Sorry. He basically told me if your motor is stock then 2.25"-2.50" is fine. He didnt exactly answer the "back pressure" thing we've all heard about over the years. He also told me if it were true they would all be using small piping on these race cars that go through his shop. I just got my NV4500 finished and will be heading back to the exhaust guy to put on my Magnaflow's and change my piping a tad cause my Mag's are a little longer. I put exhaust back on "less the tail portion" after the NV4500 just to see the difference for a week til I do the swap. More bassy for sure. Sounds quite a bit more healthy. I prefer a quieter ride for everyday though.