Why Upgrade the Turbo on Your B9 A4 2.0T?

The B9 Audi A4 2.0T (2017–present) is already a solid performer, thanks to its turbocharged 2.0L EA888 Gen 3B engine. But for enthusiasts chasing real power gains beyond a simple stage 1 tune, the factory turbocharger quickly becomes a bottleneck. The stock IHI IS12 or similar unit maxes out around 260–280 wheel horsepower on pump gas. To push past that safely and reliably, a larger turbocharger like the BorgWarner S3 is the proven next step. This upgrade not only unlocks 300+ horsepower but also dramatically improves mid-range torque and throttle response, transforming the car into a genuine sport sedan.

Understanding the BorgWarner S3 Turbocharger

The BorgWarner S3 turbo (often sold as the “IS20” or “S3/RS3 upgrade”) is a direct swap for many MQB-platform EA888 engines, including the B9 A4. It offers a larger compressor wheel and optimized turbine housing compared to the stock unit. Key features include:

  • Compressor wheel: 9-blade, 50mm inducer (vs. ~45mm stock) for significantly higher airflow capacity.
  • Boost pressure: Capable of 28–30 psi safely with proper fueling and intercooling.
  • Spool characteristics: Slightly slower than stock below 2500 RPM, but pulls hard from 3000 RPM to redline.
  • Efficiency: Improved thermal management and reduced backpressure at high RPM compared to smaller turbos.

The S3 turbo is rated for approximately 350–400 crank horsepower, making it ideal for daily driven builds aiming for 300–360 wheel horsepower on pump gas or E85.

Installation Process for the BorgWarner S3 Turbo on B9 A4

Proper installation is critical to avoid leaks, oil starvation, or boost control issues. The following guide assumes you have mechanical experience and access to a lift or jack stands. Always consult your vehicle service manual for torque specs and procedures.

Tools and Materials Required

  • Socket set (metric, 8mm–18mm), including triple-square bits (8mm, 10mm) for exhaust manifold bolts
  • Torque wrench (ft-lb and in-lb)
  • Gasket scraper and brake cleaner
  • New OEM gaskets: turbo-to-manifold, manifold-to-head, exhaust downpipe gasket
  • New oil supply line and crush washers
  • New coolant supply and return lines (if reusing lines, inspect for hardening)
  • Drain plug gasket for oil pan
  • Cat-back exhaust or downpipe removal tools (WD-40, breaker bar)
  • Hose clamp pliers, needle-nose pliers
  • Engine hoist or support bar (optional, for supporting engine if removing subframe)

Step-by-Step Installation

While each car may vary slightly due to options (AWD vs FWD, sport exhaust), the core process is consistent.

  • Step 1: Prepare the car. Disconnect the battery negative terminal. Jack up the vehicle and support on jack stands. Remove the engine cover, intake air box, and turbo inlet pipe. Drain the engine oil and coolant (or clamp coolant hoses to minimize loss).
  • Step 2: Access the turbo. Remove the front bumper cover and crash bar for better access. On the B9 A4, the turbo sits low on the back of the engine (close to the firewall). Removing the downpipe and exhaust manifold heat shields first gives you room. Detach the oxygen sensors from the downpipe.
  • Step 3: Disconnect lines and pipes. Carefully unclip the turbo coolant and oil lines. You may need to remove the oil feed banjo bolt from the top of the engine block. Disconnect the charge air pipe (turbo outlet to intercooler). Remove the wastegate actuator rod if needed.
  • Step 4: Remove the stock turbo. Support the turbo from below. Unbolt the exhaust manifold from the cylinder head (usually 8 or 10 triple-square bolts). Remove the manifold and turbo as an assembly. Separate the old turbo from the manifold on the bench.
  • Step 5: Prepare the BorgWarner S3 turbo. Transfer the heat shield, wastegate actuator, and any brackets from the stock turbo to the S3 unit. Use new gaskets and clean all mating surfaces. Reposition the wastegate actuator rod to align with the wastegate arm (adjust if needed to maintain proper preload).
  • Step 6: Install the new turbo. Bolt the S3 turbo to the manifold with new gasket, torque to spec. Reinstall the manifold/turbo assembly to the cylinder head. Torque head bolts in sequence (see manual). Reconnect oil and coolant lines with new crush washers. Tighten banjo bolts to spec (over-torquing can strip threads).
  • Step 7: Reassemble. Reinstall downpipe with new gasket, oxygen sensors, charge air pipe, heat shields, front bumper, and intake. Fill engine with fresh oil (5W-40 or 0W-40 recommended). Refill coolant and bleed the system. Reconnect battery.
  • Step 8: Initial start and check. Start the engine and let it idle. Check for oil leaks, coolant leaks, and exhaust leaks. Monitor boost pressure and look for smoke. After a short warm-up, perform a gentle test drive. Do a final tighten check of all nuts.

Expected Power Gains from the S3 Turbo Upgrade

Without supporting mods, the S3 turbo will not reach its full potential. But even with a stock downpipe and just a generic flash tune, you can expect the following gains over stock (on 93 octane):

  • Horsepower: 300–315 whp (stock is ~230–240 whp). That’s an increase of 60–85 wheel horsepower.
  • Torque: 330–370 ft-lb at the wheels (stock ~270 ft-lb). Peak torque comes on earlier and holds longer.
  • Boost: 24–27 psi (stock 18–20 psi) depending on tune and fuel.
  • Spool: Full boost by 3200–3500 RPM vs 2800 RPM stock — slight trade-off for top-end gain.

With a full bolt-on setup (downpipe, intercooler, intake, ethanol blend), the S3 turbo can push 350–370 whp on a custom tune. That’s enough to push the B9 A4 to low 12-second quarter-mile times.

Supporting Modifications for Maximum Performance

The S3 turbo upgrade alone is not a “drop in and go” solution for high power. To safely support 300+ whp, these modifications are strongly recommended:

  • Upgraded intercooler: The factory intercooler heat soaks quickly on sustained pulls. An upgraded unit (e.g., Wagner Tuning, Integrated Engineering) lowers intake temps by 20–40°F and reduces timing pull.
  • High-flow fuel injectors: The stock injectors are adequate for ~310 whp on pump gas. For E85 or higher power, upgrade to 550–650cc injectors (e.g., Bosch or Injector Dynamics). You’ll also need a high-pressure fuel pump (HPFP) for E85.
  • Performance downpipe: A 3-inch catted or catless downpipe reduces backpressure and allows the turbo to breathe. This can add 15–25 whp alone when tuned.
  • Intake system: An intake with a larger inlet pipe and high-flow filter (e.g., APR, IE, Eventuri) reduces pressure drop and improves turbo response.
  • ECU tuning: This is the most critical supporting mod. A proper tune (from a reputable shop like United Motorsport, EPL, or through OTS files from Unitronic or 034Motorsport) will calibrate fuel, timing, and boost maps for the S3 turbo. Without a tune, the S3 turbo will overboost or run dangerously lean.

Optional Upgrades for Reliability

  • Clutch upgrade (manual): Stock clutch slips around 370 ft-lb. Upgrade to a Southbend Stage 3 or DSG tuning with upgraded clutches for automatics.
  • Engine mounts: Stiffer mounts reduce wheel hop and improve driveline power transfer.
  • Diverter valve: Stock valve is adequate; but a GFB DV+ or Turbosmart upgrades can hold higher boost without leaking.

Tuning Options and Considerations

Choosing the right tuning platform is as important as the hardware. For the B9 A4, you have three main routes:

  • Off-the-shelf (OTS) tunes: Companies like Unitronic and 034Motorsport offer stage 2/2+ files that accommodate the S3 turbo with specific hardware lists. These are convenient but less aggressive than custom tunes.
  • Custom tuning via COBB Accessport: The B9 A4’s ECU is supported by COBB. You can run an OTS tune from COBB or get a custom remote tune from a shop like EQT or Stratified. This gives the most flexibility for ethanol blends or unique setups.
  • Standalone ECUs: For extreme builds (500+ whp), consider a standalone like Motec or Syvecs, but that’s overkill for the S3 turbo.

Important: Always log boost, fuel pressure, and knock retard after installation to ensure the tune is safe. Overboosting on a cold day can damage engine.

Common Installation Pitfalls and Tips

  • Oil line routing: The S3 turbo oil feed line may have a different banjo bolt size than stock. Check fitment before assembly. Use thread sealer on pipe threads, not Teflon tape.
  • Wastegate adjustment: If the actuator preload is wrong, you’ll either have creep (overboost) or slow spool. Set preload to 0.5–1mm of rod extension beyond fully closed wastegate.
  • Coolant line fitting: The S3 turbo uses O-ring seal fittings on some coolant connections. If reusing stock lines, replace the O-rings.
  • Downpipe clearance: Aftermarket downpipes may require loosening engine mounts to wiggle into place. Use a steady hand to avoid damaging the driveshaft. (AWD models have tight clearance.)
  • Bleed coolant properly: Use a vacuum filler or run the engine with heater full hot to purge air bubbles. Air pockets can cause overheating or poor heater output.

Reliability and Daily Drivability

With correct tuning and supporting mods, the S3 turbo upgrade is perfectly reliable for daily driving. The EA888 Gen 3 (and 3B in B9) engine has strong connecting rods, forged cranks, and good oiling capacity. Power levels up to 350 whp are within safe margins. However, consider upgrading the PCV system (to an RS3-style catch can) to reduce crankcase pressure at high boost.

The BorgWarner S3 turbo itself is durable, with a journal bearing design that withstands street use and occasional track days. Expect a service life of 80,000–100,000 miles if properly maintained (regular oil changes with quality synthetic, cool-down idling after hard pulls).

Conclusion

The BorgWarner S3 turbo upgrade for the B9 A4 2.0T is one of the most cost-effective ways to achieve a genuine performance transformation. With careful installation and the right supporting modifications—tune, intercooler, fuel system, and exhaust—you can expect 300–370 wheel horsepower, while retaining daily drivability and reliability. This upgrade turns your modest A4 into a serious contender against the S4 and even some newer performance rivals. As with any turbo swap, take your time, use quality parts, and consider dyno tuning to dial in the final result. For more information on BorgWarner turbo specifications, visit BorgWarner’s official site or check out community guides on Audizine and APR’s knowledge base.