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Tech Tip #2: Radiator Caps

Radiator Caps

In a cooling system, a higher pressure equates to a higher boiling point for the coolant. Higher coolant pressures also transfer heat from the cylinder heads more efficiently. We recommend using a radiator cap with the highest pressure rating that the radiator is designed to accept. In general, performance radiators will accept 22-24 PSI, and professional racing radiators will accept a 29-31 PSI.

The coolant will typically only build to 16-18 PSI, due to expansion up to 200°F. However, if the engine does overheat due to external factors, the pressure inside the cooling system could reach as high as 28 PSI. Once the radiator cap has opened and vented coolant, the engine will not cool down until it has been turned off. The radiator cap is basically a “safety valve”, so always use the highest pressure radiator cap that the radiator will tolerate. If you are unsure of the pressure rating for your radiator, check with the manufacturer for the maximum recommended operating pressure.

Radiator Cap Location

The radiator cap should always be located at the highest point of the cooling system, and on the low pressure side (after the radiator core).Cross flow radiators mounted higher than the engine are ideal because the cap is on the tank that is connected to the water pump inlet. This configuration offers 3 advantages:

  1. The cap is at the highest point of the system, allowing any air to migrate to the area just below the cap. In the event the cap vents due to excessive pressure, the air will escape first.
  2. This area has the lowest velocity within the system, allowing air to separate from coolant even at high engine RPM.
  3. The cap is located on the low pressure (suction) side of the system, so it is unaffected by the pressure generated by the water pump.

For cooling systems NOT using a cross flow radiator, mounted higher than the engine, you must use a surge tank. A surge tank is typically a 1 quart tank mounted at the highest point of the system, with the radiator cap on top. The bottom of the tank is connected to the inlet side of the water pump with a 1/2″ or 3/4″ line. A 1/4″ to 3/8″ “bleed” line from the side of the surge tank is connected to the highest point of the low pressure side of the radiator. The bleed line allows some circulation through the tank while the engine is running. The surge tank is also large enough to allow the air to separate as the coolant flows through it. Air in the system will then migrate to the area just below the radiator cap, again so that it will forced out first if system pressure exceeds the radiator cap’s rating.

In street car applications, an upright radiator (top and bottom tanks, with the cap on the top tank) represents a compromise that will work, as long as the car is not operated at sustained high RPM, like those seen in racing.

Any aftermarket thermostat housing that mounts the radiator cap directly above the thermostat location, or that mount the radiator cap in the top coolant hose, are NOT recommended. Both of those housing styles are poorly designed, and will push coolant out of the cap at high RPM.