Upgrading your 2020 Audi S4 with a high-performance exhaust system is one of the most rewarding modifications you can undertake. Beyond the immediate enhancement in sound, a properly installed exhaust can unlock additional horsepower, reduce overall vehicle weight, sharpen throttle response, and improve overall driving dynamics. However, the installation process demands careful attention to detail to avoid exhaust leaks, drivetrain issues, or poor fitment that could affect performance and longevity. This comprehensive guide expands on the basics, providing advanced tips, expert techniques, and maintenance advice to ensure your exhaust upgrade delivers maximum performance, sound quality, and durability for your Audi S4.

Choosing the Right Exhaust for Your 2020 Audi S4

The 2020 Audi S4 is powered by the potent 3.0L turbocharged V6 engine (EA839), known for its smooth power delivery and sporty character. Its exhaust layout is relatively complex, featuring a single exhaust path from the turbocharger that splits downstream after the downpipe into dual rear outlets. Selecting the right exhaust system requires understanding these factory design nuances alongside your performance and sound goals.

System Types: Cat-Back, Axle-Back, and Turbo-Back Explained

  • Cat-back Exhaust Systems: This upgrade replaces the exhaust components from the catalytic converters rearward, including the mid-pipe, mufflers, and tips. Cat-back systems are the most popular choice for street-driven S4s because they provide a noticeable improvement in exhaust sound and modest horsepower gains (typically 5–15 horsepower) without the need for ECU tuning. They maintain factory emissions controls and offer a direct bolt-on experience.
  • Axle-back Exhaust Systems: Axle-back systems replace only the rear mufflers and exhaust tips. They primarily alter the exhaust tone, providing a deeper or more aggressive sound without significant performance increases. This option is ideal if you want a subtle upgrade that does not affect emissions or require extensive installation work.
  • Turbo-back Exhaust Systems: Turbo-back kits replace everything from the turbocharger outlet through the entire exhaust system, including the downpipes. This setup eliminates restrictive factory catalytic converters and oxygen sensors, resulting in much larger power gains (20–40 horsepower or more with proper tuning). However, turbo-back systems often require ECU tuning to avoid check engine lights and are illegal for street use in many regions due to emissions regulations. They are best suited for track-focused or off-road applications.

Material and Construction Considerations

  • Stainless Steel (304 Grade): The most common material for aftermarket exhausts due to its excellent corrosion resistance, durability, and moderate price point. A 304 stainless steel system can withstand harsh weather and road salt, making it a practical choice for daily-driven S4s.
  • Titanium: Titanium exhausts offer significant weight savings—approximately 40–50% lighter than stainless steel—and produce a distinct, higher-pitched, and sharper exhaust note. These systems are often reserved for track-oriented builds due to their premium cost and unique sound characteristics.
  • Mandrel Bending vs. Crush Bending: Mandrel bending maintains a consistent internal pipe diameter through bends, reducing flow restrictions and improving exhaust scavenging. Avoid systems that use crush bending, which can cause internal pipe collapse and reduce performance.

Sound Preferences and Adjustable Valves

When choosing an exhaust, it’s essential to consider how it will sound both at idle and under load. Brands like AWE Tuning and Milltek Sport are known for refined, drone-free tones that complement the S4’s character. In contrast, companies such as Armytrix and Fi Exhaust offer electronically controlled valve systems that let you switch between quiet cruising mode and aggressive, track-ready sound with the push of a button. Additionally, some systems feature resonated mid-pipes, which help reduce unwanted drone during highway cruising at the expense of some top-end volume and aggressiveness.

Tools and Equipment: Beyond the Basics

While a basic exhaust swap requires common hand tools, professional-grade installation demands several specialty tools to ensure a smooth and damage-free process. Before starting, make sure you have the following:

  • Torque Wrench: Essential for tightening bolts and clamps to manufacturer specifications. Use both foot-pound (ft-lb) and inch-pound (in-lb) settings, especially for sensitive components like oxygen sensor bungs.
  • Exhaust Hanger Removal Tool: Designed to easily pry rubber hangers off metal pegs without damaging the hangers or your hands—much safer and faster than using a standard pry bar.
  • Anti-Seize Compound: Apply to all bolt threads and O2 sensor threads to prevent rust and seizing caused by extreme exhaust temperatures.
  • Penetrating Oil: Use products like PB Blaster or Liquid Wrench to soak rusted bolts and hangers at least 12 hours before attempting removal, reducing the risk of breakage.
  • Deep Sockets (14mm, 15mm, 16mm): Necessary for exhaust manifold studs, downpipe bolts, and heat shield fasteners.
  • Heat Gun or Propane Torch: Useful for heating stubborn or corroded bolts to facilitate loosening.
  • High-Temperature RTV Gasket Sealant: Optional but helpful for sealing flange gaskets if new gaskets are not provided.
  • Safety Wire or Zip Ties: Handy for temporarily securing hangers and exhaust components during dry-fitting and alignment.

Preparation and Safety Essentials

Proper preparation and safety measures are critical for a successful exhaust installation. Follow these guidelines:

  • Work on a level, well-lit, and well-ventilated garage floor or driveway.
  • Use quality jack stands rated for at least 3 tons to support the vehicle securely—never rely on a hydraulic jack alone.
  • Chock the rear wheels to prevent vehicle movement.
  • Allow the engine and exhaust components to cool completely—exhaust parts can exceed 600°F after driving.
  • Wear thick mechanic’s gloves and safety glasses to protect against hot surfaces, rust flakes, and debris.

Pre-Installation Inspection

Before removing the stock exhaust, inspect the underbody thoroughly for rust, corrosion, or damaged hangers. Spray penetrating oil generously on all exhaust fasteners, including bolts, clamps, and hanger studs, and allow it to soak for 15–20 minutes. Pay special attention to the downpipe-to-turbo junction, which on the 2020 S4 uses a V-band clamp that can corrode and seize. If you encounter a stuck clamp, consider replacing it with a new OEM or aftermarket one during installation for improved sealing and reliability.

Step-by-Step Installation Walkthrough (Advanced)

1. Removing the Stock Exhaust

Begin at the rear of the vehicle by unbolting the axle-back section first, then progressively work forward towards the mid-pipe and downpipe. Support each section with a jack or enlist a helper to avoid dropping or damaging components during removal. For the downpipe, access the V-band clamp from underneath the vehicle using a long extension bar combined with a swivel socket to reach tight spaces. Maneuvering the downpipe past the steering rack and front subframe can be challenging; loosening the transmission heat shield (two 10mm bolts) provides additional clearance and easier extraction.

2. Dry-Fitting the New System

Before fully tightening any fasteners, loosely assemble the new exhaust by connecting all sections with clamps and hangers. This step allows you to adjust the position of each component to ensure proper clearance around heat shields, the driveshaft, and the rear differential. The S4’s exhaust tunnel is tight, so maintain a minimum clearance of 3/8 inch (approximately 10mm) from all moving or heat-sensitive parts to prevent rattles, heat damage, or premature component wear.

3. Tightening Sequence and Torque Specifications

  • V-band Clamp: Tighten to manufacturer specifications, typically between 18 and 22 ft-lb. Over-tightening can warp the clamp flange and cause leaks.
  • Mid-pipe to Downpipe Flange: Always use new gaskets and tighten the bolts in a star pattern to evenly distribute pressure. Torque to approximately 20 ft-lb.
  • Axle-back Connection: Use a slip-fit clamp and tighten to 30–40 ft-lb. Avoid over-tightening, which can crush the exhaust tube and restrict flow.
  • Hanger Studs: For bolt-on hangers, apply blue Loctite (medium strength threadlocker) to prevent loosening from vibration and torque to 15 ft-lb.

4. Reinstalling Oxygen Sensors

The 2020 Audi S4 is equipped with two oxygen sensors: one located in the downpipe before the catalytic converter (pre-cat) and one positioned downstream of the secondary catalyst. When transferring these sensors to the new exhaust, apply anti-seize compound to the threads and torque them carefully to 25–30 ft-lb. Avoid over-tightening, as oxygen sensor threads are fragile and can strip easily. If your new downpipe lacks a bung for the secondary sensor, you may need to install a weld-in bung or use a sensor spacer (also known as a defouler) to prevent a check engine light due to incorrect sensor readings.

5. Final Checks and Test Drive

  • Start the engine and listen carefully for any exhaust leaks. To amplify potential leaks, place a damp cloth over the exhaust tips, which increases backpressure and makes hissing noises more noticeable.
  • Check that the exhaust tips are centered properly within the bumper cutouts. Adjust hangers and clamps as necessary to achieve a symmetrical and factory-like appearance.
  • When the engine is warm, monitor for any unusual burning smells. It is normal for newly installed components to outgas machining oils and coatings during the first 10–15 minutes of operation.
  • Perform a test drive of 10–15 miles, varying speeds and engine loads. After the drive, recheck all bolts, clamps, and V-band clamps while the system is still hot. Retighten as necessary since heat cycles can cause slight fastener loosening.

Tuning and ECU Considerations

Installing a cat-back exhaust system alone typically does not require ECU tuning as it preserves factory emissions components and backpressure characteristics. However, if you install a downpipe featuring high-flow catalytic converters or catless designs, the vehicle’s ECU will detect reduced backpressure and trigger a P0420 diagnostic trouble code (DTC). This can cause check engine lights and potential limp mode activation.

To address this, many owners opt for a stage 2 ECU tune, which recalibrates fuel and ignition parameters to accommodate the freer-flowing exhaust. Alternatively, installing an oxygen sensor spacer can help by slightly delaying sensor readings, although this is a less reliable fix and may not pass emissions inspections.

Trusted tuning companies such as Integrated Engineering, APR, and Unitronic offer custom tunes specifically engineered for the 2020 Audi S4 platform. For optimal results, consider combining your exhaust upgrade with an intake and intercooler system, then have your vehicle professionally dyno-tuned to maximize horsepower and torque gains while maintaining reliability.

Maintenance and Longevity of Your High-Performance Exhaust

High-performance exhaust systems are generally low-maintenance but require some attention to ensure long-term durability and performance. After installation, it is advisable to drive the vehicle for about one week and then re-torque all clamps, bolts, and hangers to compensate for heat-induced expansion and settling.

Annually, inspect the exhaust system for the following key areas:

  • Clamp Slippage: Check for any movement or shifting of the system that could cause contact with the underbody, leading to rattles or damage.
  • Corrosion and Rust: Particularly on stainless steel systems, inspect weld joints and flanges for rust or pitting, which can occur in regions where road salt is commonly used during winter months.
  • Hanger Condition: Replace rubber hangers every 2–3 years as they degrade over time. Upgrading to silicone or polyurethane hangers can significantly improve longevity and reduce noise from exhaust movement.

For those driving in harsh winter conditions, consider applying a ceramic coating or high-temperature paint to downpipes and mid-pipes to prevent surface rust. Titanium exhausts require special care to avoid exposure to brake fluid and road salt, which can cause discoloration and degrade the finish.

Sound Characteristics and Break-In Period

Expect the exhaust note of your new system to evolve during the initial 200–500 miles of use. Fresh packing material inside mufflers and resonators will settle and compress over time, often resulting in a slightly louder, richer, and more refined tone. During the break-in period, it is advisable to avoid sustained high-RPM operation for the first 50 miles to allow gaskets and sealing surfaces to properly mate, reducing the risk of leaks.

Reducing Unwanted Exhaust Drone

Some owners report experiencing exhaust drone—an unpleasant, resonant vibration—at highway cruising speeds (typically 1800–2200 RPM on the S4). Fortunately, several solutions can mitigate this issue:

  • Add a Helmholtz Resonator or J-Pipe: Some aftermarket manufacturers offer these as add-on components designed to cancel specific frequencies responsible for drone.
  • Additional Exhaust Hanger: Fabricating and installing an extra hanger near the center of the exhaust system can alter vibration modes and reduce drone.
  • Resonated Mid-Pipe: Opting for a resonated mid-pipe section instead of a straight-through design helps absorb and reduce drone frequencies without sacrificing too much sound quality.

Common Mistakes to Avoid During Installation

  • Reusing Old Gaskets: Always use new OEM or supplied gaskets with flange connections. Reusing compressed or damaged gaskets almost guarantees exhaust leaks.
  • Over-Tightening Slip-Fit Clamps: Excessive torque on slip-fit clamps can deform the exhaust tubing, causing leaks and restricting flow. Tighten clamps firmly until secure, then add approximately a quarter turn.
  • Ignoring Hanger Orientation: Some aftermarket hangers are designed to rotate for alignment. Ensure rubber hangers hang straight and are not twisted to prevent premature wear and exhaust movement.
  • Neglecting ECU Adaptation: After a full turbo-back installation, drive the car under varied conditions for about 20 minutes to allow the ECU to relearn fuel trims and adapt to the new exhaust flow. If a check engine light appears, clear codes and retest.

External Resources and References

For further insights, detailed installation photos, and user experiences, consult these trusted community and manufacturer resources:

  • Audizine B9 S4 Forum – Extensive discussions, installation write-ups, and owner reviews of various exhaust systems for the B9 Audi S4.
  • ECS Tuning Exhaust Guide – Detailed product specifications, compatibility information, and installation notes specifically for the 2020 Audi S4.
  • Akrapović Official Installation Manual – Manufacturer-provided instructions and best practices for high-quality exhaust upgrades with Akrapović products.

Performance Gains Summary

Below is a realistic estimate of power and torque gains achievable on a 2020 Audi S4 with various exhaust and supporting modifications, based on dyno results from reputable tuners. Keep in mind that results vary depending on fuel quality, altitude, and installation precision.

  • Cat-back Exhaust Only: 5–12 horsepower, 8–15 lb-ft torque increase.
  • Cat-back Exhaust + Intake Upgrade: 10–20 horsepower, 15–25 lb-ft torque increase.
  • Cat-back + High-Flow Downpipe + Stage 2 ECU Tune: 30–50 horsepower, 40–70 lb-ft torque increase.
  • Full Turbo-back Exhaust + Upgraded Intercooler + E85 Fuel Tune: 60–80 horsepower, 70–90 lb-ft torque increase.

Conclusion

Installing a high-performance exhaust on your 2020 Audi S4 is an effective and enjoyable way to enhance both your vehicle’s auditory presence and dynamic capabilities. By carefully selecting the appropriate exhaust system that fits your needs, thoroughly preparing your workspace and tools, and following a meticulous installation process, you can avoid common pitfalls such as leaks, drone, or ECU errors. Additionally, attentive maintenance will ensure your investment remains reliable and enjoyable for many miles. If you lack access to a vehicle lift or sufficient mechanical experience, consulting professional installers is highly recommended. With the right combination of components and technique, your Audi S4 will deliver an exhilarating driving experience marked by an aggressive yet refined exhaust note and improved performance.