I think because of his TT setup, they removed the coolant resevoir, making it in effect a closed system. This makes it very hard (from what little knowledge I have of watercooling PCs) to get the airlock out as the air keeps moving around and you cant "catch" it as the stock coolant tank would. So having this expansion tank, will be the high point of the system and help catch any air in there Dont know how applicaple that is to this application, but I guess it should work.
Blimey, that put it well!!
and a damn site faster than me
But it wasn't catching the air that was the issue, it sbeing able
to top up (slowly, less than a quart per minute - preventing an airlock) from the high point without having to unscrew
2 pieces of hose with hot fluid in as you go through the standard 350z
coolant filling routine.