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What are the effects of the PSI rating of a radiator cap?  What PSI is normal for a street vehicle?

ANSWER:

The PSI rating for cap indicates the amount of pressure the cap will hold until it opens to allow fluid to leave the system and enter the overflow.  This "pressure" directly affects the boiling point of the pure water in the system.  Each +1 to PSI will in effect raise the boiling point by approximately 2.5f degrees iirc.  A normal 15 PSI cap would indicate a boiling point of 250 degrees for pure water, and 15-20% higher for a 50/50 mix.

The higher the pressure, the higher the boiling point.  The job of the radiator cap is to increase pressure and prevent fluid from turning to steam at normal temperatures as it passed by hotter areas of the engine like exhaust ports.  Above its rated pressure point, the cap will release pressure through a valve.  Having too much pressure in the system can result in damages.  (cracked head gasket, warped heads)  Not enough pressure, and the vehicle may overheat.


So what PSI Cap should I use?

ANSWER:

The stock indicated pressure should always be used.  If you are going to play around with cap pressure, the goal is to get enough pressure to not boil over, while having low enough pressure to prevent the hoses from blowing off.  Most normal street vehicles use both 15 and 16 PSI caps.
Check your owner manual or contact the dealership for more information on what PSI cap you may need.