap just my 3.5 cents
For every pound pressure cap you use, it boosts the boiling point by 3 degrees. So if you run a 15lb cap, boiling point of the coolant is now 45 deg higher. Water boils at 212 + 45= 257 deg boiling point.
If it doesn't boil over, use the lightest pressure you can. I see no benefit of having a radiator that is capable of running 240 degrees without boiling over if your car only gets up to 210 on a really hot day. The pressure cap has nothing to do with how hot it runs... it only affects the temp at which the car boils over.
So, if water boils at 212*...and I think(?) most cars are running at temps lower than that...why do we run pressurized caps at all?
lower pressure caps, lower boiling points. When coolant/water boils it causes steam pockets and steam is what causes cracks.
My Griffin aluminum radiator came with a 14 # cap, my roadster never runs over 210 degrees, I've thought about putting on a 7 # cap but I'm from the school of "if it ain't broke, don't fix it ".
Because it's not a perfect world. Older cars were usually run with about 180* thermostats........but, in the real world, they sometimes gotter hotter than intended. With the higher boiling points derived from pressure caps, even if they exceeded the 212* boiling point, they didn't boil over, saving a (potentially) disasterous loss of coolant and a major pain to the driver. Later models, beginning with the advent of emission controls, were routinely run even hotter , 195* to 205* were fairly common, and they obviously required even higher pressure caps to gain the same benefit of boilover protection. Ray
If you usually run 180-195 on the road and are trying to get across the Chesapeake bay bridge tonight it will definitely run up above the boiling point. It's 104 out there today and the normal beach traffic. With a pressure cap the coolant stays put in the radiator until you get back up to speed and things return to normal temps.
My old 48 Chrysler had no pressurized cooling system...the temp gauge read cold at 0 degrees and hot at 212 degrees.....I could never drive during the summer months without overheating....no wonder all of the older cars were so unreliable...none of them had pressurized systems....my guess is that the automakers used a pressurized system sometime in the early 1950s....they were only 4 or 7 pound systems, but they made a huge difference in reliability.
There are also two different types of caps. One is designed for a "closed system" where you are using a recovery tank. It allows coolant to escape into the tank but allows it to come back in when the system cools down. The other type is "vented" and only allows coolant to go out but not back in, so if you put that kind on a closed system it will not work right. Most hot rods have a closed system because we don't like our rods puking coolant all over the place when we stop somewhere. All coolants wil expand and seep some out the overflow, so if you have a recovery tank it traps and stores the coolant then returns it to the system later on. Just be sure you buy the correct type of cap when you shop for one. Don
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