Procedure
- Soak every downpipe fastener with penetrating oil for two or three days before the job. This is the whole game on a rust-belt car.
- With the car high on stands, remove the mid-pipe connection first, supporting the exhaust so nothing hangs off the O2 sensor wiring.
- Unplug and remove the rear O2 sensor with a proper O2 socket. Transfer it to the new pipe later with anti-seize on the threads only.
- Remove the downpipe brace bracket, then the turbo-to-downpipe nuts from below with swivels and patience. If a stud spins out with the nut, run a die over it and reuse with new hardware.
- Drop the old pipe, fit the new one loose with fresh gaskets at both ends, and align everything before final torque so nothing is under preload.
- Torque turbo flange nuts evenly in a star pattern, then the mid-pipe, then the brace. Recheck after one heat cycle - new gaskets relax.
- IMPORTANT: a downpipe on a turbo Subaru changes boost behavior. Do not drive hard on the stock tune - get a protune or off-the-shelf map first, or you risk boost creep and knock.
Work at your own risk. Torque values and procedures vary by vehicle — always confirm against the factory service manual for your exact model.