If memory serves me, there are fabricated mufflers with a trap door inside, I thought they were offered by Flowmaster, did not find them at Summit or on the Flowmaster site, but may not have used the correct search term. Bullet/glass-packed mufflers have less than idea flow characteristics and don't last too long, the packing deteriorates and blows out. A "Turbo" muffler will behave a bit differently, but does not flow as well. See the muffler test on Engine Masters Show. Good luck!