So basically the differences are small enough, that it probably depends more on how the tubing is run and the restrictions of the Y's.

One thing I don't understand is if the flow rate through a radiator is lower, why does that mean less performance? It seems to me that the longer the water is in the radiator, the more opportunity there is to dump heat into the air. Won't slower flow make the temp delta (between incoming and outgoing water) greater ?

]Monty[