Rick's Free Auto Repair Advice

Water versus coolant in your engine — Which does a better job?

There’s more to the water versus coolant debate than just heat transfer

It’s true that water is better at transferring heat than coolant. In fact, at engine operating temperatures around 200°F, plain water wins the water versus coolant competition by removing 17% more heat. But there a lot more to the decision than just heat transfer, and the biggest one is corrosion.

Water corrodes; it’s just that simple

Cooling systems contain many different metals; steel,

View of clogged radiator from the filler neck

This is what water gets you — corrosion

aluminum, manganese, and copper. Any time you combine dissimilar metals with an electrolyte solution, you get galvanic corrosion; a process where a metal that’s more reactive than the other acts like an anode and another as a cathode, causing an electropotential difference.The electropotential difference causes the ion migration from anodic metal into the electrolyte and then to the cathodic metals.

When you use plain water as a coolant, the water acts as the electrolyte. The longer you leave it in the cooling system, the more it will promote galvanic corrosion. The metal transfer eventually causes thinning of the heater core tubes, radiator tubes and deterioration of the water pump.

Coolant contains anti-corrosion additives

All concentrate automotive coolants are 92-95% polyethylene or polypropylene, with the remainder consisting of organic or inorganic corrosion inhibitors, pH balancing additives and anti-foaming agents.

Older anti-corrosive additives like silicates and phosphates deposit onto cooling system surfaces to “passivate” the metals to prevent them from becoming anodic. Silicates and phosphates act quickly buy have short  lives, so carmakers have turned to organic acid technology (OAT) to prevent cooling system corrosion and extend coolant life.

However, some carmakers prefer a hybrid mix to provide quick passivation yet still provide long life.

S-Hoat is a combination of silicates and organic acid

P-Hoat is a combination of phosphates and organic acid

These newer coolants can last as long as 10 years or up to 150,000 miles. During that time, they’re protecting all the cooling system components from corrosion, leaks, and overheating.

Water and racing are a different story

The water versus coolant debate is an entirely different animal if you’re talking about racing. Because most race car owners don’t leave water in their engines when they’re done with the race. Think about, after spending all that money to build a race car, who in their right mind would leave the water in just to watch it corrode the cooling system?

So they fill it with distilled water before the race and drain it after. The race car benefits from the higher heat transfer during the race, but isn’t affected by corrosion afterward because it’s gone.

©, 2022 Rick Muscoplat


Posted on by Rick Muscoplat

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