Ron Davis's remark that you reduce cooling temps by 17 degrees with a double pass over a single pass is inaccurate. I have installed, helped install and observed many Pantera cooling systems in hot and humid Texas. In these applications we have run both double pass and single pass radiators. While both work well, back to back testing gives the nod to single pass. In our applications utilyzing by-pass thermostats, both types of radiators will cool adequately at highway speeds to hold temps at the opening temps of the thermostat. If you sensor for your fans trigger them at a temp above your thermostat opening temp, the fans will not engage. Certainly no 17 degree temperature difference there.
The nod goes to single pass on around town driving due to the shorter recovery time for fans to switch off after bringing down coolant temps. The same situation at idle.
The 17 degree difference statement is about as accurate as, another I hear, stating Watter Wetter will reduce coolant temps by 15 degrees. I don't think so.
For more info on employing a bypass thermostat, try this website: http://www.spacecitypanteras.c...Cooling-JTaphorn.pdf The article deserves updating as our latest testing proves that a single degas bottle that is large enough works great; there is no need for a separate expansion tank. This simplifies the desiagn and reduces space utilization in the engine compartment.