The Ideal Mixture

The coolant mixture you use in your engine should be such that it will offer both enough freezing point protection and enough boiling point protection. It is best to have additional freezing point protection below the coldest weather you expect simply because weather is unpredictable. Finally, it should not be necessary to modify the antifreeze concentration in the coolant for any particular season. The right mix will serve you well in both summer and winter.

Pure water will freeze at 32° F. while pure antifreeze will freeze at -2° F. However, a 50/50 mixture of antifreeze and water becomes frozen at -34° F.! Refer to the Coolant Protection graph for confirmation. The addition of water to the antifreeze enhances the antifreeze's cold weather protection (lowers the freezing point). It also improves the antifreeze's hot weather protection (increases the boiling point) A 50/50 mixture of antifreeze and water will boil at 226° F., whereas pure water boils at 212° F.

Coolant composed of mostly antifreeze or water provides less freezing and boiling protection (that is, higher freezing point and lower boiling point) than does a 50/50 solution. A 50/50 solution offers optimum hot and cold protection as well as maximum rust and corrosion prevention. Even if your vehicle is operated in climates that never reach below 32° F., the freezing point of the coolant should be maintained at -20° F. or lower just to ensure that enough antifreeze is present to prevent corrosion and scaling as well as provide adequate water pump lubrication.

Note that the pressurized environment of the cooling system further increases the high temperature protection (boiling point) even more than indicated above. Most radiator pressure caps are meant to build cooling system pressure to about 15 psi and then maintain this pressure. The boiling point of water is about 246° F. at this pressure (not 212° F.). A 50/50 solution of antifreeze and water would boil at 262° F. under this pressure (not 226° F.). Therefore, a properly pressurized coolant with the optimum concentration of antifreeze (50%) has a boiling point 50° F. higher than pure, unpressurized water. This is why it is important to have an airtight cooling system.

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