A double pass radiator requires a lot more pump pressure to flow the same volume as a single pass radiator. The effectiveness of cooling is in the difference in temperature between the cooling air and the hot radiator. If you restrict the flow and allow the outlet of the radiator to get too cool, the efficiency of the radiator is diminished. It was interesting reading the Yellow Bullet thread where in the same paragraph one guy expounded the necessity of slowing the flow of coolant in the radiator while increasing the flow of air. Flow is king on both sides of the radiator, and the hotter you can get the radiator and the cooler you can get the air, the better it will work.