On another tack there is a rule of thumb that the first 1/2 of the Rad does 3/4 of the cooling - is there some truth to that claim? 

George (down under)

Yes, George that one is true. If you think about it for a second I'm sure you will get why. Think delta T. Delta T is the greatest asset to disposing of unwanted heat. The first part of the radiator is where the AIR is the coolest, the temp difference the greatest, and therefore most effective. FWIW George, there has never been anything as effective as pure water for heat transfer. Ed was correct that there is a bunch of transfer that can be done with phase change but a gas in the "water" jacket has shown insulating qualities so the transfer from the metal isn't great. Most Phase change systems require a fairly high pressure environments too. Not really practical.

Bill Jepson