Rumble, drone, resonance in my 930
08-25-2006, 01:39 AM
#1
Advanced
Thread Starter
Join Date: Jul 2006
Posts: 68
Likes: 0
Received 0 Likes
on
0 Posts
Rumble, drone, resonance in my 930
I have been running down things that need to be fixed on my newly acquired '86 turbo. I have this issue with noise and I haven't found a clear single solution.
Here are the major mods:
K27-2700
Kokeln I.C.
~1 bar of boost
GHL exhaust
911SC cams
During my 2000mi test drive (I had to get it home somehow ) my ear drums started to touch together in the middle. It sounded as if the muffler outlets came into the car. The engine was loud, and when I turn my head it appears to be coming from many directions (in stereo no less). I quickly determined that the installer who put the 8 speaker Blaupunkt stereo in was a loony.
The speakers on each side of the jump seats were installed by cutting through the body into the area behind the fender flairs and putting a plastic cup over the speaker in the wheel well. I fully intend to weld back fitted metal into these holes and return the soundproofing and interior to original. My temporary test was to cut plexi plates to cover the hole in the body and put closed cell foam between the plexi and the grills to deaden the sound. I later added a second piece of plexi on the driver's side. I couldn't tell if this helped as I did it before more significant fixes.
I also reinstalled the engine sound pad which was left out the last time the engine was removed. I used a nice quality one with the reflective covering.
I then purchased a Fabspeed muffler (The workmanship is beautiful) which resonates less than the GHL dual out. The fabspeed is basically a single out with another tip for the wastegate. It puts the sound on the far side of the car from me. (Thanks for the tip Stephen!) The Fabspeed is better (quieter), but the car still isn't comfortable on the highway.
I then discovered that the aforementioned loon cut out the sound pad around the speakers on the rear deck. So the speaker magnets rest on the firewall, carrying the sound back to the passenger compartment. This was confirmed by contorting myself around the rear of the car while it was idling.
I installed dynamat on the bare metal of the firewall/package tray. I put down a layer of carpet pad foam (I have the official sound mat on order), put down a layer of rubber-backed carpet runner to simulate the hard rubber/fiber layers of the factory pad, and then stuffed closed cell foam in the speaker grills where the speakers used to be. (My 8 speaker stereo is now down to 4 speakers, but I can always install it correctly when I figure out this problem).
I then started looking suspiciously at the motor mounts. I replaced the rears (noting that the top washers were installed such that they bridge the gap to the body and were actually shiny around the rim where they had been touching). I corrected the washers, and checked the tranny mounts (yep, washers flipped there to and providing a vibration bridge to the body).
This is significantly better. The car is now louder with the windows down than with the windows up. This wasn't the case on the trip back. I think I'm on the right track here. I do want the growl of the flat six *outside* the car, but I don't want it to be so loud inside that I can't have a conversation at a normal volume. The sound I am still having trouble with is a low drone that peaks at about 2400 rpm. By the way, under boost it's not objectionable at all
I have eliminated the majority of the engine sound in the back seat area, low speeds are now perfect, and higher revs sound good.
What am I missing? What else would carry the resonance into the car through the body shell? I don't really want to go back to a factory euro exhaust unless it's absolutely necessary. I'm concerned that it would kill the performance. Do I need to add sound proofing elsewhere? Rumble is good, drone is bad from my perspective. Does anyone have any insight as to why I am hearing so much sound *through* the car? Should I fix the side speaker holes before I go further? I am thinking of hiring a small person to crawl around in the back of the car while I drive at exactly 55 mph...
Andy
Here are the major mods:
K27-2700
Kokeln I.C.
~1 bar of boost
GHL exhaust
911SC cams
During my 2000mi test drive (I had to get it home somehow ) my ear drums started to touch together in the middle. It sounded as if the muffler outlets came into the car. The engine was loud, and when I turn my head it appears to be coming from many directions (in stereo no less). I quickly determined that the installer who put the 8 speaker Blaupunkt stereo in was a loony.
The speakers on each side of the jump seats were installed by cutting through the body into the area behind the fender flairs and putting a plastic cup over the speaker in the wheel well. I fully intend to weld back fitted metal into these holes and return the soundproofing and interior to original. My temporary test was to cut plexi plates to cover the hole in the body and put closed cell foam between the plexi and the grills to deaden the sound. I later added a second piece of plexi on the driver's side. I couldn't tell if this helped as I did it before more significant fixes.
I also reinstalled the engine sound pad which was left out the last time the engine was removed. I used a nice quality one with the reflective covering.
I then purchased a Fabspeed muffler (The workmanship is beautiful) which resonates less than the GHL dual out. The fabspeed is basically a single out with another tip for the wastegate. It puts the sound on the far side of the car from me. (Thanks for the tip Stephen!) The Fabspeed is better (quieter), but the car still isn't comfortable on the highway.
I then discovered that the aforementioned loon cut out the sound pad around the speakers on the rear deck. So the speaker magnets rest on the firewall, carrying the sound back to the passenger compartment. This was confirmed by contorting myself around the rear of the car while it was idling.
I installed dynamat on the bare metal of the firewall/package tray. I put down a layer of carpet pad foam (I have the official sound mat on order), put down a layer of rubber-backed carpet runner to simulate the hard rubber/fiber layers of the factory pad, and then stuffed closed cell foam in the speaker grills where the speakers used to be. (My 8 speaker stereo is now down to 4 speakers, but I can always install it correctly when I figure out this problem).
I then started looking suspiciously at the motor mounts. I replaced the rears (noting that the top washers were installed such that they bridge the gap to the body and were actually shiny around the rim where they had been touching). I corrected the washers, and checked the tranny mounts (yep, washers flipped there to and providing a vibration bridge to the body).
This is significantly better. The car is now louder with the windows down than with the windows up. This wasn't the case on the trip back. I think I'm on the right track here. I do want the growl of the flat six *outside* the car, but I don't want it to be so loud inside that I can't have a conversation at a normal volume. The sound I am still having trouble with is a low drone that peaks at about 2400 rpm. By the way, under boost it's not objectionable at all
I have eliminated the majority of the engine sound in the back seat area, low speeds are now perfect, and higher revs sound good.
What am I missing? What else would carry the resonance into the car through the body shell? I don't really want to go back to a factory euro exhaust unless it's absolutely necessary. I'm concerned that it would kill the performance. Do I need to add sound proofing elsewhere? Rumble is good, drone is bad from my perspective. Does anyone have any insight as to why I am hearing so much sound *through* the car? Should I fix the side speaker holes before I go further? I am thinking of hiring a small person to crawl around in the back of the car while I drive at exactly 55 mph...
Andy
08-26-2006, 06:23 PM
#2
Instructor
Join Date: Dec 2002
Location: Carnation, Washington
Posts: 174
Likes: 0
Received 0 Likes
on
0 Posts
You could add a Vortex cone in your Fabspeed muffler. I added one in mine(same muffler), there was no noticeable power drop and it did make it a tiny bit quieter. Still not as quiet as I would like just more tolerable. A vortex cone is about $80. I put mine in the exhaust tip end, I welded a tiny lip on the flange so it slides in and then is held in place by the tip clamp, that way it can be removed easily. I do wish someone sold an aftermarket muffler that actually muffled and sounded more like the newer Porsches.
08-28-2006, 11:04 PM
#3
Advanced
Thread Starter
Join Date: Jul 2006
Posts: 68
Likes: 0
Received 0 Likes
on
0 Posts
Hey D, what does the Vortex look like? I didn't see a picture of one online... If I can't stop the resonance inside I guess I could reduce the source of the sound... I wish I could get something in the middle too. Is this proof that I'm getting old?
