Thinking about removing them and putting a straigjt pipe in on each down tube to replace them. Anyone here try it? I'm going to get a new muffler for the rear soon (aftermarket of course) and probably a new resonator to replace the pre-muffler.

Not only is it really bad for the enviorment, your exhaust smells awful.

A quality highflow cat is going to be almost as good as a straight pipe, just opt for one of those. Carsound cats are some of the best in the buisiness.

Further, it really won't open up any more HP unless you have larger diamater piping, and replace your exhaust manifold with some headers from TDL.

large diameter tubing isn't always the best. it allows gasses to expand, cool, thereby becoming more dense and harder to push out, creating more back pressure. so bigger isn't always better.

Well my cats are hollowing out, and I want to remove them so my exhaust will quit popping and back firing. Replacement cats are very expensive from Audi, and if a high flow cat is damn near the same as bypassing them, why spend the money on on the high flow. In Michigan we don't do emissions testing anyways, so who would know? I am also going to be replacing the mufflers but will probably keep the same pipe for now since its still good. If I do decide to get new pipe bent I would go with 2 1/4" thats all the bigger I'd need to go, since this car isn't turbo or supercharged, it wouldn't need as large a diameter pipe, I think 2 1/4" is plenty large.

I was thinking about the ported exhaust manifolds from TDL Racing or maybe just shelling out the $2200 for the equal length headers. I love the look of my car, its black, tinted, debadged, big wheels, soon to be lowered. Its all show and no go though, so I want to help it out. I know intake and exhaust are two easy ways to extract more hp from an engine. I'm also getting the ported throttle body and ported intake manifold. I suppose to hold me over for a while I could get the TDL ported exhaust manifolds. Eventually I want to try to put a vortech s-trim centrifugal supercharger on my car. If KamakazieP can mount an extra alternator on the car, I'm sure I can fit a centrifugal blower on mine. Plus I'm going to get a chip, and make a custom cold air intake that removes the air box and places a cone filter in behind the front bumper and fender. Shouldn't be too hard to make the piping, might have to extend the MAF wires and stuff but it doesn't look too hard to do.

You don't want to go bigger than the HP you are putting down, or planning to achive at some point. Going to small is just as bad.

Obviously, these cars were designed as luxury sedans not sports sedans like an M5. The Exhaust was designed to be quiet, and it is done by exhaust restriction.

Here are the diamater vs HP they are capable of handling

2-1/4" up to 210HP @ the flywheel (about 180-185 whp)
2-3/8" (60mm) up to 235HP @ the flywheel (about 200-207 whp)
2-1/2" up to 265HP @ the flywheel (about 225-235 whp)
2-3/4" up to 325HP @ the flywheel (about 275-285 whp)
3" big for big HP (Forced Induction: > 275 whp)

Just replacing the cats is going to have minimal gains because the exhaust system works as a package. Everything from the exhaust manifold, to the cats, to the piping, to the mufflers. You also don't want to replace the cats with stock diamater test pipes only to get a larger catback system and find that they are now a bottleneck because of the changes in diamater.

The C4 A6's have the performance problem where the catback is actual a 2-1 design. (There are two cats, which have two pipes, which merge into one.. with out increasing the diamater).

2-3/8" is really what you would want out of a C4, if you plan at some point to do all of TDL's mods (they claim a 60hp increase). Wait untill you get the rest of your exhaust system done before you do your cats, otherwise they are going to be holding your back untill you buy new ones.

Thanks for the exhaust diameter info. I don't want the car to beat a vette in the 1/4 mile but more power around town would be good. On the highway I don't really care since I usually cruise 85-90 mph anyways and the car does that just fine.

Maybe I could replace the cats with "off road test pipes" and then from the y-pipe back run a larger diameter single pipe like 2.5" to compensate and get an exhaust with a 2.5" inlet.

If and when I do the exhaust upgrades which should be in the next month or so, I'll post pictures and info incase anyone else is interested. Since not many people have done exhaust upgrades on the 2.8L I suppose I am sort of a pioneer?

Who carrys the CARSOUND cats? Do they have a website? What's the AVG cost for them?

Just wondering because one or both of my cats took a dump, and I really do not want to spend $1500 EACH for OEM ones.