Quote Originally Posted by Diverge View Post
Soon as you introduce a Y you're adding resistance, unless the bottom part of that Y, the |, is larger diameter then the 2 branches.
No not exactly, a Y halves the restriction by converting 1 intake path into 2. There is a resistance at the inlet of the Y but as long as that inlet is the same size at whats feeding it it does not add any restriction at all.

Also as long as the restriction of the inlet of the Y is less than that of the combined outputs of the Y the Y will not add any restriction whatsoever.

In otherwords the only time a Y will pose a restriction (in a computer loop) is when turning 2 sources into one. (For instance turning both loops back into one before dumping back into the res) It will never pose a restriction when turning one source into 2 (At the beginning of a loop feeding two sepearate loops) as long as that restriction is greater than the restriction at the inlet of the Y.