John, I also thought it was odd that Ron Davis would say that, since the "ultimate" Pantera rad. he helped Dennis Q. develop is a single pass. I bought my rad from Ron Davis and when the question of "passes" came up, they completely dismissed any thought I had with regard to a multi-pass rad.
That being said, I believe Jack when he says Ron Davis said a two pass rad would lower coolant temps by 17 degrees. Perhaps he was referring to rows of coolant tubes in the core and said "passes" by mistake.
This is taken from the Stewart Components web site:
The most important criteria for any radiator is it's surface area. The thickness of the core is increased only after the surface area is maximized. Adding thickness to a radiator does not increase it's efficiency the same extent as surface area, but in no case will additional thickness alone decrease the efficiency.
The radiator becomes less efficient as the coolant outlet temperature approaches ambient. Therefore, a low flow rate keeps the coolant in the radiator longer. The longer the coolant stays in the radiator the lower the efficiency of the radiator.
When baffles are inserted in the tanks to force the water to go through the radiator twice, the water spends the same amount of time in the radiator but must go twice the distance.