It's time to overhaul the exhaust system on my 2001 A4 Avant quattro 1.8t.

I've already acquired a full 3" high flow cat, and I know a local place that does quality custom exhaust work for relatively cheap. Now I need to settle on a combination of mufflers, resonators and pipe sizes.

Ultimately, my goal is silence. I want the final result to be as quiet or quieter than stock. A stock-ish look would be nice too (ie. 2 normal-sized tips, no giant outlets).

Looking under the car, there appears to be a ton of room for resonators, mufflers, etc, and I'd like to take full advantage of it.

Some things I really need to know:

- How big a muffler/resonator can I fit before the rear differential? Quick measurements I took gave me an approximate 20" long by 9.75" wide, but I'd like to hear from owners with personal experience.

- How big a muffler can I fit behind the rear differential? I came up with 16" x 9.75", but again, personal experience from others is much welcome.

I'd also love to know people's opinions on different mufflers. I've already eliminated Dynomax VT from my list of possible candidates.

My current general plan goes like this:

- 3" high flow cat
- 3" down pipe
- 3" in/out resonator (4" round) instead of the long straight pipe (14" long?)
- 3" in/out muffler (20" x 9.75") in front of the rear diff
- 3" in/2x2.5" out muffler (14" x 9.75") behind the rear diff
- resonated tips

But in the name of silence, I'm also considering going down to 2.5" somewhere along that path.

 

I bought a Borla muffler with a 19" case and it was too long to fit. Don't forget to take into account not only the case dimensions of the muffler, but room for tips and a bend to get around the diff. I would stay away from the 2nd resonator though, seems very overkill for a stock turbo/engine setup.

For stealthy looking and sounding mufflers, check into Vibrant. They make "ultra quiet" resonators and a full line of "stealth" mufflers that are powder coated black so they don't stand out like polished tips do.

 

Yeah, I've been looking at Vibrant stuff a lot for this build. I've selected a Vibrant flex coupling and clamps. Right now, for the rear muffler, I'm torn between the following:

- Vibrant Streetpower Flat Black 3" in/2x3" out (15" x 5" x 9")
- Magnaflow XL 3" in/2x2.5" out (14" x 4" x 9")

What bugs me about the Vibrant muffler is the dual 3" tips. Sure, they're black, but that seems unnecessarily large to me. Also, the Vibrant is $126.25 vs the Magnaflow at $54.74.

What a conundrum...

As for having a front resonator, I figure it isn't that much more expensive than straight 304 3" tubing anyways, as long as it doesn't reduce ground clearance too much. That's something I'll need to mock up with cardboard and clamps to figure out.

 

I can tell you that the Vibrant definitely fits, so the Magnaflow will definitely fit. I think the 3" will look better than you think, especially in single wall black like the Vibrant

 

You've convinced me, I selected the Vibrant for the rear muffler Now I need to settle on a middle muffler.

 

Well, it took over a year to get around to "completing" this project. I put that in quotes because I'm not terribly happy with the result and I plan to make a change within a week.

I ended up going with a 3" downpipe with high flow cat, 3" flex pipe into a custom-bent 3" SS 14-gauge pipe to a Magnaflow XL 3" mid-muffler and then a 3" Vibrant stealth rear muffler. So it's 3" and 304 stainless all the way through.

End result? GOBS of power, starting with a ton more low-end torque and a redline that arrives almost before I have time to shift. The turbo spools up insanely fast, and manages at least 3-4 psi more than it used to with the stock exhaust (which had a clogged up original cat). This has completely freed up the engine's rear-end breathing, it's amazing...

...at the cost of silence. It's DAMN LOUD! I really hoped that having 2 full mufflers would help reduce the droning noise of a 3" exhaust, but I was wrong. It's mostly tolerable when I'm cruising on the highway or accelerating gently, but any extra engine load brings the noise level way up. To be honest, it's kinda fun for now. It doesn't sound like a teenage fart can, it has a deep tone that sounds aggressive. But I'll be sick of it within a week, and even moreso when I start driving our upcoming baby girl around in a couple months.

So I'll be adding an 18" resonator under the driveshaft next week, which will hopefully bring the noise level back to an acceptable level. If that doesn't work, I'll cut out the Vibrant rear muffler and replace it with a 3-chambered Magnaflow XL that has 2x2.25" outlets instead of the Vibrant's 2x3". The Vibrant doesn't really fit as well as I'd hoped anyways. The 3" pipe leading to it makes it hard to fit it under the bumper properly because it requires 2x45 degree bends in quick succession. Still looks pretty sweet in flat black, though.

 

Whenever I get the chance to build a turbo B5 I will probably go 3'' also.

Right now I got a 3'' catback coming off of the 2.25'' stock piping. Its pretty cool cause the velocity is higher until it hits the 3'' catback and then the pressure increases. It creates a nice deep rumble, and best of all, no backpressure.

OBX isn't Magnaflow or Borla but its loud, and surprisingly, I like it.

 

I ended up adding a different resonator. Instead of the 18" one I was planning on, I got a Vibrant Ultra Quiet resonator. It's got a very different shape, almost like a full muffler. It fit perfectly near the transmission tailshaft. There seems to be a spot there intended for a catalytic converter on V6 cars, and it nestled right in there without affecting ground clearance.

The exhaust is MUCH quieter now, it's amazing what a difference it made. But it's still louder than I'd like. I must be turning into an old geezer... the exhaust path has a turbo, a high flow cat, a super-quiet resonator and 2 full mufflers, and I still find it too loud.

Oh well, I'll drive it around for a couple weeks and see if it grows on me. If not, it'll be time to look at alternatives for the rear muffler.

 

It might quieten down in the next couple of weeks. When I got just a custom midpipe (from downpipe to 1st muffler) it was annoyingly loud but became pretty quiet a couple thousand km later.

 

Crap, I just called Magnaflow. It turns out their XL line of mufflers is LOUDER than their regular line, when comparing equivalent sizes. I was under the impression that the XL line was quieter, based on this erroneous info:

Magnaflow Vs. Magnaflow XL | eHow

That whole page makes no sense, the XL mufflers aren't necessarily any larger than the regular ones. But I based my purchasing decision on that stupid info.

I've got a regular Magnaflow Universal muffler on order, this time it's 18" long rather than the 14" XL that's on there now. That should solve my loudness problems once and for all.