Years ago I had a set of the early Thrush 2.5 inch inlet and outlet turbo muffs on my 1969 Dart GTS with a set of Hooker BB 2.0 inch fenderwell headers feeding into the muffs under the middle of the car with a set of 2.5 inch tailpipes exiting in front of the rear tires. I was testing tire pressure on a new set of M/T 28x10.5x15 slicks at the old LACR 1/4 mile strip, it rana best of 12.23 at 109.+ MPH with the exhaust on the car, I removed it the next morning and it ran 11:39 at 117.6 MPH with no other changes, the D.A. was within 500 FT, it was lower, of the previous evening
I later did a similar test with a set of the early Magnaflow crimp muffs that had a 3.0 inch in side inlet with a center outlet , those muffs where 14 inch long ovals with 7.0 inch tall. They were on my last pump gas street and strip Duster with the same headers that were on my 1969 Dart GTS it ran a best of 7.01 ET at 93.4 MPH in the 1/8 mile at Madras, OR with the complete system on, those muffs were mounted behind the rear end and the rear bumper. I remove the complete system and that car ran6.96 at 93.8 MPH best the next day, probably due to the less weight
My message is some muffs hurt performance and some don't, bigger diameter pipes are better