I bought the Summit 3 inch system for my 1969 Dart 383 4 speed car 3 years ago, it had headers on it at the time. They worked well, I have since replaced the headers with stock exhaust manifolds and I see no noticeable difference with the manifolds Years ago I had a 2.5 inch exhaust system with Turbo muffs and side exhaust exiting in front of the rear wheels on this car, it had a 440 and 727 in it at that time, removing the exhaust made that rascal run a lot better It ran 12.23 at 109. MPH at LACR on a Friday night when it was cool and wet with the exhaust on, I removed the exhaust and the car ran 11.39 at 117. MPH, no other changes Bigger exhaust is way better usually My Duster had 3 inch exhaust, it would pick up .03 ET and .2 MPH in the 1/8 mile with the exhaust removed, it was a full system that had the muffs at the rear bumper, more than likely the weight removed made the differences, not the exhaust restrictions The Dart exhaust system was straight out of the headers into the turbo muffs and then about 30 inches of pipe to the side of the car, not much weight or length of pipes in that system