Godspeed downpipe install on STi
First and absolutely most important, you must have an STi and a Godspeed downpipe (I say this since it's specific to this downpipe but others would be almost EXACTLY the same, I suspect).
You will need either a lift, ramps or a jack and jackstands to do this install. You'll also need a socket set and some open-ended wrenches, in the 10-15mm range. I don't remember which size to which nut and bolt, so I'm including the range of sizes I used. Test-fitting is simple...if anyone knows the exact sizes needed and for what bolts, please let me know. In addition, you will need 2 washers.
1. Get car up (ramp/lift/jacked), open hood.
2. Remove the heatshield around the turbo (10mm bolts). It IS possible to do so without removing the intercooler, but it's not easy. Of course, I opted to go the hard way.
3. Once all the heatshielding is removed from around the turbo, you should be able to see a very small turbo that has 5 points where it connects to the stock downpipe. Some of these are bolts and some are studs. Remove all the 14mm bolts and also remove all the nuts from the studs, leaving the studs in place. Use a wrench to hold any nuts or bolts in place if they start turning instead of loosening.
4. Although hard to find, there should be two 14mm bolts connecting the stock downpipe to the tranny. They are tough to remove when you're reaching through the open hood. Because I got to them the hard way, I do not know how accessible they will be from below. However you can reach them, you must remove them.
5. Go under the car. You should see where the downpipe connects to the cat-back, as the bolts have springs on them. Use a wrench to hold either the nuts or bolts in place while you remove these two bolts. These are 14mm and 12mm (nuts and bolts not same size).
6. The last bolt remaining is on the hanger. It is near the cat-back connection. Remove this bolt. It should be 14mm.
At this point, all your bolts should be out.
7. (This step may be easier if done earlier.) On the top of the stock downpipe should be a sensor. It has yellow wrap around its wires and is solidly screwed into place. Unscrew it.
8. Get the downpipe off the hanger's hook. I used a large flathead screwdriver to pry them.
9. With nothing else connecting the downpipe to the car, it SHOULD slide out and off the turbo fairly easily. Remove the downpipe and set it aside.
With the stock pieces removed, now begins the easy part--the install of the new downpipe. This is most easily done with help from another person.
10. Go under the car with the new downpipe. Lift it and guide it over the studs still on the turbo. Be sure to use the gasket that was present before--no need for a new one. Which end to attach should be obvious since only one end has 5 holes. Once it has been guided onto the studs, start putting the nuts back on to hold it in place, then put the bolts back in as well.
11. The place where the hanger was before will be used again, so hook the new downpipe in to the hanger, then put the factory bolt back in.
12. The downpipe comes with a new gasket and 2 new bolts with nuts. It does not come with washers, but you will need one washer for each bolt. Line up the downpipe and cat-back pipes, then slip the new gasket between them. Put one bolt through one of the 2 holes where they attach, with the head facing the back of the car. Your washer will also need to be on the same side as the head of the bolt. If you do not use a washer, it's likely that you will only tighten the bolt and nut onto the gasket, not actually attach the cat-back and downpipe. With the washers on the proper side and nuts on the other, tighten the bolts as much as you can. The new bolts provided with your downpipe should be 17mm.
13. Your downpipe is secured now but you must attach the sensor as well. There is not enough slack for the sensor to reach its new place on the side of the downpipe. If you follow the wire, you will find a white plastic piece that attaches it under the car. You can either detach the piece of plastic or cut it, as I did. Be careful not to cut the wire. When it is detached, you will have enough slack to attach it to the side of the downpipe. Screw it into place tightly. This step could also be one of the first install steps if you do not want the wire all twisted up.
With all the nuts and bolts put back into place, your downpipe should be secured and the sensor in place. Try starting up the car. It should sound almost like stock when starting (if you keep stock cat-back). Listen and feel for exhaust leaks. Don't touch the piping since it can get hot quickly, but feel near where it connects to the turbo and cat-back. If you feel a leak, shut down the car, then see if you tightened up everything properly. When you have no leaks, the car should be fairly quiet and you should get no check-engine-light.