Custom Catback System

I got impatient and had a custom catback exhaust fabricated for me with 3.5' OD straight cut, beveled, polished stainless steel tips. The fabricator was located only afew hundred feet from theBMW/Porsche dealer and was recommended by them as he did alot of work for them. The primary design goal I gave him was tokeep it as quiet as possible while increasing the flow. With that we came up with something that looked very similar to Stratmosphere's look but with much bigger mufflers than theirs (to keep it quiet) and we decided to retain the 2nd set of cats to also reduce noise but made them high performance ones. What we ended up with looked like the stock system but with bigger tubing and smaller in height mufflers and split tips instead of the merged ones which also were individual 90 degree bends off the mufflers. It sounded great and was louder and much deeper but it had a slight and I mean slight drone from a little over 2k RPMs to just under 3k RPMs when the windows were up and you were on the gas. Above that, it purred like a throaty motor and for highway driving I can barely hear it. I've had the fabricator add a small cross pipe connecting the two tip bends to help reduce the 2 to 3k drone when on the gas which it has some, but most of the sound still goes through the bends. Next, I'm having him fabricatea single crossover tube between the tips like the rear tube that joins the two mufflers then making (hopefully) a V split to the tips. This is why I think the stock system has such a funky looking mergedand split tip; to reduce resonance issues. I had him paint the system with high temp flat black engine paint so you can't see it from the behind. Also notice that like Stratmosphere's, when you use 'normal' shaped mufflers, they are much shallower in height and tuck up underneath the transom perfectly. We couldn't easily find 3.5' double wall angle cut tips that were short enough to work with so we substituted 3.5' straight cut beveled double wall polished stainless steel ones and flush mounted them to the cutout. # inch ones looked too small and 4 inchers looked not only too big but would make the system impossible to remove without cutting the tips off. Am still looking for some short angle cut ones to give a more factory look but I am beginning to like the more serious racy GT3 look of these tips. The total price was $1422 including installation. Once it goes back for another crack at a better crossover, I'll post some underneath shots when it's on the rack. The performance pickup is very noticeable too, my butt dyno seems to tell me that along with the de-snork, I've made asolid 10% increase in power. I used to notice that it really didn't seem like I was pulling strongly until I hit about 4500 to 5000 RPMs and reached the power band, but now itfeels like it pullsmuch lowerlike anything over 1000 RPM. Though there is stilla 'shove' when it does hit the power band, below that feels much stronger now. Edited by - Coupe-de-Gras on 05/04/2006 9:53:58 PM Edited by - Coupe-de-Gras on 05/04/2006 9:55:26 PM Edited by - Coupe-de-Gras on 05/04/2006 9:56:46 PM Edited by - Coupe-de-Gras on 05/05/2006 08:33:22 AM

Cool! You should do some dyno runs just to verify before and after hp if you can, or at least some after runs if you get the chance, also if you record the exhaust upload the recording here for other to hear!

I'm already looking for a good dyno shop to test it and I'll try and get a sound clip or video going. When I took it to the dealership yesterday, the others there didn't think that the sound that bothers me is a drone. I can still hear my stereo just fine, it's just that when I apply gas at that range it becomes much louder. There are no vibrations or 'humming' or 'thrumming' sounds, just increased volume which I have experienced from every catback system I have everhad even a Dynan one I had on my old 330xi which I installed a crossover as well for the same reason. I guess it's just me. I just want better flow and deeper soundand not alot of increased volume, but it's very challenging to try and get both.

Awesome. This is the coolest thing I've seen posted. Damn, I want them too, dude!! How much of the cost was them blazing the trail on design and working out bugs? Would it cost me the same now that they've done one already? Was any of it written down for another shop to follow? More info would be great. That's way tight. Thanks much. -Steve

Ok, here is an after dyno run on a Dynojet 248-H dynamometer with Winpep 6.01 software showing Rear Wheel HP and Rear Wheel Torque with SAE correction for altitude and temperature. We made 5 runs, 2 in 4th gear and 3 in 5th gear and they were all within half a HP or half a ft/lb of each other. At first I was somewhat surprised and disappointed in that it appears that I did not gain anything in terms of maximum HP or torque. But then I looked at the chart more closely and a smile came across my face. In the lower RPM ranges the torque is impressive, showing that as early as about 2700 RPM, I'm already at 215/226.7 = 94.84% of maximum torque. The torque chart is no longer a curve but now a nearly flat plateau across the whole RPM range. Horsepower is also better in the lower RPM range but not nearly as dramatic as the torque. And that's what my 'butt dyno' was telling me, that below the upper power band in the lower RPM range, it felt like it was pulling more strongly after the catback was installed. The chart also seems to indicate that both HP and torque are not substantially dropping off at 6500 RPM which is a good thing. The fun factor has definitely increased in the lower RPM band and for me, that's where I spend most of my time. Too much attention to be over 4500-5000 RPM constantly. I did take a sound sample, but it didn't turn out very good and I will try again. The chartis indicating Maximum Powerat 268.7 HP and Maximum Torque at 226.6 ft/lbs. Edited by - Coupe-de-Gras on 05/10/2006 08:28:46 AM

It would have been nice to have before and after dynos for comparison purposes but at least there is a data point here. It looks like your HP is around the same number as mine before I did the Air Intake mod ~270 at the rear wheels. As far as HP/Torque at certain RPM ranges we'll have to post in my stock dyno run to compare. If you want to post a larger version of the photo go ahead and use the direct (non-resizing) photo upload directly into the forum if you want, the scaled down version loses some of the detail in the chart. Here is my chart for comparison:

BTW something isn't right with the torque figures, as I highly doubt that a catback only exhaust would DOUBLE the torque at 2800 RPM, that's too extreme so one dyno or the other isn't showing the torque correctly at lower RPM's. Looks like we need a third dyno chart for more comparions... /fckeditor/editor/images/smiley/MWPX/shades_smile.gif

Oh, one more thing, I did the de-snork right after you posted your article, so the dyno chart is with both the Air Intake and Catback mods. It looks like the charts do match up pretty closely. Your torque line is the blue line while mine is the red one. My horsepower is the green line and yours is the red line. At 2800 RPM you are indicating roughly 174 ft/lbs of torque (blue line) while I am showing about 215 ft\lbs (red line). That's only about a 23% gain at that RPM. It also looks like the production of torque has increased at lower RPMs as your torque at 2100 RPM is about 35 ft/lbs while mine is showing about 170 ft/lbs at 2100 RPM. Now that is dramatic at that RPM. By about 3100 RPM and above, our charts are pretty close; you show about 125 HP (red line) and about 200 ft/lbs of torque (blue line) at 3100 RPM while I am indicating about 125 HP (green line) and about 215 ft/lbs of torque (red line) at 3100 RPM. It could also be that I am noticing more dramatic low end benefits with the two 'better breathing' mods because I am up here in Reno, Nevada where the air is thinner and I do notice an increase of power when I travel down into the California valley. Edited by - Coupe-de-Gras on 05/10/2006 10:20:51 AM

According to the 'Coupe-de-Gras' Dyno charts (when compared to K-MAN S' stock chart): The 'Air Intake' mod does not add any WHP. 1- Coupe-de-Gras' chart is with a new exhaust and the De-Snork done: Shows about 270 WHP. 2- K-Man S' is a stock graph: Shows about 270 WHP. Bottom Line: Coupe-de-Gras' dyno does not support a 13 WHP gain by doing the De-Snork mod. Am I missing something or am I on to something? Shane

Could be a couple of things, could be the exhaust lost 13hp and the air intake made up for it, thus cancelling each other out, or his ECU hasn't adjusted to the increased airflow since the air intake mod, or you can't compare 2 different cars on 2 different dynos, hence the need for before and after dynos on the same machine with the same car. Most likely the latter...

Here is a larger resolution image of my dyno chart and I changed the colors to match K-man S' colors. In this chart, the red line is horsepower and the blue line is torque.

K-Man S, I agree with everything you said. Although, it does make my 'doubt' grow. I really do hope that the 'De Snork' mod will add 13WHP. We just need a few more people to do what you did. Dyno, mod, dyno same day. Drive 50 miles in two days and dyno again on same dyno with similar conditions. Shane

Ok, I managed to aquire the Porsche Tequipment Sport Round Tips and had my fabricator install them. In order for him to do so, he had to make a 'T' pipe to fit it. Bad news. The butt dyno, even after 50+ miles to have the ECU adjust was dissapointing. Although the performance and strong torque feeling was maybe a slight tad above stock and the noise level returned to stock levels, I completely lost the rich, deep, intoxicating purr and growl of the right angle bends as well as a very noticeable drop in performance at high RPMs.The guy from Stratmosphere is absolutely correct, if you combine the two exhaust pipes into one then split them out again, you lose power, period. It both felt and sounded choked when I got on it at high RPMs. The old tip system with 90 degree bends, though louder at 2 to 3k, is going back in tomorrow. I also like the look of the straight cut polished stainless steel tips better. They looked and sounded serious and racy. The highly polished chrome Tequipment ones are just too flashy for my tastes. They also stuck out a bit farther (3/4') than the stock or my custom tips. My system is going back to the above pics with a slightly increased spacing (1/4') inch between the tips so I can get a cloth in there to polish them more easily. Edited by - Coupe-de-Gras on 05/11/2006 9:42:38 PM

I would just like to thank you, Coupe-de-Gras, for the most informative topic yet on exhaust issues. You have gone ahead and made some very useful discoveries of problems, that the rest of us can now avoid. Again, thanks! Best, Wurlie Edited by - wurlie on 05/12/2006 12:47:44 AM Edited by - wurlie on 05/12/2006 12:48:52 AM

Thanks. We put back in the original 90 degree bends and previous tips today and put in a bigger and straighter crossover tube between the bends. Wow. Not only was the previous strong torque feeling back, but because of the larger crossover tube, the loudness was cut by about a third with no discernable drop in performance and the sound was noticeably smoother. It purrs and growls so nice now it sounds like it was meant to do so from the factory. I am very happy with the results.

Here are some pics of the finished exhaust system. Tubing size from the flanges was increased to 2.5' all the way to the tips with a 2' crossover connector piece connecting the two 90 degree bends. Edited by - Coupe-de-Gras on 05/17/2006 3:24:53 PM

Yes, you are correct. The flanges are north of the 2nd set of now high flow cats. The complete system is completely bolt-on, bolt-off with the stock system which I am keeping in case I ever need to put it back on for resale or whatever. I am trying to get a sound clip/video going. A friend's MP3 .ACT format clip was too close and it's range was suited only for voices. This is what was replaced:

Terrific work. What about Kaisers first question? Is it going to be offered for sale?

Oh, yes, absolutely. Total price is $1422 parts and installation. Total time is that they need it for a whole day. i.e. Drop the car off in the morning when they open, then pick it up in the afternoon when they close.

If you all want to sell this here are my suggestions for whatever they are worth: 1. Become a Caymanclub.net sponsor 2. Provide an independant dyno report from an othewise un-modifed Cayman (just to make sure there is no h.p. loss) 3. Provide anaudio file so we can listen to the change and 4.Make a kit for our local installers that is a no-brainer to install. Edited by - Santa Fe on 05/17/2006 6:10:27 PM

I will talk to the fabricator tomorrow about the possibility of providing a shippable kit. Since I could not find a suitable video/sound recording device from either a friend or co-worker, I went out and bought a Sony MiniDV camcorder. It's charging as I write this and I hope to have something soon. I will also talk to the dealer about loaning me a stock Cayman or me paying for a dyno run on one.

Ok, just spoke with the fabricator. He said he could probably rig up a jig in about a day, but would require my car again for that day. That's ok with me. He would like to more than one at a time though. Shipping it as one complete piece is possible, and preferred, but if separated into 2 or 3 pieces, he would prefer that they be cut in way for someone to weld them back together as opposed to a sleeve/collar bolt clamp.

He was too busy today to talk to but we will get together on Monday. Here is a sound video of the exhaust. At cruise, you can barely hear it. The clip is from start up with a couple of low blips thena few blips from 3000 to 6000 RPM. I will try for an in cabin run next week. http://www.planet-9.com/files/downloads/2/32/20060519213743_1010_CustomCatbackExhaust.zip Edited by - Coupe-de-Gras on 05/19/2006 9:47:12 PM

Coupe, thx for posting this great info. I love the look of the twin round pipes, and the sound is great. I would certainly be interested in a kit if they become available.

Ok, spoke with the fabricator. He is willing to start making sets and would need my car for a day or so to make a jig. The finished system would be shipped as a single piece and foam packed. He would need a minimum committment of $500 to order the parts and then we would schedule my car to come down to use my existing system as the reference. He can take VISA over the phone. I will PM his business contact information to you guys and anyone else who wants it.

It's a good time of year for a drive up to Reno. Looks and sounds great!