Studebaker applications (Avanti)
Correct applcation: A atmospheric vented, pressure-type radiator cap between 
12-15 psi.  The shop manual goes on to say that the atmospheric vented cap 
does not cause the cooling system to be sealed at all times; instead, the 
cap seals or closes the system only when coolant vapor flow is sufficient 
to close a valve in the cap; at this time, the cap becomes a full pressure 
cap. The system then remains sealed as heat (pressure) increases until the 
pressure in the system is from 12-15 psi above normal atmospheric pressure, 
at which time the pressure seal in the cap opens and pressure is maintained 
at 12-15 psi.  Excess pressure is relieved through the overflow pipe.

The manual also goes on to say that this type of pressure cap has the 
advantage  of raising the boiling point of the coolant to about 240 degrees 
F. and eliminates the disadvantage of possible hose or tank collapse from 
vacuum when the coolant temperature drops.

As for the thermostat, the manual says that all Studebaker Avantis left 
the factory with a 170-degree thermostat in production models, but that 
160-degree or 180-degree thermostats are available for cars operated in 
cooler or hotter climates.


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7 # cap in Summer?

 if a system cannot hold 12-15 psi, it is not holding 7 psi either.  The 
leak will just be slower and the car will be less tolerant of high load 
conditions that raise the thermal load on the cooling system.  Rarely does 
a cooling system get to 12-15 psi during operation.  If it did, coolant 
would be steadily pushed out of the cap.  An engine running on straight 
water with a 7 lb cap can go to 232F before water will start flowing out 
of the cap; with a 13 psi cap it can go to 245F before it starts to blow 
water.  Since your engine doesn't run at these temperatures normally, 
water isn't pushed out of the cap.  The high pressure cap gives you more 
cooling margin under load or at idle where coolant flow is low.  Cars 
boil over because the cooling system is restricted, the radiator is blocked, 
or the water level  is too low for the pump to move, not because the cap 
is low pressure.  A leaking system may appear to work with a lower pressure 
cap but it is just a bandaid for a cooling system in need of work.  
      The boiling point of ethylene glycol is 387F and it only carries 60% 
of the heat per unit volume of water.  It does not change the pressure 
developed in the cooling system versus temperature.

                                                        chuck
                                                        RQB-3553