N/A 2.5 is ideal. If you are going turbo then 3.. If you have cats. It will be fairly quiet. No cats gets noisy. Specially on 3 inch. No mater the muffler.I have a couple of cars with Greddy SP systems in non turbo for now and no cats. They are fairly noisy idle and specially at high rpm. Did not see power changes from 2.5 to 3 in N/A.