2003 LQ9, Entropy and Exp. Tank

[SilverGT3RS]

Trying to figure out if I need to go ahead and run an expansion tank like the factory did on my new Entropy radiator. I went to electric fans, and its a 2003 LQ9 with and new LS radiator from Entropy.

From what I can tell, the cooling system is supposed to run pressurized at the expansion tank, or at least looking at the diagrams it appears thats the way it was originally set up.

The expansion tank keeps the motor from getting an air bubble that could hamper the cooling, correct?

Any help would be appreciated!