Cool under pressure: The best coolants for your classic

Unless you own a Volkswagen, Corvair or other air-cooled classic, at some point you’ll need to consider the health and type of the coolant in your radiator. This used to be fairly straightforward, but today there are many more choices and considerations.

The engine releases about one-third of its heat through the cooling system, and the system can be damaged if it isn’t working properly. The coolant carries the heat from the engine to the radiator, where it is dissipated into the air.

Older cars have “open” cooling systems, where the radiator cap doesn’t completely seal the system. This allows evaporation, slowly decreasing the amount of cooling potential unless the coolant was replenished.

Around World War II, automakers began manufacturing cars with a sealed, pressurized system. Not only did this stop evaporation, but for every pound of pressure built into the system, an additional three degrees (F) of maximum coolant temperature was possible. As manufacturing quality improved, higher pressures became more and more prevalent. While a car from the 1950s may have maximum pressures of around 5-7psi, most modern cars can sustain 15psi or more.

While water is always an option for coolant, it’s never been the best one, especially in cold climates because it will freeze. Wood alcohol (methyl alcohol) became the first antifreeze, but its high rate of evaporation and corrosive qualities led to its eventual replacement by ethylene glycol, which became the standard and ubiquitous antifreeze for a long time.

Recently, antifreeze has gotten more complicated. As improved materials and alloys have been introduced, different additives have been used in antifreeze to inhibit corrosion or increase lubricating ability. Coolant manufacturers also started changing the dyes they used in various types of antifreeze to make identification easier. These developments left a key question for owners of classic cars — what’s the best option?

When shopping for antifreeze, begin by reading the label to make sure it’s formulated for older cars. You’ll likely find it to be green, the de facto color for traditional antifreeze. Therefore, if you’re looking down the radiator cap of your classic to figure out what’s in there, you can assume green is good.

The next question is whether the antifreeze in your classic is still working properly. Color is a good starting indicator again. Remember this: green is good, brown is bad. But if it’s green, don’t stop there: Buy an inexpensive antifreeze specific gravity tester and test the antifreeze to determine whether it’s still appropriately diluted for the protection you need. Generally, the best dilution is 50 percent water and 50 percent antifreeze.

What about age and maintenance?  A good flush at five-year intervals will keep your cooling components clean and working at their optimum potential. It’s never a bad idea to put a tag somewhere near your radiator cap with the date of the last flush and antifreeze replacement. You should check your coolant level several times a season to ensure none is leaking.

There are also environmental considerations regarding antifreeze. If your car is old enough to have an open system, it’s good practice to use a product based on propylene glycol instead of ethylene glycol. While it still has some risks, propylene glycol is much less dangerous to animals that might drink it if it is spilled, boils out or is inappropriately disposed of.

And speaking of disposal, dumping old coolant down the drain isn’t the best idea.  The easiest way to dispose of it is to take it to a repair shop with an antifreeze recycling system.