I would be more worried about ferrous metals. Aluminium is an interesting one as it's rust typically protects the metal underneath, same for copper, I believe oxidised copper is one of the most corrosion resistant materials there is. In ferrous metals it's tends to be invasive however, rust leads to more rust.

How sure are we that there is no ferrous metals in the coolant circuit? Even the pump shaft, thermostat housing etc.

It's true that water has a higher specific heat capacity, around 12% or so at coolant operating temps. Resulting in 12% less flow rate, less wear on the pump etc. Even with the higher boiling point of glycols water is still better. Water does expand more and it vapourizes more easily but that would only be an issue if you overheated (steam out radiator cap or burst pipping/joints for example).

Glycols are actually corrosion inducers as they form acids in water. So coolant needs more rust inhibitors.

All that said, we can all do our research on the tinterweb but are you going to trust that or are you going to trust the car manufacturers who spend millions researching these things and proscribe a type of coolant for your car?

Sure if you are 100% certain your car will never be idle in temperatures close or below zero, distilled water and a rust inhibitor would be fine. Considering the damage that might be caused should you drive to a friend further north and forget.... Or you live in California and go skiing up the mountains and forget... I don't think it's worth it.

I know in my climate (Ireland) -5*C is about the average coldest winter morning, however in the past 10 years isolated lows of -31*C have been recorded near by and -14*C recorded where I live. So, a quick reference chart tells me I would need at least a 40% solution of glycol with 50% being advisable. Toyota proscribe 50% solution (in Europe anyway) to cater for all the climates in that region. Including driving in the high mountains. 50% freezes at -34*C. If you live in Northern russia I have no idea what they use, except for leaving a fire lit under it or a block heater.

Car manufacturer don't want to deal with idiots saving on coolant / filling with water, and then forgetting it and driving/parking far north in winter. But if it's always warm climate only, and one trusts own memory, it should be safe to drive & cool engine on just water, all year round.
Your mentioned Ireland climate of course is not of such, to drive on water all-year. But eg. California?

In nothern Russia, "far east"/"far north" as they call, in soviet times, they did it in way (for big trucks) - to never stop engine from autumn till end of spring, lol :D.

I looked at the back of my 50-50 Subaru bottle and you're right they do claim a higher boiling point than 100C... At >15psi of pressure (the bottle isn't terribly clear under which conditions unfortunately), which looking up some tables, pure water at 15psi (over atmospheric pressure of course, like in a car radiator system) boils at 121C.


I was never good at material science so I won't go into corrosion hypotheses bullshitting. But Subaru coolant hasn't managed to break common knowledge on cooling fluids and water is incredibly tough to beat without spending a lot of money.