Overheating at idle

There is a LOT that contributes to rad efficiency. Copper tube rads are limited to tubes about 1/2" long because of copper's poor strength. Alum tubes are often 1 or 1.25" long. The extra length allows for much more fin area. Even though alum is not as good as radiating heat as copper, the great increase in fin area & fin design result in a more efficient radiator. The fins are what dissipate the heat...