Attention: TDR Forum Junkies 
a brief history: my truck had stacks on it when I first got it... I went to 5" straight pipe. the resonance was so bad w/ the under-truck exhaust, it would split your ear drums at 2500rpm. it was bad from 2000 up. I added 1/8" of lead sheet to the back wall of the cab and the floor below the seat and 1/16" on the rest of the floor and the firewall. made it pretty dang comfy. not a new Mercedes sedan, but pretty comfy for a 12v Cummins w/ 5" straight pipe! 
so I got bored and put a set of 5" miter cut stacks on it. the bottom of the miter cut is about 4" or so above the roof line. I've got a single cab BTW...
anyhow, the interior resonance came back to say hello!
starts bangin' on ya pretty good at 2000rpm... peaks at about 2200rpm... it's pretty uncomfortable from 2000-2500etc. IE, it's RIGHT where I cruise on the highway!!! 2200ish!
I ordered a dynomax bullet resonator that arrived Friday, but decided to try some sound deadener on the roof first... I love the sound of an unmuffled straight 6 diesel, so I was kinda sad about needing to run a resonator.
I removed the headliner and found the roof to be a two layer sandwich. the outer sheetmetal is obviously what you see from the outside... the inner follows the contour, but has some ~1/4-3/8" rectangular reliefs that form pockets between the sheets. the perimeter between the pockets seem very close to the roof sheet metal, and I think they might be glued together at the factory.
I drilled a 1/4" hole in each pocket... a couple more here and there to see where stuff was flowing. I shot 7-8 tubes of acrylic caulk into the roof. after about 3/4 of a tube in a pocket there'd be an audible "POP"
that's why I think the two pieces were glued together... anyhow, no visible distortion of the exterior metal. ideally I would have used a denser material, but I had the caulk laying around, and I figured it was better than nothing.
I added a 1/16" layer of lead to the interior roof sheetmetal. I cut it into small pieces to make it easier to work with... roughly 1ft squares... anyhow, I adhered it to the roof with Liquid Nails "Paneling & Molding". I had used Liquid Nails "Heavy Duty Construction & Remodeling" on one of my doors, and it worked well, but the pieces wanted to sag for a little while... even after peeling back the sheet to expose the adhesive to air and resticking. the Paneling glue advertised that it held very strong as soon as it was stuck, and it DID!
The Paneling glue isn't as strong as the Construction, but it's shear strength is listed at 125psi after 24hrs, 250psi after 48hrs, and 325psi after 7 days. at 4lbs/sqft, I've got . 33psi pulling on the glue. maybe a little more due to glue coverage? it's also listed with a -20 to 140*F service temperature range. the roof may get a little toasty during the Dallas summer, but I've got a white truck and like I said, there's a decent gap between a large portion of the inner and outer sheetmetal. time will tell how well it holds, but I'm pretty confident it'll last.
anyhow, on to the good stuff...
well, before, the only way to drown out the resonance at highway speeds was to run the volume on my ~300-400watt stereo at 90-95% volume...
now I can wash it away with about 65-70% volume. may not sound like a big difference, but it feels HUGE! I can cruise fairly comfortably on the highway w/ the radio off... still a little louder inside than it was before the stacks (and really it has more to do with the resonance point of the shorter exhaust vs. the actual volume at the resonance point because I think the resonance at 2500rpm w/ the straight pipe would kill the 2200 resonance w/ the stacks!), but not painfully so like before the sound deadener.
I plan on adding more to the roof... not sure if I'm going to go with another layer of lead or if I'm going to use an asphalt deadener (Dynamat, Brown Bread, GSI, etc. ) I probably would have added more lead, but I'm out, and after Greg scared me about the glue, I'm going to give the stuff some time to cure before I go hanging more weight off of it!
oh yeah, I'm lovin' the stacks... they get lots of looks/smiles/thumbs up, and w/ the AFC spring removed, they REALLY pour out the smoke! :-laf
Forrest
so I got bored and put a set of 5" miter cut stacks on it. the bottom of the miter cut is about 4" or so above the roof line. I've got a single cab BTW...
anyhow, the interior resonance came back to say hello!
starts bangin' on ya pretty good at 2000rpm... peaks at about 2200rpm... it's pretty uncomfortable from 2000-2500etc. IE, it's RIGHT where I cruise on the highway!!! 2200ish! I ordered a dynomax bullet resonator that arrived Friday, but decided to try some sound deadener on the roof first... I love the sound of an unmuffled straight 6 diesel, so I was kinda sad about needing to run a resonator.
I removed the headliner and found the roof to be a two layer sandwich. the outer sheetmetal is obviously what you see from the outside... the inner follows the contour, but has some ~1/4-3/8" rectangular reliefs that form pockets between the sheets. the perimeter between the pockets seem very close to the roof sheet metal, and I think they might be glued together at the factory.
I drilled a 1/4" hole in each pocket... a couple more here and there to see where stuff was flowing. I shot 7-8 tubes of acrylic caulk into the roof. after about 3/4 of a tube in a pocket there'd be an audible "POP"
that's why I think the two pieces were glued together... anyhow, no visible distortion of the exterior metal. ideally I would have used a denser material, but I had the caulk laying around, and I figured it was better than nothing. I added a 1/16" layer of lead to the interior roof sheetmetal. I cut it into small pieces to make it easier to work with... roughly 1ft squares... anyhow, I adhered it to the roof with Liquid Nails "Paneling & Molding". I had used Liquid Nails "Heavy Duty Construction & Remodeling" on one of my doors, and it worked well, but the pieces wanted to sag for a little while... even after peeling back the sheet to expose the adhesive to air and resticking. the Paneling glue advertised that it held very strong as soon as it was stuck, and it DID!
The Paneling glue isn't as strong as the Construction, but it's shear strength is listed at 125psi after 24hrs, 250psi after 48hrs, and 325psi after 7 days. at 4lbs/sqft, I've got . 33psi pulling on the glue. maybe a little more due to glue coverage? it's also listed with a -20 to 140*F service temperature range. the roof may get a little toasty during the Dallas summer, but I've got a white truck and like I said, there's a decent gap between a large portion of the inner and outer sheetmetal. time will tell how well it holds, but I'm pretty confident it'll last.
anyhow, on to the good stuff...
well, before, the only way to drown out the resonance at highway speeds was to run the volume on my ~300-400watt stereo at 90-95% volume...
now I can wash it away with about 65-70% volume. may not sound like a big difference, but it feels HUGE! I can cruise fairly comfortably on the highway w/ the radio off... still a little louder inside than it was before the stacks (and really it has more to do with the resonance point of the shorter exhaust vs. the actual volume at the resonance point because I think the resonance at 2500rpm w/ the straight pipe would kill the 2200 resonance w/ the stacks!), but not painfully so like before the sound deadener.
I plan on adding more to the roof... not sure if I'm going to go with another layer of lead or if I'm going to use an asphalt deadener (Dynamat, Brown Bread, GSI, etc. ) I probably would have added more lead, but I'm out, and after Greg scared me about the glue, I'm going to give the stuff some time to cure before I go hanging more weight off of it!
oh yeah, I'm lovin' the stacks... they get lots of looks/smiles/thumbs up, and w/ the AFC spring removed, they REALLY pour out the smoke! :-laf
Forrest
