Ah! engine stuff, I love this kind of topic. I don't think its so much the metal type the coolant is working on that matters, as opposed to how efficient the coolant is. I guess the best thing to focus on is giving thermodynamics a hand with how fast the coolant can absorb and release energy. I'm pretty sure water is the best coolant you can get, but without antifreeze to raise the boiling point, your water pressure will get too high and can blow off your radiator cap or damage the cooling system. I ran straight water in my old firebird back in the day, and it stayed plenty cool until I let it idle for half an hour in Phoenix summer. Heard a loud pop and then a lot of white smoke. Coolant EVERYWHERE. Then again, it was a crappy radiator cap...