DIY: Custom Cat-Back Exhaust

[Tommy R]

When I first bought my used '92 325is I was wondering if the previous owner had replaced the muffler or removed a resonator or something. Why, you ask? Because it was loud. Quite loud, as a matter of fact. It sounded pretty good, but borderline "unkept" like a car with a hole in the muffler can sound.

As I learned more about BMW's I realized it was common for 3 series cars to have their mufflers rust out, increasing their volume. Sometimes that's a good thing. Sometimes, however, the rusted innards can impede exhaust flow. This was essentially my first E36 so I had no frame of reference to make a judgement.

So I took a flashlight and peered into my muffler tips. What I saw was not pretty. It looked like a cavern with rusty stalagtites and stalagmites throughout. Great. Time to replace the muffler.

Removing the muffler and standing it on its end yielded this yummy reward:

[Tommy R]

So as you can see, it's time for a new muffler. Sure, there are plenty of cat-back options out there from simple, functional units to full bling (and silly) quad tip wonders. But, the cheapest you can get one of these units for is about $500! Pardon my reaction, but the aftermarket can kiss my shiny metal butt if they think I'm paying that kind of scratch for a stupid cat-back system.

What to do?

Simple. Screw the aftermarket! I'm building my own damn exhaust!!

A few online orders and I had some parts coming my way.... I ordered a dual 2" to single 2.5" Y pipe from Full Line Exhaust for ~$18, and two 2.5" 45 degree bends from JC Whitney for about ~$15. A local friend sold me two sticks of 2.5" tubing with slip connectors for $5. Oh, and my good buddy Richard gave me the polished Dynomax Ultraflow from his '96 VW GTI. So I'll ditch the pointless dual exhaust and run a single 2.5" pipe with mandrel bends into a single 2.5" muffler.

So I think I've got all my ducks in a row, right? Almost. I need a tip. Oh, God, the horror of trying to find a tasteful, stainless steel tip with a 2.5" inlet. I felt like such the ricer looking for this thing. But, not because I wanted something gaudy. Au contraire, bon jour!! I wanted subtle. However, the nice engineers at BMW were kind enough to put a cutout in my rear valence for a dual tip setup. So a single 2.5" would look flat-out silly in there. A 3" even looked rather lonely. Ideally, I'd find an oval tip about 2.5" x 3.5", but it was not to be.

As fate would have it, at the new Advance Auto Parts just outside my subdivision, I found a suitable tip. Prepare to gasp in absolute horror. It's an........APC brand stainless tip. It's got a 2.5" inlet which is perfect since that's the size of the muffler. The tip is a rolled 3.5" unit, which I was afraid would look too large, but as it turns out, it looked pretty decent in there. As you'll soon see. :12: Oh, and it was only $11!! For a stainless tip! Can't beat that....

So I hereby submit for your enjoyment, information, and redicule.... Tommy's Custom Cat-back Capers!!

Enjoy the ride.......
Tommy

[Tommy R]

As always, don't try this if you're a doofus or if you're unsure of your skills. There are quite a few tools in this procedure that can give you a world of hurt! So be careful and if in doubt, leave it to a professional..........or someone like me! :12:

STEP 1: Get your hoopty of German engineering onto some jackstands. I only raised the rear, but it's up to you if you want to raise the front, too. Obviously, I chocked the fronts and backs of both front wheels for safety!

[Tommy R]

STEP 2: Remove the old exhaust from the cat back. See my Diff Swap DIY for details on that. It's six bolts and comes off very easily, but it's heavy, baby!!

STEP 3: Build the new cat-back. Here's the front of the old cat-back next to the Y adapter we'll be using....

[Tommy R]

We mocked up the Y and decided that we'd use the tips from the factory cat-back so the mounts will be the same. It's easier that way. So we roughly located the Y adapter under the car and took some measurements. Next was to mark where to cut the tips with your best friend, Mr. Sawzall....

[Tommy R]

Well, we'd better go through with it now, huh?? :12:

[Tommy R]

Time to trim the ends of the tips to smooth out the flow. You'll see what I mean soon....

[Tommy R]

Here's the new Y pipe with the old inlets tack welded into position. As you can see, the inlet closer to me has a rather abrupt angle in it. So, that's what I was trimming in the above photo. Exhaust flow should certainly not be affected adversely at all....

[Tommy R]

It was this point while watching some Speed Channel in between welds that Rich noticed my TV was a tad dirty. You'd think I'd have cleaned it at least once in the 4 years I've been in my house, but I've got better things to do! :12:

You can see how filthy it was by checking out the shop towel. Martha Stewart, I ain't....at least not in the garage!

[Tommy R]

Now back to work!!

Before we went any further, I figured we should mount the tip to the muffler so we can build the rest of the system from the final muffler location. The tip was made to be used with set-screws since it's not designed for the true enthusiast, but rather for the "bLiNg BlInG! Look at my wing!" crowd. : I removed that crap with Mr. Air Saw.....

[Tommy R]

Here's the muffler and tip tack welded together......

[Tommy R]

Now to fit the rest of the system. After much eye-balling and measuring, we cut the 45 degree bends as needed. Here's Rich knocking off some of the burrs with his homemade bench grinder. Don't ask why he's not using my bench grinder 3' to his left. :

[Tommy R]

Here's everything mocked up and supported in place.... Yup, it's coming along nicely so far!

[Tommy R]

Now to get under there and tack weld all the joints in place.

Me like making sparks!!!

[Tommy R]

Now to fully weld all the seams as such....

[Tommy R]

Okay, so this whole contraption is ready to get mounted. But, how in the hell are we going to hang it?? The front will be supported by the intermediate (catalytic) pipe. But the rear was a bit of a challenge....

Stupid "polished stainless" mufflers are nothing but steel mufflers with a thin, thin, thin veneer of staniless sheet wrapped around it. Did I mention it's thin?? So welding hanging rods to it was impossible. Ah, but we were not out of ideas, oh yee of little faith!! :

We took some 3/8" solid rod and bent and formed it to shape. Here's one in the process of being beaten into submission....

[Tommy R]

Rich welded the "custom hangers" in place....

[Tommy R]

Here you can get an idea of what we did. The 3/8" rod was bent with a 90 degree angle on each end. The rod is held in the middle by the factory hanger. We beat the 90 degree angles to follow the contour of the muffler's shape (and it's unfortnately partly shaded out in the pic). The passenger side of the muffler looks like this (below) in the front and back with the rod spanning its entire length. The driver side only got one side welded as it seemed overkill to to it as robustly.

[Tommy R]

Here it is all welded and bolted up.....

[Tommy R]

Here's what it looks like with the car on the ground....

[Tommy R]

And from behind......

[Tommy R]

Overall, I'm happy with the way it turned out.

Performance-wise, it's great. Pulls stronger throughout the entire RPM range and smoother to boot! And I don't know the weight, but I'd guess no more than half of the factory weight!

Appearance-wise, I'd still prefer an oval tip, though a single in/dual out muffler would've been fine. But, this tip was cheap, stainless, and filled the cut-out pretty well. I may paint the muffler flat black at some point, if I can get primer and paint to stick to the stainless. Of course, I'd leave the tip polished. The muffler and tip also seem to angle downward ever so slightly. I may tweak this.

Fitment-wise, it's flawless. No rubbing or vibration. Installs as easily as the stock piece.

And so concludes my cat-back caper!

Tommy

[-ursa-]

nice.

necessity is the mother of invention

[ayton]

Great write up! we should gather all of these up and put them on a CD for sale ...damn a bently manual :12:

[sparco]

Great job!!

now I was wondering why did you choose the 2.5" diam? Have you done some research on the correct diam performancewise?
I have been considering a setup like this too that's why I want to know. Thank you