I wanted to add something about the purpose of a radiator cap.
The purpose of a radiator cap is to raise the boiling point of the water/antifreeze mix.
The normal boiling point of water is 212 degrees Fahrenheit (F).
Typical engine operating temperature after the themostat opens (with good air flow) is about 200 degrees F.
You can see that there is only a small difference between the two.
A radiator cap increases the boiling point of water by raising the atmospheric pressure (pressurizing) the system. When the water gets hot, it expands, but the radiator cap keeps the water pressurized by not allowing it to escape from the system untill it reaches the psi rating of the cap.
A 15 psi cap will raise the boiling point of the water by about 45 Degrees F. So now the boiling point of the water in the system becomes 212 F + 45 F = 257 F
So, the lower the psi rating of the cap, the earlier the water will boil, and the engine will overheat at a lower temperature. A lower psi cap will definitely keep the system pressure lower, but in stop and go traffic or high abient temperature, the engine will overheat faster and blow water from the overflow.
1960 Hard Top/430
Thunderbird Registry Number 1231
Last edited by bcomo : 01-27-2009 at 11:10 AM.