My exhaust consists of:
4kq manifold and downpipe - a big improvement over the stock manifold. This is a nice bunch of pipes that funnel down to about a 55 mm outlet. This displaces the OXS, so you need to drill a hole further down the system and weld in an "OXS bung", they're about $7 at speed shops, and install a heated OXS (it's too far from the head now to heat up quickly on its own). I discovered the downpipe touched an unused boss on the transmission, Mr. Sawzall and I made quick work of that. Luckily no transmission fluid came spewing out when I did this!
The next piece is a bit of a junk 1981 5kt's downpipe, I just cut a couple of pieces of it that had approximately the right curves and welded them together. One already had the donut-type flange to attach to to the downpipe, the other end required a bit of time with a big hammer at the vise. This piece is where I mounted my OXS.
Then I used the catalytic converter from the 5kt.
After that, I had a local Meineke bend up a couple of pieces of aluminised pipe to get to the muffler off that same 5kt, and a clumsy little tailpipe. Here is a photo of the results. (21 & 18 k)
This system is loud, bassy, and sometimes more than a bit nasty. I haven't heard it being driven, but I'm told it sounds damn "sporty" whatever that means. Sometimes it is quite unobtrusive, other days the rumble at lower revs gets a little noticeable. I've been stopped and hassled twice by REO's for it. Innocent passengers don't seem to notice, though, unless they are just too polite to mention it.
I probably gained 8-10 hp from this. 5 for the downpipe and manifold - that's the difference between a 4kq and Coupe of the same year. Another 3-5 for the larger diameter pipe and the less obstructive muffler and "cat". On an 82 Coupe that is 10%, which is very noticeable. This is probably the one genuine HP increasing project for an Audi coupe that can be done without opening up the engine. When the muffler goes, I'll probably buy an aftermarket free flow unit and a "cool" tail pipe and weld it on. I do a lot of highway driving, so hopefully the muffler will last a long time.
Not bad considering a cat back stock system on sale from aftermarket folks runs about $300 for my car. Sure, I made out by using parts from my junk car collection, but that's half the fun!
The next system will probably be built of stainless steel pipe pieces and I will try to detail the entire set up so it can be duplicated. While the 4kq em/dp seems to be a regularly mentioned upgrade, few ever seem to have done it - I think it is the pain of dealing with the rest of the system that makes it a chore. If I can determine the angles and lengths of piping needed perhaps more people will be encouraged to try this!