Rumble along the highway

[ayglass]

I am new to turbo's but not to 911's. I owned one years ago and worked on them 20+ years ago. I just purchased a nice '86 with some performance 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. I got to work on the problem as soon as I got home. 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 flairs and putting a 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 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, 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 with our host), 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.

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 having trouble with is that low drone that peaks at 1200 rpm and again about 2600 rpm. By the way, under boost it's not objectionable at all

What am I missing? What else would carry too much of the lows 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. Should I be looking at motor mounts? Transmission mounts? 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 almost as much sound *through* the car as behind the car?


Thanks,

Andy

[randywebb]

Replace all the rubber in ther rear suspension.

Be certain the muffler is not a source. Put car in neutral and check the noise level.

Replace the tires with new rubber; replace with less sporty type that has lower NVH.