Tap water in hard water areas in particular can cause scaling which inhibits heat transfer.
DI water and to a lesser extent distilled can actually cause corrosion in the absence of an inhibitor. That is why you don't want to run straight water without adding any protection in.
If you ran straight tap water, you would still get corrosion. Once glycol is added to the mix, there is corrosion inhibitor's added in with the glycol.
Distilled water is just re-condensed steam. That is as pure as it gets.... vs hard tap /well water at the other extreme. De-ionized water is a chemical process, which is cheaper to implement vs distilled water.
The story I got was de-ionized water is ion depleted, and will attract the ions from surrounding metal surfaces.....and start eating metal....including stainless steel, aluminum and also copper.
1 gal jugs of distilled water are cheap at any grocery store. Distilled water is all that ever goes into a battery. At the telco I worked at for 34 years, we went through dozens of 5 gallon containers of distilled water, used for the strings of 400 lb batteries for the UPS supplies for the office. 24 x 400 lb batteries per string..and up to 12 strings in parallel. If I ever caught anybody using tap water, I'd put a boot up their ass. All 3 Local Ford dealers use distilled water in eng rads and also HE-IC loops. Tap water here is excellent.
Bottled water has additives in it...so the water...'tastes good'.
RP's site depicts a 350 ci eng with a 160 deg T stat. ( 160F imo, is way too low)
50-50 water glycol = 228 F
50-50 water glycol + purple ice = 222 F
100% water = 220 F
100% water + purple ice = 200 F
The above is verbatim as what's on redlines site. IE: both claim a 5-6 deg F redux if purple ice /water wetter added to a 50-50 mix. ( I saw no change, and nobody else does either)..and a massive 20 deg F redux if RP /redline added to 100% water. ( I have never tried 100% distilled water + RP /water wetter.... however I know several who have..and they see typ a 3 deg F redux.
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Ok, I have an OEM 190F T stat on my 2010 GT with Roush M90 blower. Sitting in the drive way, eng on, idling, 75F outside day, I get the following. (eng coolant temps, as read on aeroforce gauges plugged into OBD port)
50-50 distilled water glycol = 194 F. Once temp gets to 204F... low speed fan kicks in and temps drop back down to 194 F, then the cycle repeats.
50-50 distilled water + 1 jug of redline water wetter.
194...rising to 204, then low speed fan kicks in.... drops back to 194 F, cycle repeats. Both my low and high speed fan thresholds can be independently set to anything I want via the SCT-X3 and VMP tune.
Driving around town, low speed fan never comes on, and temps sit around 198 F. Ditto on the hwy. Cyl head temps remained the same at aprx 208-210F My conclusion is water wetter does nothing. I also used a Fluke 62 point + shoot IR thermometer to take additional coolant temps in several places...water wetter did not decrease temps at all...with my 50-50 mix + 204 F low speed fan threshold.
Maybe I should re-run the test with the low speed fan threshold set as high as it will go, or perhaps temp disconnect the fan /pull fan fuse. The repeat..but using 100% water..take some readings..then add RP etc.