Introduction

The Audi RS6 4.0 TFSI (often called the 4.0TT) is a marvel of engineering, packing a twin-turbo V8 that delivers both brutal acceleration and surprising refinement. Yet even this superb powertrain has room to breathe—literally. The factory downpipes are a well‑known bottleneck, designed primarily to meet noise regulations and emissions standards rather than maximize performance. Upgrading to high‑flow downpipes is one of the most cost‑effective ways to unlock additional horsepower and torque, with many owners reporting gains of approximately 30–50 wheel horsepower after installation and a proper tune.

This guide walks you through everything you need to know to upgrade the downpipes on your RS6 4.0TT. We cover tools, safety, detailed removal and installation steps, common pitfalls, and the post‑upgrade requirements for a safe and reliable power increase. Whether you’re a seasoned weekend mechanic or a first‑time DIYer, following these instructions carefully will help you achieve a clean installation that performs flawlessly.

Tools, Parts, and Preparation

Required Tools

  • Socket wrench set (metric, including extensions and universal joints)
  • Torque wrench (0–100 Nm / 0–75 ft‑lb range)
  • Jack and jack stands (or a four‑post lift)
  • Safety glasses and mechanic’s gloves
  • Trim removal tools (for heat shields and plastic covers)
  • Penetrating oil (e.g., WD‑40 or PB Blaster)
  • Oxygen sensor socket (22 mm / 7/8″)
  • Flathead screwdriver and pry bar
  • Shop rags and a vacuum (to clean debris)

Parts Required

  • Aftermarket downpipes (catless or high‑flow catalytic converters – check local laws)
  • Exhaust gaskets (turbo‑to‑downpipe and downpipe‑to‑exhaust system; always use new)
  • Hardware kit (new bolts, nuts, and locking washers if not included with downpipes)
  • Oxygen sensor spacers (if retaining factory sensors and running catless downpipes to prevent check engine light)
  • Heat wrap or ceramic coating (optional, for lower under‑hood temperatures)

Safety First

Working underneath a 4,500‑lb sedan is inherently dangerous. Never rely on a hydraulic jack alone; always use jack stands rated for the vehicle’s weight. Disconnect the negative battery terminal before starting to avoid shorting out sensors or the ECU. Allow the exhaust and turbos to cool completely—surface temperatures can exceed 500°F even after 30 minutes of idling. Wear gloves to protect against sharp flanges and hot metal, and wear eye protection to guard against debris and penetrating oil.

Preparation: Getting the Car Ready

Park the RS6 on a level concrete or asphalt surface. Engage the parking brake and chock the rear wheels. Raise the front of the car with a jack, then place jack stands under the factory lift points (indicated by small arrows on the rocker panels). Lower the car onto the stands and verify it is stable by gently rocking it side to side. Remove the under‑engine cover (if equipped) to expose the downpipes and turbocharger area.

Disconnect the Battery

Using a 10 mm socket, loosen the negative terminal clamp and set the cable aside. This prevents accidental airbag deployment or sensor damage while working near electrical connections. Wait five minutes before proceeding to allow any residual charge in the system to dissipate.

Removing the Stock Downpipes

The factory downpipes are restrictive, with cast‑iron or stainless steel construction and one or two catalytic converters. Removing them requires patience, especially if corrosion has set in. Follow these steps in order:

Step 1: Access the Downpipes

From underneath, locate the two downpipes that connect the turbocharger exhaust outlets to the rest of the exhaust system. On the RS6, they run behind the engine block and are partially obscured by heat shields. Use a socket and extension to remove the bolts securing the heat shields over the downpipes. There may be several small shields; keep track of their locations for reinstallation.

Step 2: Disconnect Oxygen Sensors

Each downpipe typically houses two oxygen sensors (one upstream of the cat, one downstream). Disconnect the electrical connectors at the sensor bodies—do not pull on the wires. Use an oxygen sensor socket to unscrew each sensor from the downpipe. Apply penetrating oil if they are stubborn. Store the sensors in a clean place, avoiding any contact with grease or dirt.

Step 3: Unbolt the Downpipes from the Turbos

Using a deep socket and a ratchet (often 13 mm or 15 mm), remove the three or four bolts that secure each downpipe flange to the turbo outlet. On some model years, you may need a swivel joint to reach the lower bolts. Work slowly to avoid stripping the nuts. If any bolt feels seized, soak it with penetrating oil and wait ten minutes before trying again.

Step 4: Separate the Downpipe from the Mid‑Pipe

Behind the downpipe, there is a v‑band clamp or flange connection to the centre exhaust section. Loosen the clamp bolt with a socket or wrench. If the connection is corroded, use a hammer and flathead screwdriver to gently tap the clamp loose. Once freed, slide the downpipe rearward and carefully lower it down. You may need to rotate the assembly to clear the subframe or steering components.

Step 5: Remove the Driver‑Side Downpipe

The driver‑side downpipe is usually easier to remove because there is more clearance to the steering rack. The passenger side is tighter, and you may need to detach a small bracket or two to wriggle the pipe out. Take your time—forcing the pipe can damage the turbo flange or the pipe itself.

Installing the New Downpipes

Now for the exciting part: fitting the high‑flow downpipes. Most aftermarket options are made from T304 stainless steel and feature mandrel bends for maximum flow. Some are catless, while others include high‑flow catalytic converters. Choose a set that matches your performance goals and local emissions laws.

Pre‑Installation Checks

  • Compare the new downpipes to the stock ones side by side. Verify that all flange holes align and that the oxygen sensor bungs are in similar positions.
  • Apply a thin layer of anti‑seize compound to the turbo flange bolts and the oxygen sensor threads. This makes future removal much easier.
  • Install new gaskets on both the turbo flange and the mid‑pipe connection. Never reuse old gaskets—they will leak immediately.

Step 1: Position the Downpipe

Starting with the driver side, slide the new downpipe up into the engine bay from underneath. Align the flange with the turbo outlet. It helps to use a jack stand or a helper to support the pipe while you start the bolts. Loosely thread all bolts into the turbo flange before tightening any of them. This ensures proper alignment.

Step 2: Secure the Flange

Tighten the flange bolts to the manufacturer’s torque specification (typically 18–22 ft‑lb for M8 bolts, or as stated in the downpipe instructions). Use a torque wrench for accuracy. Over‑tightening can warp the flange or strip threads.

Step 3: Connect to the Mid‑Pipe

Attach the downstream end of the downpipe to the centre exhaust section using the supplied v‑band clamp or bolts. Ensure the gasket is seated correctly before tightening. Torque the clamp or bolts evenly to prevent leaks.

Step 4: Reinstall Heat Shields

Bolt the factory heat shields back into place. Some aftermarket downpipes may require slight trimming of the shields for clearance. If so, use a die grinder or tin snips to make modest cuts. Always reinstall shields to protect the chassis and nearby components from radiant heat.

Step 5: Plug in Oxygen Sensors

Screw the oxygen sensors into the new downpipes. For catless downpipes, consider using oxygen sensor spacers (mini catalytic converters) on the downstream sensors to prevent a check engine light. Hand‑tighten the sensors, then give them a gentle 1/16 turn with the socket—do not overtighten. Reconnect the electrical connectors and ensure they click securely.

Post‑Installation Checks

Before lowering the car, double‑check every bolt and connection:

  • Verify that all flange bolts are tight and that no gasket is pinched or misaligned.
  • Look for any wires or hoses that may have been pinched during installation. The downpipes can push against the steering column boot or coolant lines if not routed correctly.
  • Reattach any brackets or covers you moved.
  • Reconnect the negative battery terminal.

Start the engine. The first start may be louder than usual as the ECU adapts. Listen for irregular sounds—ticking often indicates an exhaust leak at a flange. A quick way to check for leaks is to spray soapy water on the joints while the engine idles: bubbles will appear if there is a leak. Shut off the engine and retighten any leaking connections.

Test Drive

Take the car for a gentle drive, avoiding full throttle until the downpipes have gone through at least one heat cycle. During the first few miles, the ECU will relearn fuel trims and may trigger a temporary reduction in power. After 15–20 minutes of mixed driving, the check engine light may come on if oxygen sensor spacers are not used or if the tune does not disable the rear O2 monitors. If the light appears, scan the code with an OBD‑II reader.

Tuning Considerations and Emissions Notes

Installing downpipes without a corresponding ECU tune often yields minimal gains and can even cause the vehicle to run rich or lean. The stock ECU is calibrated for the backpressure of the restrictive factory downpipes. With increased flow, the air‑fuel ratio and boost targets need adjustment. A professional tune from a reputable shop (e.g., Unitronic, APR, or a custom dyno tuner) will unlock the full potential of the downpipes, often adding 30–50 wheel horsepower and improving throttle response.

Be aware of legal implications. Removing catalytic converters (catless downpipes) is illegal in many states and countries for street use and will cause the vehicle to fail emissions testing. High‑flow catted downpipes are a better choice for daily drivers who need to pass inspections. Always check your local laws before proceeding.

  • Unitronic – Offers stage 2 ECU and TCU tunes for the RS6 platform.
  • APR – Known for reliable stage 2 packages with downpipes.
  • Audizine Forums – Community resource for installation tips and tuning advice.
  • 034Motorsport – Supplier of high‑flow downpipes and engine mounts for the C7/C8 RS6.

Common Mistakes and How to Avoid Them

  • Forgetting to disconnect the battery: This can cause short circuits or accidental airbag deployment. Always disconnect the negative terminal.
  • Reusing old gaskets: Old gaskets will almost certainly leak. Spend the few dollars on new ones.
  • Overlooking oxygen sensor wires: Ensure the sensor wires are routed away from hot downpipe surfaces. Use zip ties to secure them to harness clips.
  • Skipping the heat shield reinstallation: High‑flow downpipes radiate more heat. Without shields, you risk cooking nearby plastic components and the starter motor.
  • Not performing a smoke test: After installation, a smoke test can reveal small leaks that are invisible to the ear. Many DIYers skip this, but it is worth the effort, especially on catless setups.

Conclusion

Upgrading the downpipes on your RS6 4.0TT is a rewarding project that delivers genuine performance gains. With the right tools, a clean workspace, and careful attention to detail, you can complete the installation in a weekend. The result is a sharper throttle response, a more aggressive exhaust note, and a noticeable increase in horsepower—often enough to drop 0–60 times by a few tenths of a second. Pair the hardware with a proper tune, and your RS6 will become an absolute beast on the road or track.

Remember to double‑check all connections, break in the components gradually, and consult a professional if you encounter stubborn bolts or wiring issues. Safe wrenching, and enjoy the new breath of life in your V8.