I had one car(1969 Dart 440, 727, 3.91 and slicks) with Hooker 2 inch fenderwell headers to 2.5 turbo muffs and side pipes exiting in front of the rear wheels. It ran 12.23 ET at 109.7 MPH corked up, I remove the muffs and pipes (9 or 12 lbs each side maybe (no other changes) and the car ran 11.39 at 117.8 MPH My Duster has a full three inch exhaust to the 3 inch x 14 long Magnaflows mounted at the rear bumper, that car pick up .04 ET and .3 MPH in the 1/8 mile by removing the exhaust Those wiegh around 25 lbs per side I think I have never ran the car with the exhaust off at a 1/4 mile track, maybe some day Jet for MPH, run it the way it is on the first run(put in one or two new plugs just before that run) and then look at the new plugs at the end of the run, shut it off at the finish line if you can to get a valid read on them. make a jet change the way you think it needs to go and run it again to see if you went the right way on the jetting Go the other way if it slows down on the first jet change, I try to use two Holley jet size changes at first and then go to one size per change whenm I think I'm close Good luck,have fun and let us know your results