Hey guys, knew to this forum and was wondering if anyone could help by giving some advice. I have Milltek manifold upgrade along side my standard cat and have a piper back box with resonator deleted. . Way prefer the sound of the piper over the Milltek box. But want to buy a Milltek sports cat and have a non red link pipe however their pipe diameter is 69mm where as the piper box is connected to 55mm pipe. I want the sports cat so o can move onto more mods like cams and proper remap. Could I plug in Milltek sorts cat and link pipe then use a pipe converter sleeve to use piper box as it says it will be able to piece pipes together. I just want to know whether it’ll work or if it could lead to problems. I know nothing and have heard rumours of back pressure and stuff going wrong. Thanks in advance

With respect you can make most things fit with converter sleeves, but if this was me I would like to see the whole thing the same bore  from fount to back>For example on my focus St its 3" all the way through  downpipe to back box with a 200 cell sports cat. It could lead to problems in running if you do other mods  later but may be not. A miss match some ware  just may upset the max power you could achieve. Think of it this way some idiots just change the back box to a big bore they get lots off loud noise but the miss match actually reduces  any real power they already had just for that loud sound