Upgrading your Subaru WRX with a COBB Tuning downpipe is one of the single most effective bolt-on modifications you can make for significant power gains. The factory catalytic converter is a major restriction in the exhaust flow, creating backpressure that limits turbocharger spool speed and overall engine output. Replacing it with a high-flow COBB downpipe reduces this bottleneck, allowing exhaust gases to exit the turbine wheel more efficiently. This translates to a noticeable increase in horsepower and torque, improved throttle response, and a deeper exhaust note.

However, this isn’t a simple “remove and replace‣ job. It requires mechanical aptitude, the right tools, and strict attention to detail. An improperly installed downpipe can lead to exhaust leaks, CELs, or even damage to the turbocharger. This guide provides a comprehensive, step-by-step approach to ensure a clean, safe, and high-performing installation on your WRX.

Before starting, understand that a downpipe upgrade typically requires a professional tune (either via an Accessport or a pro-tuner) to prevent the engine from running lean and causing potential knock events. The COBB downpipe is designed to work seamlessly with COBB’s OTS (Off-The-Shelf) maps, but a custom tune will maximize the gains.

Tools and Materials Required

Having everything on hand before you lift the car saves time and prevents frustration. Gather the following items:

  • COBB Turbo Downpipe (ensure it is the correct part number for your specific model year and emissions regulations)
  • Socket Set (metric, including 10mm, 12mm, 14mm, 17mm, and 19mm sockets)
  • Wrench Set (combination wrenches in the same sizes)
  • Torque Wrench (both 3/8″ and 1/2″ drive for accurate fastener tightening)
  • Breaker Bar (for stubborn bolts, especially the turbo-to-downpipe studs)
  • Jack and Jack Stands (rated for the vehicle’s weight; never rely on a jack alone)
  • Penetrating Oil (e.g., PB Blaster or Kroil) to loosen rusted fasteners
  • New Gaskets (turbo-to-downpipe and downpipe-to-midpipe gaskets; COBB often includes them, but have spares)
  • Anti-Seize Compound (copper-based is ideal for exhaust components)
  • Exhaust Hanger Removal Tool (or a large flathead screwdriver and pliers)
  • Safety Glasses and Mechanic’s Gloves
  • Shop Rags and a Catch Pan (for any coolant or oil drips)
  • Ramps or Low-Profile Jack (the WRX sits relatively high, but a low-profile jack helps)

Preparation and Safety

Work on a flat, level surface in a well-ventilated area. The vehicle must be completely cool – exhaust components can reach temperatures over 600°F during operation. Allow at least two hours of cool-down time after driving.

Disconnect the Battery

Before any major work, disconnect the negative terminal of the battery. This prevents accidental shorts and resets the ECU, which helps it adapt to the new downpipe once the car is started.

Apply Penetrating Oil

Spray all exhaust fasteners (turbo-to-downpipe, downpipe-to-midpipe, and any O2 sensor connections) with penetrating oil. Let it soak for at least 15-20 minutes while you prepare your tools. This dramatically reduces the risk of snapping a stud, which is a common headache on Subaru exhaust systems.

Step 1: Lift and Secure the Vehicle

Using a floor jack, lift the front of the WRX at the center jacking point (the front crossmember). Place jack stands under the factory jacking points on the front pinch welds. Lower the car onto the stands and give it a firm shake to ensure stability. Do not crawl under the vehicle until you are certain it is secure.

Step 2: Remove the Factory Undertray

The plastic undertray protects the engine bay from debris. Use a 10mm socket or a flathead screwdriver to remove the retaining screws or quarter-turn fasteners. Carefully lower the tray and set it aside. You will likely have to remove the front portion to access the downpipe completely.

Step 3: Disconnect the Exhaust System

With the vehicle lifted and secure, locate the connection point between the downpipe and the midpipe (the section of exhaust that runs under the transmission).

  • Downpipe-to-Midpipe Connection: There are typically three bolts (12mm or 14mm) that hold the flanges together. Use a socket and wrench to loosen and remove them. Be prepared for some spring pressure from the gasket.
  • Downpipe-to-Turbo Connection: This is the most critical joint. There are four or five studs that mount the downpipe directly to the turbocharger exhaust housing. Loosen the nuts with a 12mm or 14mm wrench. A breaker bar may be necessary. If a stud begins to spin, apply more penetrating oil and work the nut back and forth gently.
  • O2 Sensor: Unplug the O2 sensor wiring connector located near the turbo. Use a 22mm wrench to carefully unscrew the sensor from the stock downpipe. Set the sensor aside; it will be reused on the COBB downpipe.

Step 4: Remove the Stock Downpipe

With all fasteners removed, carefully work the stock downpipe out from its mounts. It will be heavy and awkward. Tilt the pipe downward and toward the driver’s side of the vehicle. You may need to gently pry the exhaust hangers with a removal tool to create clearance. Be cautious not to damage the turbo exhaust housing, the midpipe, or the transmission components.

Inspect the removed stock downpipe for any signs of excessive carbon buildup or oil residue. This can indicate other underlying issues (PCV system, turbo seals) that should be addressed before proceeding.

Step 5: Install the COBB Turbo Downpipe

Now the fun part begins. Take your time with this step – proper alignment is crucial.

Prepare the Mounting Surfaces

Clean the mating surfaces on the turbocharger and the midpipe flange with a wire brush or a rag to remove any old gasket material or debris. Apply a thin, even layer of anti-seize compound to the threads of the turbo studs and the midpipe bolts.

Position the COBB Downpipe

Install the new downpipe gaskets. COBB typically provides a two-piece gasket for the turbo connection. Ensure the gasket is oriented correctly (the larger opening corresponds to the turbo outlet). Carefully lift the COBB downpipe into position. Align the downpipe flange with the turbo studs and the midpipe flange simultaneously. This can require some wiggling and patience.

Do not fully tighten any bolts yet. Start all hardware by hand to ensure everything is aligned freely. This avoids cross-threading and eliminates stress on the mounts.

Reinstall the O2 Sensor

Coat the threads of the O2 sensor with a small amount of anti-seize (be careful not to get it on the sensor tip). Thread the sensor into the bung on the COBB downpipe. Tighten it snugly with a wrench – do not overtighten.

Step 6: Secure the Downpipe

Once the downpipe is in place and all hardware is started, use a torque wrench to tighten everything to the manufacturer’s specifications. Follow the correct sequence:

  1. Turbo-to-Downpipe nuts: Torque to 35 ft-lbs. Work in a crisscross pattern to ensure even pressure on the gasket.
  2. Downpipe-to-Midpipe bolts: Torque to 40 ft-lbs.
  3. Downpipe bracket bolts (if applicable): Torque to 25 ft-lbs. COBB downpipes often include a support bracket. Do not skip this bracket – it prevents stress on the turbo housing and reduces vibrations.

Recheck all fasteners after a few minutes of driving to account for thermal expansion settling.

Step 7: Reinstall the Exhaust System and Undertray

Reconnect any other exhaust components you may have disconnected (O2 sensor wiring, midpipe hangers). Lower the vehicle from the jack stands and reinstall the undertray, ensuring all clips are secured.

Final Checks and First Start

With everything back together, reconnect the battery. Start the engine and let it idle. Listen for any exhaust leaks, especially at the turbo and midpipe flanges. A small leak can cause a hissing noise and will affect performance.

Check the O2 sensor connections and ensure no wires are rubbing against the hot downpipe. Allow the engine to come up to operating temperature and verify the cooling fans cycle on.

Test Drive and ECU Learning

Take the car for a gentle test drive, avoiding heavy boost for the first 5-10 miles. This allows the engine management system to relearn idle and fuel trims. If you are using a COBB Accessport, flash the appropriate stage 2 map before driving. If you are running a custom tune, follow your tuner’s specific instructions.

After the first few miles, check all exhaust fasteners again while the system is still warm. Re-torque if necessary. Monitor the Accessport or check engine light for any codes (e.g., P0420 – catalyst efficiency below threshold). The COBB downpipe is designed to minimize this, but it can still occur with some tuners.

Performance Expectations and Considerations

A COBB downpipe, combined with a proper tune, typically yields 20-40 wheel horsepower and a comparable torque increase on a stock or lightly modified WRX. The turbo will spool noticeably faster, reaching peak boost earlier in the RPM range. The exhaust note will deepen significantly.

Be aware that this modification will likely attract attention from law enforcement in regions with strict emissions testing. The downpipe removes the restrictive main catalytic converter, making the vehicle illegal for on-road use in many states (CARB regulations). Check your local laws before installing. For track or off-road use, this is an ideal upgrade.

For further reading and advanced tuning resources, consider these external references:

If you encounter a stubborn exhaust stud that snaps during removal, do not panic. This is a common issue. It can be drilled out and extracted or replaced with a fresh stud from the dealer. Patience and the correct tools are your best friends in this scenario.

Conclusion

Installing a COBB turbo downpipe on your Subaru WRX is a rewarding project that unlocks the car’s true potential. While it requires careful attention to detail, proper preparation, and a solid understanding of torque specifications, the results are undeniable. Increased horsepower, faster spool, and a more aggressive sound profile make it one of the hallmark modifications for any serious WRX build.

By following this expanded guide, you ensure a leak-free, safe, and high-performing installation. Always pair the hardware upgrade with a proper tune to protect your investment and enjoy the full breadth of performance gains.