Just thought this would help a bit, at least to fellow forum members, to shed a bit of light to downpipe disassembly. I compiled this short how-to about 2 months ago, but never posted it. Anyhow, I hope it helps. If you have any pictures or comments/corrections to the walkthrough below, feel free to post them and I'll edit them in.
Again, I compiled this quickly about 2 months ago so it may be a little bit incomplete.
The following is a short how-to guide on how to install the Cobb Downpipe on the Mazdaspeed3. I wrote the folliwing to supplement the instructions that Cobb releases with the downpipe, unfortunately the cobb's instructions do not come with pictures. I didn't take too many pics during disassembly from stock, so most of the pictures that I took for reassembly were used to illustrate some of the steps. The how-to may be also used with other downpipes.
[size=8pt]Use & Disclaimer:
Please do not cross-post this how-to guide / walkthrough and its content (pictures, textual information, etc) to any other forums or boards outside of Mazda3forums.com without first requesting & gaining my permission.
This brief how-to should not be used as an all-encompassing walkthrough. It is simply a short description of my procedures for removal & installation, and I am not responsible for any damages or injuries that may result.
Jack & jackstands
Ratchet (3/8", and 1/2")
O2 sensor remover (crow foot)
10mm, 13mm, 14mm and 17mm sockets (6-pt sockets) & a 2", 4" extension
12mm, 13mm, and 14mm box-end wrenches
penetrating fluid & torch (if needed)
1.) Remove top mount intercooler.... If you are using the stock TMIC, remove the TMIC cover by removing the (2) 10mm bolts. For the intercooler core there will be (3) 12mm nuts holding the intercooler to the top of the motor; there will be (2) at the front and (1) at the back. Next, unbolt the blow-off valve by removing the (2) 10mm bolts [circled in yellow]. Next loosen the clamps on the rubber hoses using a 10mm socket. Now the intercooler should be ready to be lifted off.
2.) Optional....Remove foam/foil heatshield mounted to the firewall by removing the two plastic nuts and sliding it up and out.
3.) Unclip wire loom from black black bracket. Also, unbolt the power-steering line from the bracket using a 10mm socket. Gently move both the wire loom and PS line to the side.
4.) Using an 8mm socket remove the (5) bolts that hold the upper heatshield to the exhaust manifold. Slide the heatshield towards the firewall. Position the heatshield like shown in the second pic, this allows for easier access to remove the rear intercooler mount bracket. You will find it hard to pull the heatshield out of the engine bay w/ the intercooler mount there.
5.) By moving the heatshield out of the way you can now remove the rear TMIC bracket from the block. To gain more access to the (2) bolts that hold the mount to the block, we first have to disassemble the brackets from the intercooler mount. Remove the power steering line bracket from the mount using a 10mm socket.
6.) Next, grab a 13mm socket and remove the (2) bolts holding the mount to the engine. I remember having to use a 13mm box-end wrench to remove the bolt that is straddled by the exhaust manifold.
6.) Using an 8mm socket, remove the (3) bolts and remove the lower heatshield from the exhaust manifold
7.) Unclip the lines to the first O2 sensor from the downpipe heatshield. There should be one spiral clip and two pinch clips that hold the O2 sensor wire as it snakes around the downpipe. Remove the O2 sensor using a 7/8" oxygen sensor removal tool. I used a crowfoot type tool. Soak the threads to the sensor by spraying penetrating fluid under the O2 sensor 'hat'.
8.) Remove the (5) nuts that hold the downpipe to the turbo using a 14mm 6-point socket and various extensions. Be sure to soak the nuts w/ PB blaster before attempting to remove the nuts. Also be sure to use a breaker bar to make removing them easier. I had access to an impact gun, just in case I needed it, but the breaker bar was more than sufficient. Also my car only had 7000miles on the odo at time of disassembly, so the nuts weren't seized on the studs. Temporarily re-install 2 of the top nuts to hold the downpipe in place as you continue disassembly under the car.
9.) Lift the car and place the car on jackstands. Remove the mid-chassis bracing by removing the (8) bolts using a 12mm socket.
10.) Remove the chassis bracing just behind the front crossmember. There will be (4) 14mm bolts and (1) 10mm bolt.
11.) Disconnect the O2 sensors from the ECU harness. Sorry for not having a pic of the sensor location with the stock midpipe in place. The only pic that I have that are useful are of the tunnel w/o the downpipe, but at least it'll give you a general idea of where to look. Follow the oil return line from the oil pan to the bottom of the turbo. The connectors circled are the two connectors to the first O2 sensor (blue & grey and the black connector). The second O2 sensor, which is not in the picture, is to the left of the first O2 sensor connectors (grey connector). Remove the first O2 sensor from the car, and set it aside in a very safe place.
12.) Remove the (2) 14mm spring bolts between the rear of the midpipe and the main exhaust tube. Then remove the (2) 14mm bolts that hold the midpipe to the stock downpipe.
13.) I chose to remove the second O2 sensor w/ the midpipe off the car. Once the sensor is disconnected from the main harness, slide the hanger rods out of the rubber exhaust hangers and remove the midpipe from the vehicle. Once off the vehicle you are free to try your hand at removing the second O2 sensor.
14.) Now go back under the car and remove the (2) 14mm bolts from the factory downpipe to the engine mounting bracket.
15.) Remove the (2) 14mm bolts which hold the bracket to the engine. I recall the outboard bolt being the only one accessible w/ a standard 3/8" ratchet & socket, so remove that one first. The second one I loosened w/ a standard 14mm box end wrench. Once loosened it allowed me to pivot the bracket slighty enough that I could remove it w/ a ratchet.
16.) Remove the two bolts temporarily hold the downpipe to the turbo, that we re-installed in step #8. You will need to rotate and pivot the downpipe to get it out, so try not to tear up the foam heatshield from the exhaust tunnel too much. I had some trouble w/ the downpipe heatshield snagging/catching in a few areas. So i took my 10mm socket and started to remove some of the heatshield bolts, thereby allowing me to move the downpipe heatshielding around to slide it out. Once you have everything out, you should have a parts pile that looks like the following...
Now that everything is removed, just work backwards to reinstall the new downpipe.