Again, very straightforward steps. The first thing to do is to position the
downpipe in the car. As with the collectors, the downpipe needs to be installed from
under the car. The exhaust gas leaves the turbo via the downpipe so the pipe
connects to the turbine outlet (side of turbo) and then to the cat.
Back in the engine bay, it is now time to bolt the turbo down to the collectors. Make sure the studs are high enough to protrude through their respective holes in the turbine housing. Set the turbo over the studs and bolt it tight.
After the turbo is securely fastened to the collectors, the downpipe needs to be bolted to the turbine housing. The downpipe flange and the surface on the the housing it mates to should be machined true to provide an exact seal. However, if there is any doubt to how they bolt up, a little Ultra Copper RTV around the inside of the flange will help cure any potential leaks.
Line up the downpipe flange with the turbine housing and insert the bolts
with washers. Hand tighten each until the two surfaces meet. At that point,
torque each bolt.
Back under the car, the other end of the downpipe bolts to the front of the cat just like the y-pipe did. I had an sensor fitting welded on to the end of the pipe to accept the OBD-II sensor on my '97.
After all of the pipes are in and secure, the cross member and lateral member can be re-installed.