i was just wondering when i had my y pipe installed the max seemed to have 3 cats and the y pipe replaced one of them.
I wanted to buy the high flow cat from r/t but which one or both do you replace the one before the y pipe or the one after

It'll replace the main one (behind the y). You still have 2 cats left...

what, still have two? There is two on the Y-pipe (pre cats) and one main cat? No? I know the Cattman unit will leave you with one, which is one too many

One is on the y pipe and the other is right before the y pipe, so the y pipe removes one cat leaving you with 2 (one pre-cat and the main cat).