I've been using distilled for years because I assumed there would be less corrosion. I wrote he guys at Royal Purple because I wanted to use their Purple Ice coolant additive and they had this to say:

"Using distilled water will increase the radiator's susceptibility to damage as distilled water is not electrochemically balanced; it is stripped of impurities and therefore ionically hungry, which will cause it to attack soft metals more aggressively. Drinking, spring, or softened are the recommended water types for use in a radiator, particularly in racing applications."

Doesn't make sense to me, what do you guys think?

I would opine that these guys have their head in a dark place, and in my mind it makes all their products and advice suspect.

Antifreeze has is own carefully blended mix of chemicals that function as corrosion inhibitors, and it's best not to introduce any other chemicals. This is best done by blending distilled water 50/50 with antifreeze. Soft, potable water is usually okay with most OEMs, but distilled water is best since it minimizes the introduction of minerals or other "contaminants". Chlorine is especailly bad!

Some racing organizations disallow antifreeze in cooling systems because it is hard to clean up spills. In such cases soft tap water or distilled water with a proper dose of a commercial corrosion inhibitor formula is the correct choice.

Of course, for a normally driven highway automobile, you should run a 50/50 mix of the OE recommended antifreeze and (distilled or soft tap) water. The corrosion inhibtor packages that are part of all commercial antifreezes are as important as their freeze and boilover protection.

Duke

Zerex recommends distilled water with their G-05 (MB approved) coolant.

they probably meant to say "deionized" instead of "distilled". Distilled water is probably ideal, "deionized" is indeed stripped of ions and as such will attempt to electrochemically balance itself by removing metallic ions from what it is in contact with, and if that is the cooling system of your car, it basically doesn't care.

I use distilled water. For an extra dollar or 2 clean water in there won't hurt anything.

__________________
Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record