[patrickt]

... and the higher the water to coolant ratio, the better it dissipates heat. But, the freeze point is higher, so be careful if you're in a state that sees a real winter. If you go with straight distilled water, then make sure you add a supplement, such as Water Wetter, to help lubricate & protect the water pump and what not. In fact, I use Water Wetter along with my 50/50 mix of plain old green coolant. And I flush it all out every three years.