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I wad looking at getting rid of the cats ghetto way by just removing what's inside them, but doing some research on here it seems that low end torque is affected negatively and someone even reported drop in power after decat, so now I'm not entirely sure if it's a good idea.
What have people experienced after decat, pros and cons?
'93 Audi 100 Avant - R.I.P
'92 Audi UrS4 Avant - SOLD
'93 Audi UrS4 Avant
'15 Audi A7 quattro 3.0 TDI
not gutted an existing cat to achieve a de-cat but on cat replacement pipes on my 3b its all good. No issues with low end torque, car breathes easier/free-er and then there's the sound
Hollowed cats always sounds like a bad idea to me as you are creating a large void for the air to expand, then trying to squeeze it out a smaller hole again, i wouldn't be surprised if it performed worse.
I've always run by pass pipes on mine and only put the cats in for the yearly test, but i was too lazy to swap them out last year and left the cats in. It seemed to go better low down with the cats, less laggy if you like. My next test is due at the end of the month so once that is out of the way i will put the bypass pipes back in and see what difference i notice. It does without doubt go better from 4k rpms up with the bypass pipes, or so my butt dyno tells me.
91CQ20v - Gone to a new home
93UR-S4 - The Magic Carpet
94S2Bus - The Emerald Express
Decat pipes rather than empty cats for meand. There is i noticeable difference in pick up, sound, top end and butt dyno. A good exhaust manufacturer should be able to make them cheaply and easily.
The Perfectionist
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Audi Class Concours winner Stoner Park 2014, ADI Concours D'Elegance Class Winner 2008, Runner up 2014. Winner, S2Forum Show'n'Shine Billing 2006 & 2008.
Thanks for the input guys. Ideally I'd like pipes but they're €200 and really I can't justify spending that on the old rusty exhaust which will be replaced in the future completely.
I have a gas-less mig welder which I could use to make the pipes but again, don't know how to deal with the bends.
I will also be removing middle silencer completely and replacing with straight pipes and gutting the rear one just so it's not too loud.
'93 Audi 100 Avant - R.I.P
'92 Audi UrS4 Avant - SOLD
'93 Audi UrS4 Avant
'15 Audi A7 quattro 3.0 TDI
Been thinking about that too. Something to consider. I will take it off and see whether it's possible to have it made up of straight pipes and some ready bent stuff you can buy.
'93 Audi 100 Avant - R.I.P
'92 Audi UrS4 Avant - SOLD
'93 Audi UrS4 Avant
'15 Audi A7 quattro 3.0 TDI
I'd go for de-cat pipes every time over gutted originals, I tried a gutted original on one of my Corrado G60's and it sounded awful compared to the straight through pipe I fabricated to replace it.
Your old cats are worth at least £50 each in scrap if you find a fair decent scrap metal dealer and probably a bit more than that to someone who wants to use them.
What age is your car as getting it through an MOT could be a problem if it's manufactured after 1993/4, unless of course you have a 'friendly' tester.
1990 B3 2.0 80 quattro.
1992 C4 2.8 Avant quattro
1994 MTM RS2.
1998 B5 TDI quattro Sport.
2000 B5 TQS.
I'd go for de-cat pipes every time over gutted originals, I tried a gutted original on one of my Corrado G60's and it sounded awful compared to the straight through pipe I fabricated to replace it.
Your old cats are worth at least £50 each in scrap if you find a fair decent scrap metal dealer and probably a bit more than that to someone who wants to use them.
What age is your car as getting it through an MOT could be a problem if it's manufactured after 1993/4, unless of course you have a 'friendly' tester.
I would prefer pipes to replace buthe as I mentioned I don't want to be paying hundreds to make it, I rather leave them in in that case.
Ah that's good to know, maybe can get material cost back they're very rusted out so doubt they're any good for anyone
I'm running LPG and on last MOT emmisons were 1/3 of maximum so that shouldn't be a problem.
'93 Audi 100 Avant - R.I.P
'92 Audi UrS4 Avant - SOLD
'93 Audi UrS4 Avant
'15 Audi A7 quattro 3.0 TDI
I would prefer pipes to replace buthe as I mentioned I don't want to be paying hundreds to make it, I rather leave them in in that case.
Ah that's good to know, maybe can get material cost back they're very rusted out so doubt they're any good for anyone
I'm running LPG and on last MOT emmisons were 1/3 of maximum so that shouldn't be a problem.
Cool, how are you finding the performance with LPG, what software are you running?
I think it's mandatory for car's manufactured after 1993/4 to have cat's if they were originally fitted. Though I could be wrong.
Yeah they have good scrap value as long as the gut's are in reasonable shape, that's what's worth the money, doesn't matter what their like on the outside.
I got £50 each for some I removed from of various four cylinder Audi 80's as well as one from a NG Coupe and their a fair bit smaller than the S2/S4 units. Conversely I got a fiver for a crappy after-market one I removed from a V6 couple, it's the originals that have value. They must have a lot more of the valuable metal in them.
1990 B3 2.0 80 quattro.
1992 C4 2.8 Avant quattro
1994 MTM RS2.
1998 B5 TDI quattro Sport.
2000 B5 TQS.
Cool, how are you finding the performance with LPG, what software are you running?
I think it's mandatory for car's manufactured after 1993/4 to have cat's if they were originally fitted. Though I could be wrong.
Yeah they have good scrap value as long as the gut's are in reasonable shape, that's what's worth the money, doesn't matter what their like on the outside.
I got £50 each for some I removed from of various four cylinder Audi 80's as well as one from a NG Coupe and their a fair bit smaller than the S2/S4 units. Conversely I got a fiver for a crappy after-market one I removed from a V6 couple, it's the originals that have value. They must have a lot more of the valuable metal in them.
Personally see no difference in performance. Big difference is when I fill up every week For LPG I have KME Diego G3 ECU and car is stage 1 mapped.
Dad works at the garage that do MoT so I'll ask him to check whether cats are needed if emmison levels are sufficient. I remember taking half out of each cat on my old S4 and passed MOT no problems, and it was clear it was cut open at some point.
Mine seem to be fully working, and pretty big too so I ain't throwing those out
'93 Audi 100 Avant - R.I.P
'92 Audi UrS4 Avant - SOLD
'93 Audi UrS4 Avant
'15 Audi A7 quattro 3.0 TDI
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they're very rusted out so doubt they're any good for anyone 
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