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      11-22-2010, 03:29 AM   #1
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Arrow DIY: OEM x-pipe secondary cat removal/gut

Looking for an after market x-pipe with secondary cat delete and resonators for your car? How about modding your OEM exhaust! There is VERY LITTLE difference between the aftermarket x-pipes and the stock one! BMW has built a very good quality exhaust system from the factory and it needs only a few minor tweaks to squeeze out the extra hp. I was looking for a x-pipe with 2nd cat delete with resonators but could not understand how the manufactures could justify there pricing, they must think we have $ to burn or that most M3 owners just believe the hype and buy

The secondary cat delete (Gut the factory cats) is very easy and only took me about 3 hours total including removal, modifying and reinstall. This is one of the best improvements i have done to the car. It will make the engine more responsive and should add around 8-15hp for $0.00 and a little bit of your time. If you are not the DIY type then an exhaust shop should be able to do this for less than $150-200.

+ The best part is the dealer will have no clue it is modded, it looks stock


So here is the process of doing a secondary cat gut / delete:

1. remove the x-pipe portion of the exhaust from car, you can leave the rear sip-on exhaust on the car
2. you will need to mark the exhaust pipe with 2 or 3 parallel lines on each pipe because you will use these later when you are realigning the exhaust tubing for welding. Now using a cut off wheel, cut the slip joint right behind the weld at the front of secondary cats. Be careful and only cut through the outer pipe wall, not all the way through


3. now this is what the 200cell secondary cats look like


4. just for reference, these are the tools that i used

5. (PUT ON DUST MASK) take your 1/2 HD drill and use a 3/4" +/- self feed spiral drill bit to remove the majority of the catalyst. If you try to just pound it out it will compress into a hard metal mass that will lodge itself inside the exhaust tubing(trust me) Remove most of the catalyst material and then use the pipe to scrape and push out the left over material. You should have a pile of catalyst like this from the first cat

and this is what it should look like with the first cat removed/gutted

6. now do the same for the next one and your pile of catalyst on the floor should look like this

7. you will need to rig up a wire brush on a piece of pipe and clean-up the inside of the exhaust pipe, it doesn't need to be perfectly clean just get most of the catalyst out


8. now that you have the gut complete you are ready to reassemble

9. you should now be able to realign the exhaust using the alignment marks you made at the beginning. I used a small MIG welder to weld the exhaust tubing back together. You will need to do a series of tack welds around the exhaust tubing as to not burn through, or set your welder to the lowest setting, this is very common for exhaust tubing.



10. if you are trying to be stealth then nows the time to clean up the welds

and this is what it should look like, once it gets a Little heat coloring on it the dealer will have no clue that it is modded!


In addition to what I wrote at the beginning, I did this mod so that I would not have to deal with a CEL and then a tune, and the inevitable chance that the dealer will reflash the car while in for service. I would then lose my tune and have to pay for it to be reflashed to the ECU, its just not worth it to me for an extra 10-15 whp that a tune gives you. I will probably in time gut the main OEM cats also, I just need to find a good CEL fix that the dealer won't find. Most of the hp people are getting out of our cars are from the removal of the cats (20-30 whp) NOT THE TUNE! (10-15 whp). I drive my car hard at the track and I want to know that if something goes wrong with the car, the dealer will have less to complain about and no reason to say I voided my warranty.

Hope this helps some of you that are thinking of doing this mod.......
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Last edited by erics02wrx; 11-22-2010 at 11:52 PM.
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      12-07-2010, 11:55 AM   #2
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This is awesome... Going to save me 1k.... Thanks....

Quick question... How exactly did you clean up the welds? I want to do that also but dont want to mess it up...
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      12-07-2010, 12:54 PM   #3
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Great Write up!

Do you notice any increase in power after doing this?
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      12-08-2010, 10:16 AM   #4
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This is incorrect information from my dyno findings. A proper custom tune will yield 15-20whp. And the secondary cats are maybe worth 10hp and some noise. The primaries delete are worth alot more as the design is far more restrictive.
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      12-08-2010, 12:09 PM   #5
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Quote:
Originally Posted by BT M3 View Post
This is incorrect information from my dyno findings. A proper custom tune will yield 15-20whp. And the secondary cats are maybe worth 10hp and some noise. The primaries delete are worth a lot more as the design is far more restrictive.
Your right on and that is what I was saying! I was not stating that the secondary delete alone was going to get you the same hp as no cats+tune or even main cats+tune. What i was saying is that the power that people are getting out of tunes is directly related to the cats....if you left the cats in place and got a reflash you would only pick up 7-12 whp max ( less than just doing a secondary cat delete and rear slip on, which would not cause any issues from a warranty standpoint like a tune could) Remove the main and secondary cats and even without a tune you will pick up 20whp+.....add a tune to the cat removals and you will add an additional 10-15whp! My point is most of the gains people are getting are not from the tune but from the removal of the cats(65% from secondary and main cat delete and 35% from tune). There is not a lot of hidden hp in this engines tunning parameters stock, you have to really start changing things(cams, pulleys, intake, exhaust, TB, headers, etc....) before you are going to see big gains from a tune.
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      12-08-2010, 12:19 PM   #6
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Quote:
Originally Posted by 1sikbmr View Post
This is awesome... Going to save me 1k.... Thanks....

Quick question... How exactly did you clean up the welds? I want to do that also but dont want to mess it up...
Look at #10 and you will see me using a right angle die grinder with a 100grit sand paper wheel on the end of it. It is not quite as fast but way cleaner than using a grinder which would leave large grind marks that the dealer could see. If you have a lot of weld to remove, start with a grinder and then finish it up with the sanding disk.
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      12-08-2010, 12:36 PM   #7
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Quote:
Originally Posted by andrew270681 View Post
Great Write up!

Do you notice any increase in power after doing this?
I have not had it dynoed, but probably only 5-8 rwhp. The big diferance was it makes the engine feal more responsive. I also have the Corsa⁄UUC Exhaust on my car and i noticed a lot more gains in top end pull with the secondary cat delete over just the rear slip on.
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      12-09-2010, 12:15 AM   #8
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Very nice DIY, thanks for sharing. How is the sound afterwards? Hopefully not too much increase in loudness (unless you wanted that).
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      12-09-2010, 01:53 AM   #9
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Very nice DIY, thanks for sharing. How is the sound afterwords? Hopefully not too much increase in loudness (unless you wanted that).
Maybe 5% louder than before, no change inside the car. It did add a little more rumble, sounds awesome!
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      12-09-2010, 07:03 PM   #10
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Quote:
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Maybe 5% louder than before, no change inside the car. It did add a little more rumble, sounds awesome!
Really? Not noticeable in the car? Cool. My aftermarket cat back Vorsteiner set up added some volume and I am really hesitant to make it any louder. The sound quality was really improved for me but I am definitely concerned about doing something that will make long freeway trips and road trips annoying to unbearable.
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      12-10-2010, 01:36 AM   #11
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nice write up. would the process be the same if i wanted to gut the primaries too? would those be harder to gut and would you be able to put it back together as cleanly.
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      12-10-2010, 04:26 AM   #12
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Quote:
Originally Posted by spoonf20c View Post
nice write up. would the process be the same if i wanted to gut the primaries too? would those be harder to gut and would you be able to put it back together as cleanly.
You can make the cut at the same place to do the primaries, but you will get a cel light.
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      12-10-2010, 12:18 PM   #13
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Quote:
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Really? Not noticeable in the car? Cool. My aftermarket cat back Vorsteiner set up added some volume and I am really hesitant to make it any louder. The sound quality was really improved for me but I am definitely concerned about doing something that will make long freeway trips and road trips annoying to unbearable.
I have the UUC / Corsa exhaust and I was afraid of the same thing (being to loud) but it was barely noticeable. Its a little louder from the outside of the car but on the inside its maybe 5% louder, if that. I love it!
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      12-10-2010, 12:25 PM   #14
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Quote:
Originally Posted by spoonf20c View Post
nice write up. would the process be the same if i wanted to gut the primaries too? would those be harder to gut and would you be able to put it back together as cleanly.
Same process and very easy. You will get a CEL just like alpina527 said..... so you will have to get a CEL fix or tune, I guess you could also just drive it with the cel on, it wont hurt anything but would be very annoying.

Quote:
Originally Posted by alpina527 View Post
You can make the cut at the same place to do the primaries, but you will get a cel light.
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      12-24-2010, 02:41 AM   #15
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This was great. I just finished it flawlessly... didn't take any pics but do have a vid... I do feel a really different car now. It is way more responsive than before and the exhaust note is just 10x better.
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      12-24-2010, 11:40 PM   #16
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Quote:
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This was great. I just finished it flawlessly... didn't take any pics but do have a vid... I do feel a really different car now. It is way more responsive than before and the exhaust note is just 10x better.
Great job, this is one of the best mods I've done also and your right, the sound is awsome......post your vid!
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      12-25-2010, 06:03 AM   #17
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Quote:
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Great job, this is one of the best mods I've done also and your right, the sound is awsome......post your vid!
Hey just made a thread with the video we made... we also did a little mini shoot... nothing professional just friends hanging out... Check it out...


http://www.m3post.com/forums/showthr...65#post8560765
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      02-20-2011, 08:34 AM   #18
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I finished mine yesterday and im not impressed by the gains or the sound difference. My car is completely has a complete stock exhaust. I noticed a small gain in responsiveness and small rumble after 3500rpms. overall if you can weld at the house then maybe its worth it. I would not pay a shop 200 smackers to do this. I'm not discrediting what the original post says but I was excited as he was until I drove it and the performance described just was not there. Not being negative, just informative.
Thanks for the write-up, it did go well. One alternative we saw would be to do the cut and weld on the other side of the factory weld so you can put the straps over it and it does not have to be perfect, just flat.
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      03-09-2011, 08:46 PM   #19
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I finished mine yesterday and im not impressed by the gains or the sound difference. My car is completely has a complete stock exhaust. I noticed a small gain in responsiveness and small rumble after 3500rpms. overall if you can weld at the house then maybe its worth it. I would not pay a shop 200 smackers to do this. I'm not discrediting what the original post says but I was excited as he was until I drove it and the performance described just was not there. Not being negative, just informative.
Thanks for the write-up, it did go well. One alternative we saw would be to do the cut and weld on the other side of the factory weld so you can put the straps over it and it does not have to be perfect, just flat.
Thanks for the feedback, its interesting to see that everyone has different outcomes. Main differance is that you still have a OEM rear section. I have a Corsa exhaust, the improvement was noticeable for me and the sound improved also.
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      03-22-2011, 11:32 PM   #20
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hey i'm thinking about doing this mod! but.. i have already taken out the resonators.. do you think if i do this mod will i run into a cel issue>? and how is the idle is it any different ? and will u pass inspection ? or emissons ?
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      03-23-2011, 12:57 AM   #21
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hey I'm thinking about doing this mod! but.. i have already taken out the resonators.. do you think if i do this mod will i run into a CEL issue>? and how is the idle is it any different ? and will u pass inspection ? or emissions ?
As long as you don't touch the main cats you will have no issues at all. There will be no change in idle, no CEL and it has no effect on passing your inspection. If you get a sniff test as part of your emissions test I believe you will still pass no problem because the secondary cats are only for start-up emissions reduction. WA state does a OBD2 test only, so there is no problems with the mod as its post the o2 sensors.
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      03-23-2011, 01:59 AM   #22
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YUP! See, but watch people will be to lazy to do this and will start whining and crying to find a reason to buy a 2500 dollar setup lol.
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