expansion tank

[Whaler]

The radiator cap you choose is rated for so many pounds pressure. That spring therefore is the force the system pressure must overcome to pop off and spray green stuff on the ground at your next carshow. There are 3 common pressure caps. The cap, if and when it pops, will open from within and "vent" thru that small overflow and downspout. If you have a hose on it with a little length, it will at least go to either the catch can you speak of or to the ground in a more orderly and cleaner fashion.
Pressurizing our cooling system and using the 50/50 mix of coolant and water allows us to get the water above boiling point in terms of temperature but not allow boiling. We effectively raise the boiling point. Anything on this side of that open rad cap is non pressurized and likely piddliung into your catch can. If you have one, fine.

I was told by my rad guy that the level of the overflow should be just above the baffle plate. The extra space will allow for raising and falling when heating and cooling. aka "expansion"
This is also your high point in the filled system..allowing filling and venting of entrapped air.

I am throwing a nice length of whatever is handy and looks nice onto that niipple on my car and directing it to the floor...

Tim

here's some info on the newer style idea of a catchcan IE the plastic ones you see now.

cooling

rad caps