Will OEM cats fit with Long Tube Headers?
Thread Starter
Senior Member
Joined: Sep 2010
Posts: 432
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From: BayArea, CA
2013 C63 AMG
Will OEM cats fit with Long Tube Headers?
After 12 years of ownership with LTH and no cats, I'm tired of jumping through hoops to pass California smog. I bought it new originally, so I was exempt from smog tests the first 8 years, but the last 4 years I've had to find shops that will pass it with a wink and a few hundred dollars. I actually had 200cell high flow cats installed before my last smog test thinking that would solve my problem, but I still had to run an extension for the o2 sensor to prevent getting a CEL and that failed visual inspection. Anyway, it's getting more and more difficult to find a shop that will "pass" it these days and I've decided to throw in the towel and go back to a legal setup so that I can sell the car.
My question is, will OEM cats actually fit on a setup with LTHs? I realized that the spot where the cats normally would be, is occupied by the LTHs. Has anyone ran LTH with stock cats somehow? I'm questioning if going back to stock cats means I'd actually need to install the OEM headers. At that point I'd probably just install the entire stock exhaust...
My question is, will OEM cats actually fit on a setup with LTHs? I realized that the spot where the cats normally would be, is occupied by the LTHs. Has anyone ran LTH with stock cats somehow? I'm questioning if going back to stock cats means I'd actually need to install the OEM headers. At that point I'd probably just install the entire stock exhaust...
Senior Member
Joined: Oct 2023
Posts: 473
Likes: 240
From: New Hampshire
ML 63 AMG
After 12 years of ownership with LTH and no cats, I'm tired of jumping through hoops to pass California smog. I bought it new originally, so I was exempt from smog tests the first 8 years, but the last 4 years I've had to find shops that will pass it with a wink and a few hundred dollars. I actually had 200cell high flow cats installed before my last smog test thinking that would solve my problem, but I still had to run an extension for the o2 sensor to prevent getting a CEL and that failed visual inspection. Anyway, it's getting more and more difficult to find a shop that will "pass" it these days and I've decided to throw in the towel and go back to a legal setup so that I can sell the car.
My question is, will OEM cats actually fit on a setup with LTHs? I realized that the spot where the cats normally would be, is occupied by the LTHs. Has anyone ran LTH with stock cats somehow? I'm questioning if going back to stock cats means I'd actually need to install the OEM headers. At that point I'd probably just install the entire stock exhaust...
My question is, will OEM cats actually fit on a setup with LTHs? I realized that the spot where the cats normally would be, is occupied by the LTHs. Has anyone ran LTH with stock cats somehow? I'm questioning if going back to stock cats means I'd actually need to install the OEM headers. At that point I'd probably just install the entire stock exhaust...
GESI Gen2 UHO cats would likely satisfy your check engine light issue but long tube headers will always fail a visual inspection…especially in Commiefornia…!
MBWorld Fanatic!
Joined: Oct 2007
Posts: 2,299
Likes: 380
From: Texas
10 C six trizzle
different diameters depending on header size, ground clearance would be reduced and quite a bit of exhaust modification would be necessary. much easier to just go with a smaller high flow cat. It makes a huge difference.
Super Member
Joined: Mar 2009
Posts: 657
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From: Boston
2020 E63s Wagon, 2012 C63 AMG Coupe, 1989 560SL, 2023 911 Carrera T
nope, the long tubes basically go where the OEM cats were. i actually put my car back to stock headers around 10 years ago because of passing emissions problems in my state, 200cell hi flow cats did not do the job for me. since then i got another set of long tubes that have been sitting in my basement. in 2027 my car will be 15 years old and therefore exempt from emissions in massachusetts and the long tubes will finally go back in. I know this isnt an option in california unfortunately.
Super Member
Joined: Jul 2013
Posts: 742
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'12 C63BS "SlickTop", '08 CLK63BS, '16 E63S wagon, '12 CLS63, amongst other non-Merc toys
with that kind of car collection, look into registering your cars out of CA; say bye-bye to smog checks, ridiculous reg fees & c2c sales tax. 
Member
Joined: Aug 2021
Posts: 176
Likes: 59
From: NYC
500E - CLK BS
bay area CHP will stalk him and hunt him down. they are judge, jury, and executioner of under body panels and wheels if they tow the car
