Decatting an S8 (D2)

[Abs]

Hello all,

Just wondered if anyone has decatted their standard S8 exhaust system?

If so, any possible advantages or disadvantages in doing so?

Cheers

[treetotreef]

I thought about this also. My '98 S8 has 2 cats and the 2 lambda sensors are before them, so there shouldn't be any issue with the ECU...but i haven't worked out yet why the CAT light in the dash sometimes comes on. It must be the lambda sensors up front detecting temps outside the usual range for whatever reason (unburnt fuel burning in the cats due to poor spark/coil pack i suspect in my case).

But to answer your question, you'll probably loose some low end torque and gain some top end power. the opening part of the cat also doubles up as an expansion chamber, which helps when the gas speed is low. when the gas speed is high, as you might guess...the cat is seen as a restriction. And you'd have to put them back for MOT i suppose...

[Abs]

Thanks for the reply! I'm aware of the error you'll get due to the lamda probe's. Had this error on all my previous audi's when I decatted them. It's not a problem.

Just wondered if there were any other advantages or disadvantages in doing it? I'm guessing the exhaust noise will be considerably increased too?

[treetotreef]

When i've done it to other cars it certanly increased throttle response, and a little louder too. sound proofing on these cars seems to be pretty dam good, so it shouldn't be too bad inside.

here's an interesting thread on exhaust note from the V8:

AudiWorld Forums: The x-pipe removes the raspy tone that normally results from the uneven firing order of a V-8.

this guy appears to know what he's talking about when it comes to pulse tuning.

[LostKiwi]

There are a couple of us here already with X-Pipes... I have one, Figure11 has one and so does Botang (A8_Tony may have one too but I'm not sure).

Does it make a difference? I started my exhaust mods with an H-Pipe then found out about the X-Pipe. Moved to the X-pipe (had to get it specially made) and it definitely made it smoother to the point where at certain revs it 'howls' as opposed to the more usual beat of a V8. Mid range torque seemed slightly better but economy seemed to suffer slightly (but that could have been me listening to it!).

There is a bit of information in the 'More Exhausts' thread here:

http://www.tyresmoke.net/forum/audi-a8-s8/96085-more-exhausts.html

Also some excellent info here:

http://www.planetsoarer.com/pdf/xfactorcolour.pdf

[Abs]

Excellent stuff! So the X-Pipe is in an "X" shape pipe, which is an alternative to having 2 seperate decat pipes?

Do you have the contact details of where you got yours done from? I normally have all my exhaust work done from MIJ in Walsall, so I will have a chat with them soon. Do you have any pictures of your x-pipe?

[mkoneman]

you decided to keep it in the end then abs?

[Figure11]

If your going to decat, don't get to excited about your MOT. have a read of this. As for the CAT light, as I understand it doesn't tell you anything about your Cats as it only measures Oxygen content in the exhaust gas as well as it's temperature. Removing your cats should cause any problems.

Have a read of this. Especially the second post.

Mike

Edited August 15, 2008 by Figure11

[Abs]

you decided to keep it in the end then abs?

lol I keep changing my mind! Some days I love it, and some days I don't! It's the only car I have ever owned which I haven't modded, so I think I'm just bored of it. Hence, the idea of decatting. I ideally would like to keep the existing set up, but its just tooooooooooo quiet. So I thought of decatting, and just wanted to know if it was a good idea?

I'm having it MOT'd this week anyway, so I will have a full year without having to worry, and when MOT time comes, I can always put the cat's back in.

[Sven]

Don't do it, it'll be a waste of time and money. I tried it when I fitted the Quads, had some nice exchangeable replacement pipes made (for our version of the MOT), but ended up with less power/torque and way too much noise. The S8 likes a bit of back pressure.

With the Quad pipes, X-crossover and the cats in place it sounds great and performs great.

The X pipe however is a very good idea. I happened to be the first here, and several others followed. It'll give you a bit extra power (I've had the regular 'H' before the X was fitted, and tested both - the X gave better results), plus it sounds nicer (less USA style rumble, more howl).

See also this thread.

Noise wise, best way is a custom exhaust obviously. If you want to fiddle with the OE exhaust you can try to lose some dampers, but keep the cats.

Edited August 19, 2008 by Sven

[A8 Doc]

I removed the main resonating box on my D3 A8 and it made a nice subtle difference to the sound of the car. Just enough to hear the V8 but without sacrificing motorway noise.