I just completed an mbrp 4" turbo/back minus the cat this weekend. Im glad I did not just for the sound but taking the cat off exposed a problem. After the install I noticed some puffs of WHITE smoke when taking off from stop lights. After checking the usuall like level in the degas bottle (which was good). I called a serv. tech from Ford and explained what was happening and he said looks like my turbo is going out. So, what im getting at is the cat was stopping the smoke before it exited the tip.