Why Upgrade the Downpipe on Your Audi S4?

The factory downpipe on the Audi S4 (B8/B8.5 or newer, depending on engine code) is a significant bottleneck in the exhaust system. It contains a restrictive catalytic converter and often incorporates a narrower diameter than aftermarket alternatives. Installing a Turbosmart Turbo Downpipe reduces exhaust backpressure, allowing the turbochargers to spool more quickly and efficiently. This translates to noticeable gains in horsepower and torque across the powerband, especially when paired with a flash tune. The upgrade also improves exhaust gas flow, which lowers exhaust gas temperatures (EGTs) under high load, contributing to engine longevity during spirited driving or track use.

Beyond performance, a quality downpipe like Turbosmart’s is built from high-grade stainless steel or T304, offering superior corrosion resistance compared to the coated mild steel of the stock unit. Many aftermarket downpipes also include a high-flow catalytic converter option (often 200-cell or 100-cell) that still keeps emissions reasonable for street use while dramatically improving flow. If you are in a region with strict emissions testing, a catted downpipe is usually the wiser choice. A catless downpipe, on the other hand, offers maximum flow but may require tuning to prevent a check engine light and is not legal for on-road use in many areas.

Before starting the installation, consider that a downpipe swap can change the exhaust note of your S4, making it deeper and more aggressive. Some drivers pair this with a cat-back exhaust system for a fully opened exhaust path. For more information on the specific product line, visit the official Turbosmart website for fitment details and technical specifications.

Tools and Materials Needed

Having the correct tools and consumables on hand before you lift the vehicle saves time and prevents frustration. Here is the complete list:

  • Turbosmart Turbo Downpipe (with gaskets and hardware included in the kit)
  • Socket set: 10mm, 13mm, 14mm, 15mm, and 18mm sockets (deep and shallow)
  • Wrench set (combination wrenches 10–19mm)
  • Torque wrench (capable of 30–80 ft-lb range)
  • Breaker bar (1/2-inch drive) for stubborn bolts
  • Jack and two heavy-duty jack stands (minimum 3-ton capacity)
  • Rubber mallet
  • WD-40 or penetrating oil (PB Blaster recommended)
  • Safety glasses and mechanic gloves
  • Exhaust gasket (Turbosmart includes one; carry a spare OEM gasket for backup)
  • Anti-seize compound (for threads)
  • O2 socket (22mm) or oxygen sensor removal tool
  • Trim panel removal tools (if accessing the downpipe from above)
  • Shop rags and a drain pan (for any residual coolant if removing a water-cooled turbo)

Check the Turbosmart box to confirm all included parts. Some vehicles may require additional V-band clamps or adapter rings, but the B8/B8.5 S4 downpipe installation typically uses standard flange connections. Ensure you have a clean, well-lit workspace with enough room to move under the car.

Preparation Steps

1. Secure Your Audi S4

Park the vehicle on a level concrete floor. Engage the parking brake and chock the rear wheels. Disconnect the negative terminal of the battery using a 10mm wrench to prevent any electrical shorts or accidental airbag deployment while working near the exhaust system.

2. Raise the Vehicle

Use a jack to lift the front of the vehicle. Place jack stands under the reinforced jack points (the pinch welds with rubber pads on the unibody). Lower the car gently onto the stands and verify stability by pushing the car side to side. Never work under a car supported only by a hydraulic jack.

3. Cool Down the Exhaust

If the engine has been run recently, wait at least one hour for the exhaust system to cool to a safe temperature. Hot components can cause severe burns and make bolt removal more difficult due to thermal expansion. Use penetrating oil on all exhaust fasteners while the system is warm (but not hot) to help break loose corrosion.

Removing the Stock Downpipe

This is the most labor-intensive part of the job. Patience and methodical work are key to avoiding broken bolts or damaged components.

Step 1: Access the Downpipe

From underneath the car, locate the downpipe. It connects the turbocharger outlet to the front of the exhaust system, just behind the engine. You will likely need to remove the under-engine splash shield (held by 10mm or 8mm bolts) and possibly a heat shield over the turbo/downpipe area (often 10mm fasteners). Set these aside in a safe place, keeping the hardware organized.

Step 2: Disconnect the Oxygen Sensors

The stock downpipe has two oxygen sensors: one pre-turbo (upstream, in the downpipe itself) and one post-turbo (behind the catalytic converter). Use the O2 socket or a 22mm wrench to unscrew them from the downpipe. Be careful not to damage the wire pigtails. You may need to unclip the sensor connectors from brackets on the transmission bellhousing or along the firewall. Label the sensors if you plan to reuse them; typically the pre-cat sensor is reused in the Turbosmart downpipe’s bung, while the post-cat sensor might require an extension harness (often included with the downpipe or available separately).

Step 3: Unbolt the Downpipe from the Turbo

The downpipe flange is attached to the turbocharger outlet flange with three or four bolts (typically 13mm or 15mm). Apply penetrating oil to the threads and let it soak for five minutes. Use a breaker bar to break them loose, then remove them by hand. A wobble socket or universal joint may help reach the rear bolt near the firewall. If a bolt is seized, carefully alternate tightening and loosening with penetrating oil. Do not apply excessive force as the studs can break inside the turbo housing.

Step 4: Remove the Downpipe from the Exhaust System

There is usually a slip-joint or flange connection where the downpipe meets the rest of the exhaust (midpipe or catalytic converter). Loosen the clamp or bolts (14mm or 15mm). The downpipe may be hung up on the subframe; some B8 S4s require rotating the downpipe to clear the front crossmember. Wiggle and pull the pipe toward the rear of the car. If it is stuck, strike it gently with a rubber mallet. Once free, lower the downpipe out of the vehicle. Note that the stock downpipe is heavy due to the integrated catalytic converter.

Step 5: Inspect and Clean the Mounting Surfaces

With the stock downpipe removed, use a razor blade or wire brush to clean the turbo outlet flange and the midpipe connection flange. Remove any old gasket material, carbon deposits, or corrosion. Ensure the mating surfaces are flat and free of debris. Clean the threads of the turbo studs (if you didn’t remove them) with a tap or a thread chaser to ease installation of the new hardware.

Installing the Turbosmart Turbo Downpipe

Follow the manufacturer’s instructions included with the kit, as specific mountings may vary by vehicle model year. The general procedure is:

Step 1: Position the Gasket and Downpipe

Place the new exhaust gasket (provided by Turbosmart) onto the turbo outlet flange. The gasket is usually a multi-layer steel design. Slide the Turbosmart downpipe up into position, aligning the three holes with the studs or bolt holes on the turbo flange. You may need to angle the pipe to clear the subframe and steering column. Do not force it.

Step 2: Connect to the Exhaust System

Attach the downstream end of the downpipe to the exhaust midpipe using the provided clamp or new bolts. Do not fully tighten yet – leave some play to allow for final alignment. Some Turbosmart downpipes include a V-band connection; if so, install the V-band clamp and hand-tighten the nut.

Step 3: Bolt the Downpipe to the Turbo

Apply a small amount of anti-seize to the threads of the turbo studs (if reused) or insert the new bolts supplied with the kit. Tighten the bolts in a star pattern to 25–30 ft-lb (or as specified by Turbosmart). Use the torque wrench for accuracy. Over-tightening can warp the flange or strip the threads in the turbo housing.

Step 4: Tighten Exhaust Connections

Once the turbo flange is secure, go back and fully tighten the midpipe connection. Use a torque wrench if possible; slip-joint clamps typically require 35–40 ft-lb, while flange bolts may need 25–30 ft-lb. Inspect all joints to ensure the gasket is compressed evenly and no gaps exist.

Step 5: Reinstall the Oxygen Sensors

Screw the pre-turbo oxygen sensor into the Turbosmart downpipe’s bung (usually located in a similar position as stock). Apply anti-seize to the sensor threads only – avoid getting any on the sensor tip. Install the post-cat sensor into the second bung (often after the integrated high-flow cat). If the wiring is too short, use the extension harness provided. Route the sensor wires away from moving parts and heat shields; secure them with zip ties to the factory brackets.

Final Steps and Testing

1. Reinstall Heat Shields and Splash Shield

Refit any heat shields you removed, ensuring they do not contact the new downpipe (if clearance is tight, you can gently dimple the heat shield with a hammer). Reattach the under-engine splash guard.

2. Reconnect the Battery

Reattach the negative battery terminal and tighten with a 10mm wrench. Reset the date/time and radio presets after powering up the car.

3. Check for Leaks and Start the Engine

With the car still on jack stands, start the engine and listen for any exhaust leaks – a hissing or ticking sound around the turbo flange or midpipe joint. Use your hand to feel for escaping air (careful: exhaust gets hot quickly). If you suspect a leak, turn off the engine and retighten the affected fasteners. Also check for any abnormal smoke from oil or coolant that may have dripped onto the exhaust during the swap (it should burn off quickly).

4. Lower the Vehicle and Test Drive

Remove the jack stands and lower the car. Perform a test drive of at least 15 minutes, including city and highway speeds. Listen for rattles (shields touching the downpipe) and watch for any warning lights on the dashboard. If the check engine light comes on, scan the code – it is often related to the post-cat O2 sensor requiring a spacer or a tune. Many aftermarket downpipe installations require a stage 2 tune to calibrate the fuel mixture and disable the secondary O2 sensor readiness check. Consult a reputable tuning platform such as Unitronic or APR for compatible calibration options.

Common Mistakes to Avoid

  • Not applying anti-seize: Stainless steel bolts can gall (cold-weld) with aluminum turbo housings. Always use anti-seize on fasteners and O2 sensor threads.
  • Over-torquing the turbo flange: The B8/B8.5 turbo housing is cast iron but the studs can snap if over-tightened. Follow the manufacturer’s torque specs exactly.
  • Skipping the gasket inspection: Reusing a crushed or damaged stock gasket will cause exhaust leaks. The Turbosmart kit includes a new gasket – use it.
  • Neglecting to support the downpipe: The aftermarket downpipe is lighter than stock but still requires the hanger bushings to be properly seated. Ensure the rubber hangers are not twisted or stretched.
  • Forgetting the ECU tune: Running a high-flow downpipe without a tune can lead to boost spikes, lean conditions, and a persistent check engine light. A flash tune is strongly recommended to realize the full power potential and keep the engine safe.

Expected Performance Gains

With a Turbosmart Turbo Downpipe and a stage 2 tune on the Audi S4 (3.0T engine), owners typically see gains of 35–55 whp and 40–60 ft-lb of torque over a stage 1 tune alone. The powerband becomes broader, with peak torque arriving earlier and holding longer. Turbo spool time is noticeably reduced, and the engine revs more freely to redline. The sound becomes more aggressive – a deeper growl under load with a subtle whistle from the turbo – without being obtrusive on the highway.

For those who want even more, pairing the downpipe with an upgraded intercooler and a high-flow air intake can push the 3.0T into the 450–500 whp range on pump gas. A well-known resource for S4 performance builds is the Audi Forums community; check out Audizine’s B8 S4 section for real-world user experiences and dyno charts.

Maintenance Considerations After Installation

After installing the downpipe, monitor your intake valves for carbon buildup more frequently. The increased exhaust flow can affect the PCV system’s performance; consider upgrading to a catch can if you plan extended track use. The high-flow catalytic converter (if equipped) will need occasional inspection – quality units like Turbosmart’s are designed to last but can be damaged by consistent misfires or extremely rich fuel mixtures. Finally, remember that a louder exhaust may attract more attention from law enforcement in areas with strict noise ordinances. Keep the factory mid-muffler or resonator in place for a more civil daily-driving tone.

Conclusion

Installing a Turbosmart Turbo Downpipe on your Audi S4 is a rewarding project that significantly enhances the car’s performance and driving character. By following the steps outlined above and using the proper tools, you can complete the installation in a weekend with careful preparation. The key to a successful job lies in patience – especially when removing stubborn factory fasteners – and in respecting torque specifications during reassembly. Once the downpipe is fitted and paired with a proper tune, your S4 will deliver a thrilling boost in power and a soundtrack that matches its new performance potential. For further technical support, refer to the Turbosmart support page or consult an experienced VAG technician.