I have seen some engines that can push the coolant any place. I have seen others that barley move it. Most cooling pumps are centrifugal type, NOT positive flow. So, the question has a LOT to do with the pump in use and how good it is, and how high the very uppermost part of the loop is.

Without any air at the top of the loop, the falling liquid on the down side helps pull the fluid up the loop and any pump will work. Sort of like how you can siphon the fuel out of your friends gas tank...

With any air trapped in the loop, how high it is matters a lot!

Greg