The system starts with the manifold and downpipe from a 4000 Quattro.
First I welded a spare flange to this flexible piece.
The next piece is 1/2 of a 90 degree bend, followed by its other half, slightly cut down so the two angles would result in a straightened pipe.
The OXS bung is welded in to this section.
This catalytic converter was bought new from JC Whitney.
The section after the catalyst should run straight down the center of the tunnel, making sure it has adequate clearances to the parking brake parts, etc.
By the time I was done with these flanges ($5 each at a local chain auto parts store) and the curves needed to get over to the side, there was no room for a resonator. Oh well.
The section over the axle was built from three ninety degree bends (ells). After the flanges is the first, an ell to the left. This is followed by an ell (plus a little extra), angled upwards at 45 degrees. This mates with the third ell, which fits perfectly into the coupe floor pan. Then a 45 degree piece, half the ell I got the "little extra" bit out of, levels it off to get to the muffler.
This muffler is the Flowmaster I originally installed, which was louder than a straight pipe. I removed it and installed a 1974 Cadillac Coupe DeVille muffler. That started rusting and leaking within three days. I then installed a Super Silly Turbo muffler, which works pretty well.
See the next file for how I built a tail pipe to finish this off properly.