Secondary Cat removal questions - Mercedes Benz SLK Forum

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

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#1 Old 08-17-2010
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Secondary Cat removal questions

Please no links to other threads. My exhaust set-up right now is a muffler delete only. I heard that removing the secondary cats will make it louder but also is there a hp gain?
-How much louder will my exhaust get and how will the sound change?
-How much power gain?
-Will it negatively effect my engine in the long run?
Also I wont gut them because when emissions testing comes around I'd like to have working cats.
Thanks!
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#2 Old 08-17-2010
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I estimate 8 dB louder at WOT
maybe 5-10 HP. they are pretty restrictive
no
My guess is both secondary cat and muffler removal will be harsh/annoying at times.
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#3 Old 08-18-2010
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You might want to try installing metallic racing cats in place of your secondaries, they should allow better sound and airflow, but still allow you to pass california's strict emissions laws. It won't be as good as removing the secondaries all together but it's a good compromise. The fact that you had your mufflers removed makes me think doing a secondary delete would be too much. I think you can do one or the other, but not both, otherwise you will probably attract some unwanted attention. There comes a point when the sound goes from loud to simply obnoxious. More isn't always better.

I personally want to get my secondaries out but will leave my resonator (as it acts as an H-pipe) and mufflers. The resonator and mufflers are AMG and actually pretty good, the stock headers, primary, and secondary are standard mercedes and horrible. The secondaries are ceramic and very restrictive. My issue if finding a place that will do it, I have visited several shops and asked (in a hushed voice, as if I were trying to gain entrance to a prohibition-era speakeasy) if they will take them out and no one will touch them. It seems funny because I could go to any number of shops and get long-tube headers installed, which requires removal of the primary cats. But try to cut the weak secondaries out of the stock system and you'll hit a dead end. Perhaps I should try some less reputable shops?
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#4 Old 08-20-2010
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I don't mean to hijack the thread, but I'm looking to remove the secondary cats as well and was wondering how much and what type of piping to buy. I'm thinking about 4 feet of 2.5 inch diamter 304 stainless would suffice? Do any of you know if the pipes need to be treated in any special way for the heat? Or can I just go to a steel shop and buy them off the shelf?

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#5 Old 08-20-2010
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A custom muffler/exhaust shop will have it in stock. There's no need to treat it.
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#6 Old 09-10-2010
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I have removed my Secondary Cats AND the Resonator off an R171 SLK55 AMG. I replaced them with straight piping and a Supersprint X-Pipe respectively.

I do not find the sound too loud, or annoying ... In fact I absolutely love it, my car now sounds like it has the proper high performance V8 that the fine folks at the AMG plant imagined when they built it.

I rev faster with less lag now that I've stripped away all that restriction / back pressure, even my fuel economy has gone up about 1 MPG as well.
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#7 Old 09-10-2010
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That is my configuration, plus Green filters. I am looking to add headers and hi-flow cats now. Enjoy your beast!!
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#8 Old 09-11-2010
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Originally Posted by Nullsign View Post
I have removed my Secondary Cats AND the Resonator off an R171 SLK55 AMG. I replaced them with straight piping and a Supersprint X-Pipe respectively.

I do not find the sound too loud, or annoying ... In fact I absolutely love it, my car now sounds like it has the proper high performance V8 that the fine folks at the AMG plant imagined when they built it.

I rev faster with less lag now that I've stripped away all that restriction / back pressure, even my fuel economy has gone up about 1 MPG as well.
Sweet but I'm worried about getting f***ed if I get pulled over. I hear that the cops go really anal over cat removal
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#9 Old 09-11-2010
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Sweet but I'm worried about getting f***ed if I get pulled over. I hear that the cops go really anal over cat removal
You may have an issue since you live in the Republik of Kalifornia... Most states have done away with emission inspection. Here in Florida I see a lot of "emission modified" cars and trucks.
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#10 Old 09-11-2010
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Sweet but I'm worried about getting f***ed if I get pulled over. I hear that the cops go really anal over cat removal
How would cops even know you removed them? It's not like they can get under an SLK w/o it being jacked off the ground to visually inspect the bottom, the Primary Cats can't be seen from the bottom anyways, only the Secondary Cats.

And even then - what cop would know what the exhaust system of a stock SLK is supposed to look like in the first place?
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#11 Old 09-17-2010
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UPDATE: Got a ticket yesterday for no mufffler. It's a fix-it ticket and the only thing I have to pay for is putting my stock mufflers back on. My guy is gonna charge me $80. I'm gonna miss my beast exhaust and I dont know if I'm gonna put it back on after the ticket gets cleared. ****
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#12 Old 09-17-2010
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Was the primary reason for being stopped the noise? Or was the fix-it ticket a secondary offense? Is your car louder than a Harley?
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#13 Old 09-17-2010
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Originally Posted by xopher View Post
Was the primary reason for being stopped the noise? Or was the fix-it ticket a secondary offense? Is your car louder than a Harley?
That was the only reason he pulled me over. It's pretty loud but when I saw him I was driving like a grandma. It was probably the gurgle sound that gave it away
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#14 Old 09-18-2010
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That was the only reason he pulled me over. It's pretty loud but when I saw him I was driving like a grandma. It was probably the gurgle sound that gave it away
Weird...

You removed the resonator I thought, not the muffler tips .. so it's odd to get a ticket for 'no muffler' ...

I hope you didn't admit to changing anything (rule #1 - never talk to the cops!), what you've done to your SLK is no different then the 'stock' exhaust system of an exotic or supercar, so this smells like a ******** ticket to me.

I guess it depends on your state with how they define things.


Personally, I'd fight it...
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#15 Old 09-18-2010
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Much cheaper, plenty of noise and legal:
http://nyti.ms/aHVyEz
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#16 Old 09-18-2010
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Originally Posted by lkchris View Post
Much cheaper, plenty of noise and legal:
http://nyti.ms/aHVyEz
You just made me throw up a little in my mouth ...
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#17 Old 09-18-2010
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question???

so u guys remove the catalytic or resentor, will the engine check light on?how to prevent it to on?what you guys did to the sensor to prevent it?
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#18 Old 09-18-2010
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Originally Posted by Harold View Post
so u guys remove the catalytic or resentor, will the engine check light on?how to prevent it to on?what you guys did to the sensor to prevent it?
On my SLK55 (V8 Engine) - I removed both the SECONDARY cats and the Resonator (they are located next to each other) and replaced it with an X-Pipe and Straight Piping.

I have no check engine lights, I am throwing no engine codes when I run the diagnostic from my OBDII reader, and according to my shop, I'm still Texas emissions legal so I can pass yearly state inspections.

Note: Removing/Replacing the PRIMARY cats is more difficult in that you must relocate an O2 sensor, so it's a more complex change out. However this is NOT the case for the SECONDARY cats (which only exist because of California)
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#19 Old 09-18-2010
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?

what if i'm using a stock ecu?will the engine check light on?anyway what is Xpipe?what the different after putting a Xpipe?
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#20 Old 09-18-2010
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Originally Posted by Harold View Post
what if i'm using a stock ecu?will the engine check light on?anyway what is Xpipe?what the different after putting a Xpipe?

I am using a stock ECU. I've had no issues.

an X-Pipe is like the name suggests, it's a stright piping for dual exhausts that crosses both exhaust lines from each side of your headers. making an "X"

They are used for V8/V10 engines, due to how they fire, they increase efficiency and reduce backpressure when the engine firing sequence hits the point where more pressure is being sent down Pipe A than Pipe B, by crossing the pipes the exhaust can escape down either channel.

In your case, I would just remove the resonator and replace it with Straight Piping. I have not heard of any real performance gains with X-Pipes on V6 engines, but I could be wrong.

Attached are the pictures of a BEFORE and AFTER of these exhaust changes to an SLK55. (Resonator replaced by X-Pipe, and the Secondary Cats removed and replaced with stainless steel straight piping)

By removing all that useless restriction out of the mid-exhaust, I was able increase noise (it sounds beautiful), increase fuel efficiency (+1.5~ mpg), increase rev rate (it spins up so fast now!) ...

this is a giant win for a SLK55.
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