2nd cat delete on 200k - Mercedes Benz SLK Forum

Performance MODS for the R171 Tuning and Performance modifications for your SLK

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#1 Old 05-22-2018
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2nd cat delete on 200k

Hi,
did someone try this on 200k (I saw you did on 350 or 55amg)? Any performance gains (I plan to do do puley+remap too)?
I deleted 2nd cat few days ago (swap with straight pipe for now). I like the sound now, it is little louder between 3000-4500rpm. I don't know if it is better to put sport cat (100cell or 200cell) or some resonator now instead of just straight pipe?

Does this harm the engine or any other parts on the long run?
Will I pass next year CO emission test without 2nd cat, does primary cat do enough ''job''?
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#2 Old 05-22-2018
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Could someone please explain what the purpose of the second catalyst is (or is it a CC ?). The secondary O2 sensor is between the two catalysts and on three cars I have never seen the second vary more than one point off center. If the first stops working it set a code. They do not check the second one. So what is it for please ? Would not surprise me if just a resonator.


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#3 Old 05-22-2018
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Originally Posted by Padgett View Post
Could someone please explain what the purpose of the second catalyst is (or is it a CC ?). The secondary O2 sensor is between the two catalysts and on three cars I have never seen the second vary more than one point off center. If the first stops working it set a code. They do not check the second one. So what is it for please ? Would not surprise me if just a resonator.
My understanding is as follows (on unmodified cars) - please feel free to correct this if I'm wrong!
@i860 corrected me and so I've updated this post with the upstream o2 sensors and marked the resonators as secondary Cats/resonators as he's confirmed that these are used to help warm up the main Cat from cold.


V8 (AMG)
Manifold - o2 sensor- Cat - o2 sensor - secondary Cat - Xpipe* - rear boxes
*I think the 172's have a Xpipe as std where the 171 don't.

V6 (350,280)
Manifold - o2 sensor - Cat - o2 sensor - secondary Cat/resonator - Xpipe - rear boxes
Not sure if this is the same on the 170

4cyl (200) supercharged
Manifold - o2 sensor - Cat - o2 sensor - secondary Cat/resonator - Y pipe - rear boxes .

Now, I think the 'resonator' should really be called an 'anti- resonator' as, since I've had mine removed, I get a much better exhaust note but the pipes do resonate (or rasp) a little when you put your foot down hard at low revs.
I still really like this mod and think they should have done it as a standard option.

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Last edited by mike350; 05-26-2018 at 03:04 AM.
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#4 Old 05-22-2018
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Quote:
Originally Posted by Padgett View Post
Could someone please explain what the purpose of the second catalyst is (or is it a CC ?). The secondary O2 sensor is between the two catalysts and on three cars I have never seen the second vary more than one point off center. If the first stops working it set a code. They do not check the second one. So what is it for please ? Would not surprise me if just a resonator.
The second cat is there to control emissions during warm-up of primary cats. It's not checked, presumably, because at present all tests are performed on a hot engine.
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#5 Old 05-22-2018
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My O2 sensor locations



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#6 Old 05-22-2018
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Mike: Really the sequence on my Merc (and XF and Jeep) is manifold-O2-cat-O2-cat-muffler.

The first O2 is used for feedback to the mixture control. The second sensor is to determine how well the cat is functioning and should not deviate from the midpoint. The second cat does not seem to do much or trigger a code.
My past experience is a single cat makes a good muffler if you like to hear your exhaust.

On the whole I like Mercs and while they do a lot of neat things, others seem really strange (e.g. in an electronic world, a suck/blow door lock ? They had a bunch left over from the 1950s ?).
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#7 Old 05-23-2018
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Are you sure that 200k has resonator and not 2nd cat? Because inside it looks just like the cat?
If I put sport cat, will the sound be quieter than now with just straight pipe?
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#8 Old 05-23-2018
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I’’ll probably empty 2nd cat box and put perforated pipe trough it, so it won’t be visually noticable. Or do you have any other suggestions what to do?
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#9 Old 05-23-2018
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Originally Posted by nikon93 View Post
Are you sure that 200k has resonator and not 2nd cat? Because inside it looks just like the cat?
If I put sport cat, will the sound be quieter than now with just straight pipe?
The simple answer is 'no', as I'm basing my comments on what I've read on this Forum and what's on page 89 here... https://books.google.co.uk/books?id=...system&f=false

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#10 Old 05-23-2018
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The op car if he has a 1.8 petrol has got a resonator not a second cat .

So cat - resonator and then rear twin back boxes .
So if he wanted to can remove it but it will sound diff and poss a performance hit in certain areas .

His best bet is do the back boxes replacement for sound and poss performance tweak
If he has the facelift version he allready has the pulley upgrade done .

But unclear as he is on the year change for the start of facelift .
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#11 Old 05-24-2018
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Would the removal of the resonator cause any performance issues. I thought it's sole purpose was to reduce vibration an noise ??
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#12 Old 05-24-2018
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Resonators usually reduce a drone at certain speeds. They usually do not have much restriction. Putting a muffler at the back of a car is where the exhaust has already cooled and slowed down a bit. Have run cars with just a cat and turbos with a straight pipe, both sound good to me.


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#13 Old 05-24-2018
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Mine is pre-facelift model. I plan to do custom made mufflers in the future, but for now I am satisfied with the sound I got from just 2nd cat delete. I'll just put smaller pulley and do a remap to get a bit more power from my slk.

So if this is resonator and not cat, than it definitely won't take any effect on emission test. But I really thought that I've had 2nd cat, because of the appearance (it has ''cells'' inside of it).
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#14 Old 05-25-2018
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Originally Posted by nikon93 View Post
Mine is pre-facelift model. I plan to do custom made mufflers in the future, but for now I am satisfied with the sound I got from just 2nd cat delete. I'll just put smaller pulley and do a remap to get a bit more power from my slk.

So if this is resonator and not cat, than it definitely won't take any effect on emission test. But I really thought that I've had 2nd cat, because of the appearance (it has ''cells'' inside of it).
We had this done to our 2010 350, and were told they looked like cats from the outside but were in fact resonators, so had no secondary cats only primary. May be your model is the same.


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#15 Old 05-25-2018
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Quote:
Originally Posted by mike350 View Post
My understanding is as follows (on unmodified cars) - please feel free to correct this if I'm wrong!


V6 (350,280)
Manifold - Cat - o2 sensor - resonator - Xpipe - rear box
Not sure if this is the same on the 170

V6 350 r171
Manifold -o2 upstream (regulates stft ) -1st cat -o2 downstream ( ltft ) -2nd cat ( used on cold start to speed up the temp in 1st cat, some country require cold emission test, not like in UK) - resonators - x pipe - back boxes.
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#16 Old 07-10-2018
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Hey folks, alternative for resonance, a J-pipe resonator. It's just a tube attached to the exhaust (1 for each line) that bends in a J, capped at the end. Lightweight, and is what the manufacturers are using these days to control resonance. This link has information on how to properly tune a J-pipe to maximize dampening, if anyone wanted to look into it to solve a resonance problem. According to this link they are popular in Europe, so there may even be folks in Europe who make a kit for these cars.
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#17 Old 07-10-2018
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May I suggest a study of Helmholtz resonance ?


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#18 Old 10-04-2018
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I was reading about how the J pipe admits sound waves, but not exhaust gases, then bounces them back again, negating the ones coming down and cancelling the drone, or suchlike. I couldn't help but think about what effect, if any, putting a lipop baffle in a tail pipe would have?
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