I read about this months ago before picking up my 991. As you all know, we have a "sound symposer" valve that picks up engine/intake vibrations and noise and channels them into the cabin via a tube located just under the rear seat headliner.

Here's a quote from SAE:

Anyway, I immediately wondered what the cabin would sound like if I blocked the tube. I love and enjoy the engine/exhaust note, but I couldn't resist toying around with it.

1. Recline the passenger side seat and jump into the extra-roomy rear seat area. The symposer is located just under the rear window, below the plastic cover. You can grab the left side of the plastic panel and gently pull it until you hear it "snap" out. It requires a bit of gentle tugging, but the piece eventually pulls out and swings out toward the right side. I believe the entire panel is removable, but this was quicker/easier for me.



2. Here's what it looks like with the panel pushed to the side.



3. The tube is located toward the mid-right side of the panel, just below the window.



4. Here it is.



I stuffed it with a paper towel and pushed the plastic panel back into place. It requires a bit of pressure, but you eventually hear it snap into place.

Does it make a difference? Yes. The cabin is quieter and you hear less "drone" and intake noise from the engine. It's not a bad idea if you love hearing the pure exhaust note of the car without it being overwhelmed by the symposer.

That said, I don't see a typical 911 driver doing this unless you really, really appreciate a super-quiet cabin without intake noise. I've since removed the paper towel after installing my Cargraphic x-pipe to see if it made a difference. Since the x-pipe increased the cabin noise by about 60,000%, I don't think the paper towel affects the cabin noise anymore.

Regardless, I thought I'd post this up in case anyone is curious about the symposer's function and its location.

Cheers,

Chris

 

Thanks for posting. An alternative is to disconnect and put a plug on the 'symposer' on the other end. It is easily accessible on the upper left part of the engine bay (just need to remove the engine cover). I gave it a try when I was running muffler deletes and taped a yellow lid on it but the exhaust was so loud (and there was so much drone) that it did not make any difference.

 

That's my conclusion, too, as stated above. With the x-pipe, I honestly hear no difference with or without the sound symposer. It's very, very loud.

 

Excellant.. I was wondering how that all was piped into the cabin...

 

Porsche does not have BMW's active noise control though

 

ROFL.

"Not really cricket is it".

Do they sell stock?

 

Thanks for this write up guys! Very good!

 

Serge - where did you find this. You are the master!

 

Hahah, I remember watching that on April Fools last year. Too funny .

 

Chris - if the symposer doesn't make a real difference anymore with the x-pipe, have you then considered a cold air intake? I had been thinking about it a while back, and didn't go for it since it would eliminate the symposer. Wonder if anyone has any experiences with intakes on the 991s?

 

Tom,

The symposer hasn't been a real deal-breaker to begin with for me, so I can't base my decision on the lack of any noticeable difference now. (I hope the actual valve still works! After reading about Serge's problems, I'm starting to second-guess everything on the car )

In terms of aftermarket intakes, I haven't really considered getting one. I don't know if spending the $800 + removal of the rear bumper is worth the small gain in hp/tq. It all depends on how restrictive the stock intake is; I'd love to see real-world before/after dyno tests from a member here.

 

The rear bumper is easily removed: lights come out with removal of a few screws as does the bumper. I asked my tech about after market oil infused or coated air filters and he cautioned against them as they may foul the sensor.

 

Interesting, I hadn't thought about the sensor ... Could catch oil I guess. Just saw the discussion on intakes on Serge's thread also. With previous cars I've had mixed experiences. Had an Injen cold air intake on my e46 m3 and that really made a difference. On earlier cars, I had some bad experiences of losing low end torque ... Need to make sure length supports the right resonance. Looks like the stock air box is somewhat nifty. But then the cup car has a simple big filter....now they don't care as much about low end torque I guess ;-)
Will hold off and see if someone posts some experiences. For now, I have the Bmc filters inside the stock air box and will leave it at that...

 

I took things a bit further to get rid of the Sound Symposer.
My 1st attempt was Plugging the Cabin Inlet with Weather Stripping.

Cut 1' fold it in half and roll it up like pretzel dough.
Feed into cabin inlet and work it through to the engine side.
It expands and provides a good seal.
If I did it again, I would fold it over a loop of string w/ a strip of reinforcing tape, to help with pulling it through.

It made a difference, but I felt like that tube was still transmitting sound to the cabin.

 

Phase 2 was to remove the tube and cap the cabin inlet and the intake outlet.

Silicone Blanking Cap from eBay (seller: kinz-import) did the job.
(Caps had Greddy stamped on them, so they must be good quality)
I added a 1-1/2" Hose Clamp to the Valve Cap, just for good measure.
Ideal cap sizes are 32mm & 28mm (but 30mm & 30mm work too).

Note that the intake sound valve has an arm, that attaches it to the air cleaner.
Make sure it is snapped into its retaining slot, or the valve will flop around loosely.
If it feels tight, it is already done.
I knocked mine loose and had to snap it back in.
You can only do that by feel as it's hard to see down there.

That got it.
The car is real quiet now.
I might even understand why Porsche wanted to make it louder.
However, I highly prefer the quieter cabin, free of piped in intake drone.

If anyone wants a kit, let me know.
I have enough parts for 8 more cars.
The Hose Caps came in a 10 Pack.

If you do the caps, you don't need to do anything inside the car.
You can still use some weatherstrip, inserted from the cabin side of the duct, for additional noise dampening.

If you do the weatherstrip (without the caps), then you need to remove the interior panel.
I found that interior panel a bit stubborn to clip back on.
You have to really work the ends to get it properly clipped back in.
It's easy to think it is clipped, when it is still loose.