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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Catalytic Converter Question - Downpipes, Aftermarket exhaust



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      04-09-2012, 07:37 PM   #1
desean81
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Catalytic Converter Question - Downpipes, Aftermarket exhaust

Hey everyone, I need some suggestions for my situation. Awhile ago I bought an aftermarket exhaust after falling in love with the sound. It was a downpipe-back exhaust, so it removed the secondary cats.

Next, I fell in love with the idea of catless DPs for the performance gains. So once I install my new DPs I'll be completely catless which is a no-go for me as I can't feel right about just dumping all those fumes out the pipe when they could be treated.

I need a catalytic setup that I can weld into the aftermarket exhaust that hopefully won't harm the performance gains from the catless DPs. I'm of course willing to sacrifice some performance in the name of treating the exhaust gases, and passing the sniffer test we have here in Ontario (very similar to California regulations as far as I know).

I was thinking of getting a shop to cut the factory cats back a bit on their flanges to widen the opening to the size of the aftermarket exhaust (2.5") and weld the original secondary cats back in rather than purchasing something like the Magnaflow hi-flow cats as from what I've heard, they don't work all that well.

Can anyone see any issues with this setup? Is there another setup I should be considering that will keep the car legal on the sniffer and not dumping really smelly fumes out the back?

Any input would be appreciated! Thanks very much for your help!
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      04-09-2012, 08:09 PM   #2
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I would just keep your current setup and when its time for emissions, just put the DP back on
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      04-09-2012, 08:22 PM   #3
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Yea that. Boats motorcycles oldercars etc don't have cats. One car running catless wont have any effect on how much emissions are in the atmosphere.
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      04-09-2012, 08:41 PM   #4
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Staying catless isn't an option for me. I hate the smell of a catless car and I just think it's irresponsible. Not to mention that if our finest catch a whiff of something like that I'll be walking away with a hefty fine for running catless and rightly so. One car might not make a large difference but it makes a difference. I'd like to try and do it legal, and responsibly because I'm sure there's a way - I just need some direction on how.

Anyone with any input on what cat setup I should go with (keeping catless DPs, but placing cats in the exhaust) let me know. Magnaflow hi-flow from what I've heard don't work that well with the sniffer, so how about cutting back the flange on the OEMs to make them a match for the exhaust diameter and weld those up? Would that affect the flow substantially?
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      04-09-2012, 08:55 PM   #5
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Not to be rude, but with your philosophy, you should just go back to stock downpipes. The secondary cats don't "catch" the majority of pollutants like the primary's do.
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      04-09-2012, 09:06 PM   #6
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Originally Posted by E90Company View Post
Not to be rude, but with your philosophy, you should just go back to stock downpipes. The secondary cats don't "catch" the majority of pollutants like the primary's do.
Why does everything have to be about proving a point? Not to be rude either but did you read what I wrote?

I want to be as responsible as possible while staying catless on the downpipes because of the performance gains that are possible there. I agree secondary cats in their stock configuration don't "catch" the majority of pollutants like the primary setup, but that's mostly because the primary's are doing 99% of the work and the secondary setup is there to catch the remaining amounts.

If I were to reweld the secondary cats in an aftermarket exhaust (by cutting the flange to widen and match diameter) would this result in a passable setup by the sniffer? I understand that cats work best hot, so I imagine that I may have more initial emissions while the cats heat up as they're not as close to the engine as the DPs are. Would a catalytic preheater setup solve this?

I'm just looking for options for my setup, I don't need to be swayed to reinstall my stock downpipes.

Thanks!

**EDIT:

So that I'm clear, I don't believe there's a physical difference between the primary and secondary cats? They don't appear to be different but if they are, and I'd be better off welding in the original primary's I could do that instead?
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      04-09-2012, 09:46 PM   #7
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Cats require heat in order to function. They don't "filter" the fumes, they cause a chemical reaction that changes the fumes into CO2. Unless the cats are near the turbos then they won't have the heat necessary to function properly.

But yes, you can just weld some high flow cats in the midpipes to help with the smell.
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      04-09-2012, 09:51 PM   #8
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Quote:
Originally Posted by rader1 View Post
Cats require heat in order to function. They don't "filter" the fumes, they cause a chemical reaction that changes the fumes into CO2. Unless the cats are near the turbos then they won't have the heat necessary to function properly.

But yes, you can just weld some high flow cats in the midpipes to help with the smell.
Understood - hence the word catalytic

I would imagine the mid-pipes get hot eventually? That's why I'm wondering if a pre-heater of sorts would help get them working well quickly?

How do non-turbo cars have their cats setup? I'd think they'd end up further from the motor in an NA configuration?
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      04-09-2012, 10:22 PM   #9
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Originally Posted by desean81 View Post
Understood - hence the word catalytic

I would imagine the mid-pipes get hot eventually? That's why I'm wondering if a pre-heater of sorts would help get them working well quickly?

How do non-turbo cars have their cats setup? I'd think they'd end up further from the motor in an NA configuration?
The cats in the midpipe don't heat up enough to work properly, that's why the N55 doesn't even have cats in the midpipe.

On a NA car the first set of cats are typically mounted in the exhaust manifold just after the collecter.
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      04-09-2012, 10:31 PM   #10
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Originally Posted by rader1 View Post
The cats in the midpipe don't heat up enough to work properly, that's why the N55 doesn't even have cats in the midpipe.

On a NA car the first set of cats are typically mounted in the exhaust manifold just after the collecter.
Hmm so basically if you run catless DPs there's no way to control emissions? I know that people have passed sniffer tests with only the OEM exhaust installed along with catless DPs, hence my wanting to try this approach of welding in the original cats.

Disappointing that they won't reach temperature, are you reasonably sure about that or guessing?
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      04-09-2012, 10:35 PM   #11
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Quote:
Originally Posted by desean81 View Post
Hmm so basically if you run catless DPs there's no way to control emissions? I know that people have passed sniffer tests with only the OEM exhaust installed along with catless DPs, hence my wanting to try this approach of welding in the original cats.

Disappointing that they won't reach temperature, are you reasonably sure about that or guessing?
More or less guessing. I've seen a few people pass sniffer tests and I've seen a few people not pass with just secondary cats. I guess in certain situations it will work but not others.
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      04-09-2012, 10:41 PM   #12
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More or less guessing. I've seen a few people pass sniffer tests and I've seen a few people not pass with just secondary cats. I guess in certain situations it will work but not others.
Interesting. Perhaps driving distance to the testing centre has something to do with that.

I remember reading somewhere that some 7 series were equipped with catalytic "pre-heaters" to help initial emissions on startup. I haven't been able to find any kind of aftermarket "pre-heater" through searches so far though.

I'll have to look up an exhaust shop locally, maybe they'll have some more knowledge or have run into this setup before..
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      04-10-2012, 05:32 AM   #13
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For anyone with just midpipe cats, how's the smell?
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      04-10-2012, 06:38 AM   #14
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Quote:
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For anyone with just midpipe cats, how's the smell?
also interested in this question.
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      04-10-2012, 07:50 AM   #15
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I dont think people are being rude, but being realistic. If you are truly interested in running a clean emissions vehicle, keep all the cats stock. If somehow you feel running without the secondary cats makes you 'clean enough'...Ok... Others feel their car without any cats is clean enough.

If you care more about the smell and if you can get caught, that is another story.
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      04-10-2012, 07:51 AM   #16
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Not bad. You can only smell it I your standing at the trunk while idling.
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      04-10-2012, 12:21 PM   #17
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Cars with no cats on them STINK. I'm too old for it, and the minimal performance gain just isn't worth it. Drive one in DC traffic then get out of your car an hour later. You will smell like exhaust fumes.
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      04-10-2012, 07:06 PM   #18
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Cars with no cats on them STINK. I'm too old for it, and the minimal performance gain just isn't worth it. Drive one in DC traffic then get out of your car an hour later. You will smell like exhaust fumes.
Thank you! Finally someone who's making sense. The smell is horrid with no cats at all. I can't believe people run like this in their daily driver.

I think I've decided on an approach to try. I'm going to get some cats welded in to the exhaust section near where the original secondary cats were. I'm also going to take some advice from others that have suggested heat wrapping the exhaust to the cats to keep most of that exhaust heat in so it reaches further back in the exhaust more quickly. I think this should cure the smell problem and as I've heard it's possible to keep it legal (pass sniffer test) with just secondary cats I think the heat wrap will help go the extra mile and heat the cats up more.

Hopefully it works out!
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      04-11-2012, 07:01 AM   #19
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I have an AE cat back with high flow cats. Just ordered VRSF catless Dp's. With just the high flow AE secondary cats in place do you guys think that will smell bad as well?
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      04-11-2012, 07:34 AM   #20
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Quote:
Originally Posted by branabolic View Post
I have an AE cat back with high flow cats. Just ordered VRSF catless Dp's. With just the high flow AE secondary cats in place do you guys think that will smell bad as well?
I have AR catless DPs with AE HFC's and there is almost no smell. When the car was fully catless, it smelled extremely bad.
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      10-02-2012, 03:21 PM   #21
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AE Peformance Exhaust with AE 2nd HFC's

Hi guys,

Any photos of system installed ?

Any vids of the sound ?

Tried calling **********s and AE performance to ask abt the AE exhaust c/w 2nd HFC's and no joy.

Do you AEP part numbers to help me out

Thanks,

Dave

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I have AR catless DPs with AE HFC's and there is almost no smell. When the car was fully catless, it smelled extremely bad.
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      10-03-2012, 11:36 AM   #22
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OP,

So did you not know this prior to getting your catless DPs installed? What were you expecting? Lol.....sigh.

If it were me, I would just put back on the OEM exhaust, sell the catless exhaust and get an axle back exhaust.
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