
Originally Posted by
niky
*hitomi: Aluminum will still corrode if you don't use coolant. Must be a fluke. Another mechanic was complaining of extensive corrosion in an aluminum block because pure water was used. If there is significant corrosion in that Honda's engine, the guy wasn't changing his coolant often enough. Whatever coolant you use, if you don't change it often enough, it can't prevent corrosion.
While water has a higher specific heat capacity, pure water or 70:30 water/coolant mix will boil over more easily than a 50:50 mix. Coolant resists boiling and pressure rise better than water. At the same temperature, pure water will be at a higher pressure, and more likely to be "puked" out at high engine loads and temperatures. Also, there is always the danger of flash points (don't know the exact term) inside the engine, sometimes at the water pump, or at any protruding edge which bubbles can cling to, wherein water is converted instantly to vapor... all of a sudden, you have a cooling system bubbling with hihg-pressure steam, which ain't a good thing.
I think Top Gear or Tech&Tuner Philippines did an article on this, and they found that higher coolant levels gave better cooling than higher water levels. I can't find that article online, though, and it was two years ago.
Some new automotive systems, like on the Focus, use pure coolant, and are touted as completely maintenance free, as no corrosion (theoretically) can occur in the system.