4.2 Piggy Pipes

[Battle_Tested]

I want gut the pre-cats on my 4.2. I currently have a after market resonator welded right after the main catalytic converter's. Is it possible to drop the down pipes while the two are connected or do they need to be independent of each other for removal?

[rollerton]

You'll need to separate the exhaust just past the lower / main cats and then you can remove the downpipes. Drivers side is pretty easy, just the 3 bolts and you can drop and lower the downpipe. Passenger side I'm fairly sure you need to remove the trans mount for enough room to get it out.
FWIW those upper small CATs flow pretty well and the pipe is expanded/ larger diameter. If you were going to do anything I'd gut the main cats first. The sound is also EPIC with the big cats emptied out.

EPIC!

[rgtorque]

this is just wrong. gut the precat or both. the precat is to close to the engine and allows for a lot of heat buildup in the motor which leads to cylinder scoring and other issues. read up on this on the b6 s4 forum.

[rollerton]

If you say so. B6 has a V8, but otherwise that engine is 100% completely different in design, materials, etc..But sitting here looking at thes and you can literally see through those tiny upper cats. Not saying there's zero restriction there...but they're nothing compared to the foot long brick/ pyramid cats underneath....which I also have here to compare directly.

[a4kamila6]

I am curious how gutting the pre-cats affects cylinder exhaust gas scavenging. N/a needs a little back pressure.

If i were you, id leave the pre cats and go straight pipe right after all the way back.

[Battle_Tested]

Ive seen pics of the precats of a B6 S4 4.2 and those look pretty restrictive. My current setup is factory cats (pre/main) into a Super Sprint resonator. I removed the rear mufflers which gives the car a nice deep tone without any drone. Car isn't loud but makes enough noise to let you know its a got a V8. Since the 6 speed I think the car seems quieter than what it was with the stock trans. I do want some extra noise but really want free up the car to gain some HP/TQ I might just gut both pre/mains and run a X pipe to the rear mufflers or run gutted cats to resonator only...

[rollerton]

I'd say either/ and /or all/both.

If I had to do one, it'd be the big one underneath. But one of the things Audi did to get some HP was eliminate the upper cats all together on the S6..so I don't think that would have a negative impact. But if you were starting anywhere, don't just leave the big ones. I think any gains from gutting the uppers would be undone in the lower ones.
Here's the upper pipes/ cats.
I mean there's CAT inside the pipe, but I doubt it's slowing much airflow in reality. It's N/A after all. They're only like 4" long.

[rgtorque]

If you say so. B6 has a V8, but otherwise that engine is 100% completely different in design, materials, etc..But sitting here looking at thes and you can literally see through those tiny upper cats. Not saying there's zero restriction there...but they're nothing compared to the foot long brick/ pyramid cats underneath....which I also have here to compare directly.

im not really sure we are on the same page here... the allroad 4.2 v8 and the s4 4.2 v8 are damn near identical engines, as is the 05 and 06 a6 4.2. all 3 engines are alusil blocks with chain drive. diff in injectors/programming, maybe a cam but otherwise the same.

the piggie pipes topic head been beaten to a bloody pulp.

read about it here

http://www.audizine.com/forum/showth...Piggies%29-FAQ

and here

http://www.audizine.com/forum/showth...ONG?highlight=

[rollerton]

im not really sure we are on the same page here... the allroad 4.2 v8 and the s4 4.2 v8 are damn near identical engines, as is the 05 and 06 a6 4.2. diff in injectors/programming, maybe a cam but otherwise the same.

Good point.. he didn't say whether it was an Allroad or not. Not many people mod the 4.2 Allroad, I always assume it's a belt driven C5 4.2. ....if it's an Allroad then who knows. With the gearing those things have what the heck is it even about.

[happy2B5]

[02 A6 2.7T]

Good point.. he didn't say whether it was an Allroad or not. Not many people mod the 4.2 Allroad, I always assume it's a belt driven C5 4.2. ....if it's an Allroad then who knows. With the gearing those things have what the heck is it even about.

I believe you assumed correctly, since the OP's profile says ...

Daily Driver::
2001 AUDI A6 4.2 6MT

[Battle_Tested]

Update.
Got the main cats replaced with test pipes and what surprise on how much it woke up in the sound department. I expected it to add some rumble but its ridiculous raspy on full throttle in 1st & 2nd gear Current set up is factory pre-cats with test pipes to a super sprint resonator mid section without the super sprint rear mufflers. Full throttle car sounds pretty raw and no drone cruising. Car does feel more lively but sound DB are close to the limits I think for a DD. Got the new set up flange so I can drop the DP to free up the pre-cats next.

[rgtorque]

Again, people don't really gut main cat. Tgey gut the precat or both.

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[rollerton]

Again, people don't really gut main cat. Tgey gut the precat or both.

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In the 'S' V8's there is only one cat on each side underneath, no pre-cats. So Those for surely get get gutted. But the normal 4.2 the pre-cats don't look like they cause much restriction/ flow problems at all. The downpipe diameter increases a bit through the cat, but they're round, short and actually not very large.
If I was doing any of it on a normal 4.2 I'd do the main cats first, I doubt there's much to gain from the small cats. But every bit counts I guess.

But have to say when I did it on my old S6, JUST gutting the main cats gave it awesome sound. But I left the cats there, didn't weld in a pipe or anything, just opened up the cat, emptied it out and welded it shut. And it sounded awesome. Actually very quiet unless you stomped on it.

[JoelKnauber]

I am curious how gutting the pre-cats affects cylinder exhaust gas scavenging. N/a needs a little back pressure.

lol

[Battle_Tested]

Pre-cats are done. Car is finally running full catless piggy pipes. The pre-cats do look much more free flowing and are smaller than the main cats. But once you knockout all the material in pre-cats, you realize how restrictive they still are. Car revs so much better on the top end now. I definitely would recommend having both or one of the cats removed for any 4.2 owners.