[sherlock9c1]

This is a weird question, but I wondered if anyone's had experience on this. I just had the muffler replaced on my wife's 99 Chrysler minivan. From the outside, it sounds fine - nice and quiet. From the inside, the cabin drone is unbelievably loud. The only thing I can see differently is that the old oval-style muffler was setup at a 30 degree angle from the floorpan, and the new one is now parallel to the floor pan. Also, they removed the exhaust heat shield as it was loose and rattling on the muffler.

I'm taking it back to the shop today, but any thoughts on this? Is it possible that the parallel muffler is causing cabin resonance that the angled muffler didn't? Did the heat shield help break this up as well?

 

[angelo]

it is 11 years..... the new muff was exact same demensions ?
maybe internals have changed....?

I would expect this from a tail pipe attached to the muff being pointed in a new direction, not the muff itself....?

Curious what you find out !
-ALF out....

 

[sherlock9c1]

Let me clarify - the exhaust shop installed a new muffler in 2008 with a lifetime warranty, so this is most likely a direct replacement for THAT muffler.

The other thing that changed was that when they installed it in 2008, the tailpipe was angled to the right about 30 degrees (and melting the bumper cover, doh!). I specifically asked that the tailpipe be straight this time, and now it's perfectly aligned such that it is pointing straight downward at about a 30 degree angle from horizontal.

 

[angelo]

so i interpret this as 2008 to now you had the same setup and you just noticed the sound change.
my guess is pointing the tail pipe to the ground will cause the sound to bounce on ground and floor pan repeatedly givin' ya the ever so popular DRONE..... 8-\
-ALF out....

 

[crerus75]

I'm not an expert in this field, but I'll give this one a whack. Maybe someone with more experience in vibrations can tell me where I'm wrong.

It's possible that the new muffler has a relatively thin case. If that is true, then it can resonate as the exhaust pulses cause pressure changes in it. This can manifest itself as an annoying buzz or hum inside the passenger compartment, especially if the sheetmetal above it resonates as a result. I've had this happen to me with cheap turbo mufflers.

I suppose the muffler angle could conceivably have something to do with this, as the angle of attack of any resonance or sound waves is now more perpendicular to the floor. I'm not well-versed in NVH (noise, vibration, harshness) so I'm kind of out of my depth on that aspect. The heat shield may have also attenuated some of the vibrations.

First, make sure the muffler shop didn't forget to reinstall any of the rubber exhaust hangers. Also make sure that when they installed everything, they didn't misroute a pipe. A pipe rubbing on some sheetmetal can make you go crazy after a few minutes.

If you're bored (or desperate), you can temporarily add some mass to the outside of the muffler. Strap a small (non-combustible!) weight onto the top of the muffler with some hose clamps (a 5-lb barbell weight might do it, though some lead sheeting, if you can find it, would be better) and see if the resonance diminishes. Even a few hose clamps wrapped around the muffler might do it if they can keep the sides from flexing. If the resonance diminishes, I would guess that the relatively thin muffler case is at least part of the problem (ever notice how thick the case is on, say, a Hooker Aerochamber?)

If you can replace the heat shield, I would do that as well. I don't think the tailpipe has much to do with the resonance unless it exits underneath the body instead of at or just past the bumper. I found out about this one the hard way just two days ago. The tailpipe broke on my 95 9C1, just behind the muffler. I limped the car home and sawed both pipes off just after the mufflers (Aerochambers). The exhaust now dumps in front of the rear axle. I knew it would be louder, but I cannot believe how much drone there is now. The resonance is enough to split your head. All those sound waves are reverberating against the floorpan just beneath the rear seat. I was going to drive it as-is, but I may have to bite the bullet and get tailpipes.

 

[Slayerdies]

Few things here.
First always replace heat shields on stock setups. They r there for reason and some have padding on the rear to deaden noise and the shield alone will bounce some of the noise off and back towards the ground.
My guess is by changeing the angle of the muffler from 30degress to flat also put the rear exit point and entire muffler closer to the floor pan as well, thus increaseing vibrations inside the van. Always turning the tip down will increase sound and vibrations. Being this is a van I would say the hangers and heat shield will have the most effects as far as in car sound goes.
I had a shop weld the exhaust up too tight once putting constant straina dn pressure on a rubber hanger. Made whole car shake.
A muffler at a slight angel will flow differently as in speed of heat release and water vapors but not enough to affect drone. My experience is that the closer the muffler is to the floor the more drone. Also check the shops muffler. I have seen mufflers look identical but be different inside. Cheaper construction leaving out internal baffles or the padding stuff.
I always buy name brand mufflers and lookup the internal baffling b4 I make a choice.
Over all if the drone was not there b4 and is now I would change something. Check hanger and put the heat shield back first. That dont work get different muffler.
How close is the exhaust tip to the ground? If its like 4inchs or less and pointing down the would create some drone.

 

[sherlock9c1]

Well guys, the shop fixed the problem. It turns out the muffler had shifted while he was welding it and one of the clamps was touching both the muffler and a mounting bracket, bypassing the donut. He fixed it, and now it's nice and quiet.

FWIW, the tailpipe exit is no longer straight; it's cocked about 15 degrees to the right. If it ain't one thing, it's another.