When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
This project got out of hand quickly. The 4-bolt flange on my stock cat has corroded to the point of leaking quite a bit. And I discovered that my secondary muffler is leaking along a seam. All of the aftermarket options add noise, which I'm not interested in. So I thought that I would play around with cobbling something together. Starting with some handy measurements
Stock exhaust:
2.75" pipe cat converter flange= 10mm thick, 90mm square, 9mm diam bolt holes (for 7.7mm diam M8 bolts), 70mm between bolt hole centers, 2.75" center hole stock cat= 442cfm (from B&B), 17lbs (me), roughly: 19.5" total length, 8.75" core length, 17.5" core circumference, ~5.5" core diameter primary muffler= 396cfm (from B&B), 22lbs (me), roughly: 26" core length, 38" total length, 8.5" height, primary is kind of a turbo style muffler with perforated chambers and baffles supposedly not great for flow secondary muffler= 462cfm (from B&B), 19.6lbs (me), rougly: 19.5" core longest length behind tire, 10" shortest length to wheel well, 8" wide, 5.5" tall secondary is basically a straight-through design: perforated tube in a U shape with glass/sound absorbing packing all around. Supposedly a good flow design tip= 12" long from inlet edge to topside of tip, 9" long from inlet edge to bottom side of tip, slash cut oval style
Attempt #1, let's just stick some cheap stuff in there is see what happens, keeping close to stock diameters. Everything is stainless steel (t-304 pipe/flange, t-409 Magnaflow parts)
The new parts list: Magnaflow #59928= 2.75" inlet/outlet, 2.8lbs (me), metallic sport cat, 3" core length, 4" core diameter, 11" total length Magnaflow #12267= 3" inlet, dual 2.5" outlets, 13.2lbs (me), camaro/firebird style crossflow straight-through style muffler, 18" core length, 24" total length, 5" height, 8" width, inlet comes into the top of the muffler then splits into a Y for the left and right outlets Magnaflow #14210= 2.5" inlet/outlet, 11.4lbs (me), reverse direction U straight-through style muffler, 14" core length, 17" total length, 5" height, 8" width 2.5" Vacuum actuated diverter valve Default position is closed 2x exhaust tips Stainless, wide oval, rolled edge, 2.5" inlet size VW vacuum switch Simple 12v switch with vacuum in/out, plumbed into manifold vacuum and mounted near the other vacuum switches. Turned on/off via a stock push/pull switch on the dash where the RoW rear fog light switch would occupy. Pipes & bends= cat flange 3/8" thick, 2.75" straight pipe with o2 bung into cat, 2.75" 45deg pipe, 2.75" 90 degree pipe into primary, 2.5" 90 degree pipe x2 out of primary. Clamps= Walker stainless lap joint band clamps on everything at the moment. Even with my rough-cuts on the pipes and rough lap-joint flares, these clamps are sealing very well
Result:
With the 2nd outlet on the primary muffler blocked off and everything routed through the secondary muffler, it's still louder than stock up to ~4500rpm and has a nice deep note. After 4500rpm is much louder and it develops a hollow/echo sound in addition to the deep note. Seat-of-pants impression is pretty similar to stock power with a little extra at the top. No interior resonance problems though.
Opening the 2nd tailpipe is noticeably louder. You start to hear the burble of the exhaust pulses and it has a reverberating deep tone. Perfectly fine for competition, but too much for the street for me. Sounds neat though. Seat-of-pants impression is that there is some noticeable power everywhere through the revs.
So that was fun, but I didn't meet the quiet criterion. Here are some pics of the mock up and progress-
I thought there would be more interest considering this setup is about 1/4 the price of the aftermarket sport cat + muffler options. Oh well.
I made some sound recordings but they really don't tell you much. I've run it with the 2nd outlet open for an autocross and a little around town lately and can give a pretty good summary.
My gt3-owning buddy was working the autocross start this weekend and said that it sounded much deeper than the 3.2 cars (each is running a different exhaust), and he liked it. Compared to most of the cars running aftermarket exhausts, or the stock gt3/4 volume, it wasn't loud at all. It's the low-tones that make it project to my ears.
The idle has more overall volume and creates a bass-heavy resonance. Light throttle and cruise are pretty quiet and you can just hear the nice sounding lower exhaust note. WOT is a lot of low tone interior resonance which amplifies the volume. Decel, is all resonating low tones. However, with the top-down, a lot of the resonance disappears. During a cruising top-down country drive last night, I'll admit, it sounded very nice and I could see the appeal. But pulling into the detached garage at near-idle, I felt like I must be vibrating the whole house.
Seat of the pants power is unquestionably up a bit. I think you'll figure out which video is which-
Yep, I have a flowmaster that arrived the other day. I'll report back when I try it out in place of the magnaflow primary. I'm waiting to find time to build a knock-logging circuit and then I'll see if I can get any more juice out of any of these exhaust configurations with ignition timing changes. Stay tuned
Another update. I swapped the Magnaflow #12267 primary/center for the Flowmaster #53083. Same 3" in and 2.5" dual out transverse design, but the Flowmaster is a chambered style vs the Magnaflow straight-through style. Unfortunately, Flowmaster doesn't make a stainless version of this muffler, but they're cheap enough to replace them every so often if needed. Result: much improved! I don't notice any hp difference and the Flowmaster cuts the total db down and reduces the resonating bass tones .
With one side capped, and the other side forced through the Magnaflow secondary, the overall sound is closer to stock but retains more lower tones. The booming idle is lessened and the exhaust note can almost disappear with very light throttle. At WOT, it makes more sound than stock, but nothing dramatic. The hp difference having the 2nd outlet capped/uncapped with the Flowmaster does not seem as significant as it was when using the Magnaflow primary. This is all seat of the pants.
To note- the Flowmaster had some kind of coating that burned off with an impressive stink and smoke rolling out. The smoke disappeared after 20minutes. The stink took a few hours. There is also a #43083 version that has a slightly smaller body but that would just mean more exhaust dB. The #53083 is a tight fit and is much larger than the Magnaflow. I made some 1/16"x1" stainless straps that wrap around the muffler body and bolt to the engine carrier in place of the stock primary muffler hanger bar. The mount is plenty solid.
I have a/x video with the Flowmaster, but it sounds exactly the same as the Magnaflow video that I posted above. The mic on the iphone is probably not picking up the Magnaflow low tones which makes them sound similar. Here is a video from outside the car at the start line. Flowmaster primary in "loud" mode: both outlets open.
Found some time to devote to finishing up this little project. I posted a thread in the pelican bodywork section about the FVD left side exhaust cutout. Anyone here spend much time working with fiberglass? It's been an easy fit so far.
Finally have this all finished up. I used a simple Bosch vacuum switch tee'd into the vacuum reservoir lines. Then ran switched 12v to the vacuum switch from a push/pull rear window defrost switch on the dash (upside-down, the wavy defrost arrows kind of look like exhaust flow...).
Noise just from the single outlet is slightly louder than stock, but not by a lot. With the second outlet open, there is more volume and much more bass. Iphone mic isn't amazing, but this should give a good idea. Will probably be a while before I play around with dyno runs to see how it compares to stock, and the effect of opening the second outlet. Stay tuned for that.
You can hear me open the left outlet around 38sec, and around 1:10 I open/close the outlet a few times to compare the difference.
Finally have this all finished up. I used a simple Bosch vacuum switch tee'd into the vacuum reservoir lines. Then ran switched 12v to the vacuum switch from a push/pull rear window defrost switch on the dash (upside-down, the wavy defrost arrows kind of look like exhaust flow...). Noise just from the single outlet is slightly louder than stock, but not by a lot. With the second outlet open, there is more volume and much more bass. Iphone mic isn't amazing, but this should give a good idea. Will probably be a while before I play around with dyno runs to see how it compares to stock, and the effect of opening the second outlet. Stay tuned for that. You can hear me open the left outlet around 38sec Video Link: https://youtu.be/QIEdTtoLEo0
Great work. After I finish my rebuild I'm going to fit a Y pipe cutout into my Cat bypass, a bit more rudimentary than your set up.
Nick this a really awesome. I'd love to do something similar but here in California we have to keep the factory cat. Guess I could do the rest from the cat back.
Spokes- I have the stock heat shield in front of the cat, the shield that bolts to the end of the heat exchanger. That gives plenty of protection to the tire. Not sure why you couldn’t find the parts, google brings them right up for me.
Another addendum, since my secondary muffler had a partly blown out seam, and it isn’t really worth much of anything, a dissection seemed in order.
In the follow pics, you can see the double wall construction. There was a thin wrapping of fibgerglass(?) between the inner and outer shell. The inner shell also has some 1/8” holes in it by design. Moisture relief maybe?
Once the outer shell was cut away, you can see some significant rust damage to the inner shell.
With the inner shell cut off, you get to the heart of the matter. Only some very small pieces of the original steel wool and fiberglass sound absorbing mat is left in a few corners! I always thought the exhaust was loud for a stock car...
I might use the inner guts and make a new single-wall shell, packed with stainless steel wool and rock wool to replace the magnaflow secondary that I have now. In “quiet” mode, the custom setup is still a bit much. Enjoy!
I had heard that the Magnaflow (or any brand) turbo mufflers get louder over time. This is from the packing getting blown out. So I made a hatch for re-packing. It's packed fairly well, but on closer examination, the steel wool wrapped around the pipes (near the inlet/exit of the case) had gotten pushed back. This allows inlet exhaust to flow directly to the outlet without going through the full sound-absorbing U-turn. The solution was to add more stainless steel wool wrapping around the pipes in that area, and pin the wool so that it wouldn't slide back. Then I added a healthy addition of rock wool in the void between the pipes for more sound absorption. Result: No difference to my ear! It was worth a try...
9 Vehicles Porsche Helped Engineer that Aren't Porsches
Slideshow: Long before engineering consulting became trendy, Porsche was quietly helping other automakers build everything from supercars to economy hatchbacks.
9 Features and Characteristics That Only Porsche People Understand
Slideshow: Some brands build cars. Porsche builds traditions, obsessions, and a few habits that stopped making sense decades ago but somehow became part of the charm.
This Builder Is Turning Heads With Its Slantnose 911 Creation
Slideshow: A small Polish tuner has reimagined the Porsche 911 Slantnose for the modern era, blending 1980s nostalgia with widebody tuning culture and serious performance upgrades.
Porsche 911 GT3 Artisan Edition Pays Homage to Japanese Culture
Slideshow: Porsche has created a Japan-only 911 GT3 Artisan Edition that blends track-ready hardware with design cues inspired by traditional Japanese craftsmanship.
Porsche Reveals Coupe Variant of the Electric Cayenne With a Fresh Look
Slideshow: Porsche's latest electric Cayenne Coupe blends dramatic styling with supercar acceleration, turning the brand's midsize SUV into a 1,139-horsepower flagship.
custom exhaust playtime
