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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > Successfully Gutted Primary Cats!!!



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      06-26-2013, 08:00 PM   #1
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Successfully Gutted Primary Cats!!!

I finally gathered enough courage to gut my primary cats. It was simple; I cut the manifolds to expose the cats, destroyed and removed the cats, and took the manifolds to a shop to get them welded back together. My only mods before doing this were a Rogue Ikon muffler and a homemade ram air with KN filter.

What kept me from doing this before? well, it looks like nobody has done a simple gutting of the primary cats in this forum, and there are lots of haters with enough arguments of why it can't work. Having said that, these were my main concerns upon doing this mod:

1) the hollow cat cavity in the manifolds would create a turbulent flow that would prevent noticeable power gains.

2) gutting the cats would eliminate back pressure and cause a reduction of low end torque.

3) exhaust would sound awful

So, I bolted the gutted manifolds back together earlier today and fired up the engine. The engine sounded deeper but not intrusive or offensive. What I really liked is that there was a noticeable and satisfying burble while in idle. I took the car for a 10 mile drive and these were my first impressions:

1) Improved and noticeable low end torque gains.

2) mid range (3500-5500 rpms) was flat, felt like the engine needed more back pressure.

3) high end power gains were more than noticeable

4) throttle response is instant

5) engine sounds like a vintage bmw straight six with webers (love it!!!!)

6) engine sound inside car is more noticeable but not intrusive.

Overall, the car felt faster, wasn't sure why the mid range felt flatter than the low end power but I found a solution. After I drove the car I went to wash my baby girls, changed their diapers and put them to bed. Immediately after this, I took the car for a second spin. This time I noticed power gains throughout the entire rpm range!!!. I think the reason is because the ECU is adjusting for the no longer existent restriction of the cats.

Conclusion: This is not the most ideal thing to do to the manifolds in terms of flow efficiency but for $50 I have what I feel like a whole new car. I definitely recommend this. All the car needs right now is a tune to adjust the timing/intake/cam and make this mod even better. The check engine light hasn't lit up but the tune will take care of it. Enjoy.





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      06-26-2013, 08:38 PM   #2
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Nice! What will you do come smog if OH has any?
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      06-26-2013, 09:35 PM   #3
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dyno.. im pretty skeptical because of reason 1 stated above
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      06-26-2013, 09:48 PM   #4
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Sound clips and dyno would make this thread complete! but great info and risk in doing this for the NA community
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      06-26-2013, 10:38 PM   #5
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dyno.. im pretty skeptical because of reason 1 stated above
Weld in a straight pipe inside the old Cat cavity.
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      06-27-2013, 02:09 AM   #6
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Do you have have a baseline dyno so you can see the difference? The only problem I can see would be the turbulent air flow. The courage is admirable though. A straight pipe cat delete that I created confirms the most gains in the mid range rpm and it is pretty well dead stock in sound. Yours might be louder because of the cavity you created. Looks clean though.
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      06-27-2013, 07:34 AM   #7
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Sorry guys, I have no baseline dyno and I'm not thinking of doing any dyno till I get the AA tune. I can confirm form this morning's commute that the flatness in the mid range has dissapeared. As the ECU keeps adjusting, the power curve feels a lot more predictable than the first run yesterday. Believe me when I say the power gain is noticeable in low, mid and high ranges.

I did think of welding a pipe inside but its tricky cause the CAT cavity inlet and outlet are not concentric or aligned so you would need to bend the pipe to get it right.

My exhaust sounds louder but not by a lot, I think its mainly because of the Ikon exhaust. Sound/video clips added.



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Last edited by cuco; 06-28-2013 at 09:41 PM.
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      06-27-2013, 08:58 AM   #8
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Nice! What will you do come smog if OH has any?
There are no emission inspections in OH
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      06-29-2013, 07:22 PM   #9
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I like this idea a lot... I think you live very close to me as well, I'm just off Tylersville Rd. in West Chester.
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      06-29-2013, 09:08 PM   #10
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Sounds good. I would love to see a dyno even if it is after an AA tune.
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      06-29-2013, 09:46 PM   #11
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I will provide a dyno soon enough, for now I'm waiting to receive the dp fix next week to turn off the CEL.

I did reset the transmission adaptations today and got the car to shift better when driving in DS mode. It looks like every time you do something that changes the engine's power curve, the tranny has to re-learn the shift points and adapt to the new curve. Before the reset it was shifting right around 3000rpm with the throttle down at 75%.
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      06-30-2013, 12:09 AM   #12
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Quote:
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I will provide a dyno soon enough, for now I'm waiting to receive the dp fix next week to turn off the CEL.

I did reset the transmission adaptations today and got the car to shift better when driving in DS mode. It looks like every time you do something that changes the engine's power curve, the tranny has to re-learn the shift points and adapt to the new curve. Before the reset it was shifting right around 3000rpm with the throttle down at 75%.
Guys at the n54 forum say to use dp fix only get past emissions and to remove it after as it messes with the lambda adaptations. If you have no emissions testing then I would just get the AA tune that turns the light off.
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      06-30-2013, 05:41 PM   #13
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Guys at the n54 forum say to use dp fix only get past emissions and to remove it after as it messes with the lambda adaptations. If you have no emissions testing then I would just get the AA tune that turns the light off.
Yes, the secondary o2 sensors also monitor AFR, the potentiometer (dial) on the dp fix helps adjust the sensor readings and in turn helps the ECU adjust the mixture when its too rich or lean. Its no big deal but ideally the tune is better. I got it used from another member so I'll stick to it until I get the AA tune.
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      06-30-2013, 06:50 PM   #14
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Sorry if this is a stupid question, but from the initial concerns you posted up, wouldn't the turbulent flow in the cavity help with back pressure? See as the air won't flow directly through like it would in a complete Catalyst Delete, it should still hold some pressure in it, making it a better solution.

I've always wondered what the benefits would be for the NA cars with a high-flow cat vs a complete catalyst delete as i know these cars need the back pressure to achieve the proper gains. Never really thought about just gutting them. Hopefully this will prove to be the best route for improving the flow up towards the cats.
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      06-30-2013, 06:52 PM   #15
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I really want to do this..but we have smog checks in CA. Also if I ever decide to sell the car, I'd need to replace the gutted cats. Anyone know how much that is?
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      06-30-2013, 07:04 PM   #16
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Quote:
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I really want to do this..but we have smog checks in CA. Also if I ever decide to sell the car, I'd need to replace the gutted cats. Anyone know how much that is?
Same here, maybe it'd be good to get another cat for smog purposes. I plan on keeping all my stock parts specifically for smog inspections. Damn California, but I want my kids to breath slightly cleaner air too LOL.
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      06-30-2013, 07:30 PM   #17
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Quote:
Originally Posted by chaFT328i View Post
Sorry if this is a stupid question, but from the initial concerns you posted up, wouldn't the turbulent flow in the cavity help with back pressure? See as the air won't flow directly through like it would in a complete Catalyst Delete, it should still hold some pressure in it, making it a better solution.

I've always wondered what the benefits would be for the NA cars with a high-flow cat vs a complete catalyst delete as i know these cars need the back pressure to achieve the proper gains. Never really thought about just gutting them. Hopefully this will prove to be the best route for improving the flow up towards the cats.
Back pressure is not a problem on this engine. That maybe the case on old 4 cylinder civics though. If you gut the cats you would still have enough back pressure from all of the other exhaust components.

Regarding the turbulent flow question, here is my theory (could be wrong);
The cat cavity is positioned right were the exhaust of the engine block ports meet (three per manifold). At this point the velocity of the exhaust gases is pretty high, meaning that the gases will escape the cat cavity pretty fast down to the rest of the piping were there are no more things causing turbulent flow. I can say for sure that the gains I'm experiencing are very noticeable, I wouldn't be surprised if there isn't much power gain difference between this setup and a custom made header (would need a dyno to prove it though).

After I did a ECU reset the engine feels like it has more mid range torque (between 3k-5.5k), and I think thats what the guys with the headers also experience.

Quote:
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I really want to do this..but we have smog checks in CA. Also if I ever decide to sell the car, I'd need to replace the gutted cats. Anyone know how much that is?
You'll find used ones in Ebay between $200-400.
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      06-30-2013, 08:32 PM   #18
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Ive been considering this here in the UK but concerned about emissions and it throwing codes. How easy were yr dps to remove? Did you remove them yourself?
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      06-30-2013, 09:59 PM   #19
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Ive been considering this here in the UK but concerned about emissions and it throwing codes. How easy were yr dps to remove? Did you remove them yourself?
It was actually pretty easy but very uncomfortable. I was able to take all 16 copper nuts from the top. There is enough space for a 3/8" ratchet and small extension with a deep socket. It takes around 1.5 hours to take apart. There is a forum or two around that describe the process.
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      07-05-2013, 01:44 AM   #20
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Do you have the secondary cats gutted too? Or just the primary cats?
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      07-05-2013, 08:52 AM   #21
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Only the primary
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      07-05-2013, 09:57 AM   #22
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Quote:
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Weld in a straight pipe inside the old Cat cavity.
+1 perforated pipe and put some exhaust iron wool around it. And you will have resonated downpipes with deep sound
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