3/8" compared to 1/2"

[Onyx1]

What kind of temp differences should be expected between the two?

[Diggrr]

Temp differences would depend on the other components in your system.

Say I had a good running 3/8 system (which I do) and switched to 1/2. If the increased flow caused more backpressure, or carried water through the radiator too fast for it to be effective, my temps might get higher. (could happen especially with a cube style radiator). It could also happen that the increase in water velocity would improve performance via the Reynolds number increasing (more turbulent flow=better heat transfer)

I personally think that the best arguement for running 1/2 inch over 3/8 is pump life. My Danner Supreme is built for 1/2 tubing (as are many magdrive pumps) but running 3/8. Switching to 1/2 inch tubing would lower the load on the pump allowing it to run cooler, thus lasting longer.
If the pump running cooler happened to drop my temps a degree or two, then hey...bonus!