What made you stick with a perforated core design muffler? 'straight through design' perforated core is 50 year old design...The install looks great though and it does sound very trick butt....
You should try a real "new" muffler, flowmaster outlaw race mufflers. no CORE, thats right a real no core and a "new" design, not simply just a muffler which is called new but still rides off 50 year old tech! a few simple rings surround the lining, or where the muffler is swollen in the middle is all there is on the inside of the outlaw. on the shorty version there are 2 or 3 DB reducing rings and on the regular outlaw there are 5 rings. you could sell your SLP's and get the outlaws and I bet not spend much extra. flowmaster outlaws only reduce the sound from open header 2-3 DBs!
