I've seen results of using headers with a 2.5 inch exhaust system with a set of the old Thrush Corvair turbo muffs making three runs back to back and then with no other changes removing the exhaust system from the headers back, about five feet long with side exit pipes in front of the rear tires and around 15 lbs. weight per side.
With exhaust and new 10x285x15 slicks inflated to 12.0 lbs. it ran 12.31 at 109.?? MPH (this test was done back in 1993 in November on a Friday night at LACR with a D.A of around 3300 FT) at 5:30 PM, 2nd run was with 11.0 lbs. and it ran 12.27 ET at 109.?? MPH at 7:30 PM with almost the same D.A. a little colder and more humidity 3rd run was at 9:30 PM with 10.0 lbs.in the slicks and it ran 12:23 at 109.?? MPH. LACR had a 10:00 PM curfew so the next runs where done the next morning starting at 10:30 AM with almost exactly the same D.A. with less humidity and a little more air temps. with similar barometer readings. I remove the pipes and muffs and it ran 11.39 at 117.6? MPH with 10.0 lbs. pressure The next run was in the left lane as I was going to run it in the bracket race that day, (I had made all three runs Friday night in the right lane to test the tire pressures) I couldn't really believe that the car had ran the much faster and quicker when I got the time slip, I didn't feel it being that much quicker and faster during that run Same tire pressure with no changes other than running in the left lane, LACR had right at .003 quicker n the right lane compared to the left lane back then, it was 11:30 AM and maybe 80 ft. higher D.A. and it ran 11:42 at 117.6? MPH I made one more TT and it ran 11.39 again in the right lane a t 117.6? MPH before starting racing. I made it to round 3 before breaking out in the left lane by running 11:42 on a 11:45 dial on the brakes at 3:30 PM
I decide to put on a set of race bullet muffs that were 14x3.5 around twenty inches long hooked up directly to the 3.5 Hooker header collectors and it ran 11:52 at 116.?? MPH 40 minutes later, I thought it would run quicker and faster with them acting like collector extensions, it didn't
Theory is one thing and actual results are the truth
I had a complete 3.0 inch exhaust system on my old pump gas Duster exiting at the rear bumper with a set of the older crimp style Magna flows 14x7x3.0 side inlet and with center outlet with a 3.0 inch perforated pipe connecting both ends mounted behind the rear end with the same Hooker fender well 1 7/8 with 3.5 inch collectors headers as was on my old 440 1969 Dart, I tested that system at the local 1/8 mile track at Madras several years back and saw only .030 ET with .2 MPH differences. 7.01 ET with 95.25 MPH with the system on and it ran 6.98 ET at 95.45 MPH, not worth removing it
I'm sure the weight loss (around 45 Lbs. total) was the real reason it ran that much different