I used a set of 3 inch I.D. 14x7 inch oval Magnaflows with the side inlet and center outlet mounted behind the rear end exiting at the rear bumper, it was fairly quiet at part throttle, not so at WOT Those muffs only slowed my car down minimally at the drags, .030 ET and .25 MPH in the 1/8 mile On my other street and strip car with a set of 2.5 inch turbo muffs with side pipes exting in front of the rear axle slowed that car down .8 ET and almost 9 MPH in the 1/4 mile If your main interest is quietness than longer is probally better My main interest was not being slowed down by the ehxaust and then getting busted for open exhaust and getting harrassed by the law on the street , that never happened thankfully I usally did look around carefully before throttling that bad boy to WOT though