Why Does Radiator Cap Pressure Matter?

A change in pressure of 1 psi results in a 3-degree change in temperature” says Cochran, “If your engine calls for a 15 PSI cap, but the one that is actually on the radiator is only holding to 7 PSI, you can see engine temperatures 27 degrees higher than you should.” The opposite is also true, as water boils at 212-degrees at sea level, but at 6200 ft, it boils at 200, this is the difference in pressure. While a higher-pressure cap will push the boiling point higher, the pressure inside the system can reach unsafe levels. Remember, the hoses, pump, and gaskets are under the same pressure and temperature, ballooning the hoses with higher pressures is a recipe for disaster.Another key point on caps- Stay away from the cheap overseas caps, buy a quality US-made cap so you can be sure it will actually vent at the correct pressure.  

This is a common issue for gearheads, especially when swapping transmissions from manual to auto. All automatics need a cooler to maintain the 175-degree optimum running temperature. The best way to do this is to use an internal transmission cooler in the radiator. The reason for this is that transmissions do not like cold temperatures, ATF is much thicker at room temp than at 175, in fact AFT doesn’t thin out to the correct viscosity until it reaches 160 degrees, so if your transmission is running colder than 160, the fluid is putting more strain on the valves and seals. An internal cooler helps bring the ATF temp up faster so that your transmission is running at the optimum temperature.  

External coolers are unregulated, so they will cool as much as possible, often taking the temps too low. If you are experiencing higher transmission temps or tow, then you may need both. An auxiliary cooler mounted after the internal cooler to reduce the fluid temps a bit more before they go into the transmission.