The B9 S4: A Platform Ripe for Big Power Gains

The B9 Audi S4, produced from 2016 to 2019 (before the B9.5 refresh), surprised many enthusiasts when it swapped the supercharged 3.0L V6 (from the B8 generation) for a turbocharged 3.0L V6—the EA839. This single change unlocked massive tuning potential. The stock engine produces 349 horsepower and 369 lb-ft of torque, but its closed-deck block, forged steel crankshaft, and robust cooling system are built to handle significantly more. With the right modifications, owners have reliably pushed past 500 wheel horsepower on the factory turbocharger.

This guide breaks down the cost to transform your B9 S4 from a luxury sport sedan into a genuine street predator. We’ll cover modifications ranging from a simple $500 ECU tune to a fully built $3,500 setup that delivers over 100 horsepower gains. Each section explains what the part does, how it affects power, and what you can realistically expect on the dyno. Whether you are a weekend warrior or a track-day regular, you’ll find a path that fits your budget and goals.

Entry-Level Power: Under $1,000

If you are working with a tighter budget but still want a meaningful bump in performance, the sub-$1,000 segment offers excellent return on investment. These modifications focus on removing factory bottlenecks that restrict airflow and timing.

ECU Tune (Stage 1) – $500 to $800

The single most cost-effective modification for the B9 S4 is an ECU (Engine Control Unit) tune. Factory calibration leaves substantial headroom for safety and emissions. A Stage 1 tune reprograms boost pressure, ignition timing, and fuel delivery. Tuners like 034Motorsport, Integrated Engineering, and EPL offer flash tuning that you can perform at home via an OBD2 cable or handheld device.

Power gain: +40 to +60 wheel horsepower and +60 to +80 lb-ft of torque. Expect approximately 390-410 wheel horsepower on 93 octane pump gas.

Cost breakdown: The tune software alone is $500-$800. Some tuners charge extra for a cable (around $100-$200) if you do not already own one. No hardware installation is required.

High-Flow Air Filters and Intake System – $100 to $400

The factory airbox is not terribly restrictive, but replacing the paper filter with a high-flow oiled cotton unit (like aFe Power or K&N) provides a modest gain. For a more noticeable effect, a full cold-air intake system (such as APR’s Carbonio or Eventuri) reduces inlet restrictions and adds induction sound.

Power gain: +10 to +15 wheel horsepower on a stock tune; with a tune, the gain can increase to +15 to +20 when combined with intake calibration.

Cost breakdown: A simple drop-in filter costs $50-$100. A complete intake system ranges from $300 to $600. Installation is straightforward and takes about 30 minutes.

Upgraded Intercooler – $800 to $1,200

The B9 S4’s water-to-air intercooler is adequate for stock boost levels, but it heat-soaks quickly during repeated pulls or hot weather. An aftermarket unit (like APR’s or Wagner Tuning) features a larger core and more efficient heat exchanger. This keeps intake air temperatures lower, allowing the turbo to produce consistent power without pulling timing.

Power gain: Minimal peak gain (+5 to +10 HP) on a stage 1 tune, but significant gains in sustained power—recovering up to 20-30 horsepower on a hot day compared to the stock intercooler.

Cost breakdown: Intercooler upgrade kits run $800-$1,200. Installation is moderately complex, requiring removal of the front bumper and coolant system bleeding. Expect 2-4 hours labor if professionally installed.

Mid-Range Performance: $1,000 to $2,000

This bracket moves from simple bolt-ons to component upgrades that work synergistically with a tune. These are the most popular builds for daily-driven S4s that see occasional track time.

Stage 2 Tune – $1,000 to $1,500

A Stage 2 tune (also called “Stage 1+”) typically adds a higher boost target than Stage 1 and requires one supporting modification: either an upgraded intercooler or a downpipe (or both). Tuners adjust ignition timing more aggressively when they know intake temperatures will stay lower.

Power gain: With an upgraded intercooler and stock downpipe, expect +55 to +75 wheel horsepower over stock (approximately 420-435 whp).

Cost breakdown: The tune is typically $200-$400 more than Stage 1, bringing total software cost to $700-$1,200. Add the intercooler cost if not already purchased. Some tuners offer a Stage 2 package with the intercooler for around $1,500.

Downpipe – $400 to $800

The factory downpipe contains a close-coupled catalytic converter that restricts exhaust flow. Replacing it with a high-flow or catless downpipe (make sure your local emissions laws allow catless) dramatically reduces backpressure and spool time. This piece is essential for making over 450 wheel horsepower.

Power gain: +20 to +30 wheel horsepower on a Stage 2 tune compared to stock downpipe. Spool-up can improve by 300-500 RPM.

Cost breakdown: A catted downpipe (with high-flow GESI cat) costs $600-$800. A catless downpipe is $400-$600. Installation is tedious and often requires removing the driveshaft support bracket, so budget $300-$500 for labor.

Cat-Back Exhaust System – $800 to $1,800

A cat-back exhaust replaces the piping from the downpipe back to the tailpipes. While it offers minimal peak horsepower on a tuned car (maybe +5 to +10 HP), its primary benefits are weight savings (stainless or titanium vs. heavy factory steel) and sound volume. This is a personal preference mod for most owners.

Power gain: +5 to +10 wheel horsepower. The real gain is audible.

Cost breakdown: Brands like AWE Tuning, Milltek, and Akrapovic range from $1,000 to $2,500. Installation is straightforward, 1-2 hours DIY.

Transmission Tune (DSG/TCU) – $500 to $1,000

Often overlooked, the ZF 8-speed automatic (or seven-speed DSG in European models) benefits from a transmission control unit (TCU) tune. This raises shift pressure, reduces torque limiters, and shortens shift times. Without it, a tuned S4 can hit torque limits that force the transmission to shift sluggishly.

Power gain: No direct horsepower, but the car feels 20-30 HP stronger because shifts are faster and allow the engine to stay in the powerband.

Cost breakdown: TCU tunes from 034Motorsport or Integrated Engineering cost $500-$800. Many tuners offer a bundle with the ECU tune for around $1,200.

High-Performance Upgrades: $2,000 to $3,500

This is where the B9 S4 transforms from a fast daily driver into a serious performance machine. These modifications require professional tuning and often additional supporting mods.

Stage 3 (Hybrid Turbo or Larger Turbo) – $2,500 to $3,500 for the turbo alone

If you want over 500 wheel horsepower, the factory turbo (the Mitsubishi TD04-HL) is a bottleneck. Companies like Pure Turbos and EQT offer hybrid or upgraded turbochargers that fit the stock exhaust manifold. These turbos use larger compressor and turbine wheels and sometimes ported housings to flow more air.

Power gain: With proper fueling and tuning, expect +150 to +200 wheel horsepower over stock. Numbers of 520-560 whp on pump gas and 600+ whp on E85 are common.

Cost breakdown: The turbocharger itself runs $1,800-$2,500. You will also need an upgraded low-pressure fuel pump ($500-$700), high-pressure fuel pump internals ($400-$600), and injectors ($500-$1,000) if going full E85. Total package can reach $3,500-$5,000.

Upgraded Fueling – $1,000 to $2,000

To support a larger turbo, you must increase fuel delivery. The B9 S4 uses a combination of a direct injection (DI) high-pressure fuel pump (HPFP) and port injection (PI) that is added via a manifold spacer. For ethanol blends (E30, E50, E85), you need a HPFP upgrade and often a secondary port injection kit.

Power gain: Fueling by itself adds no power, but without it you cannot safely run the boost needed for 450+ whp. On E85 you can gain 15-30 HP purely from the fuel’s cooling and higher octane.

Cost breakdown: HPFP internals (bossell or larger plunger) cost $400-$700. A port injection kit (e.g., from CSF or Syvecs) ranges from $800-$1,400.

Full Exhaust System (Downpipe + Cat-Back) – $1,500 to $3,500

Instead of buying a downpipe and cat-back separately, some owners opt for a full turbo-back exhaust. This includes a downpipe, midpipe, and cat-back. The benefit is a tuned system that flows from the turbo outlet to the tips, maximizing both low-end torque and top-end power.

Power gain: Combined with a Stage 2+ tune, expect +40 to +50 wheel horsepower over Stage 1. On a larger turbo setup, the exhaust is essential for reaching peak numbers.

Cost breakdown: A full turbo-back system from AWE, Akrapovic, or Milltek costs $2,000-$3,500. Installation is 4-6 hours labor.

Putting It All Together: The 500+ WHP Build

Reaching a true 500+ wheel horsepower on a B9 S4 requires a systematic approach. Below is a realistic build order with estimated costs and expectations.

  1. Start with Stage 1 ECU+TCU tune – $1,200. Experience 410-420 whp. This is the daily baseline.
  2. Add intercooler and downpipe – $1,800. Stage 2 tune upgrade – $200. Total so far: $3,200. Horsepower: 440-460 whp.
  3. Install hybrid turbo (e.g., Pure 700) – $2,200. New Stage 3 tune – $500. HPFP upgrade – $600. Total now: $6,500. Horsepower: 520-540 whp on pump gas.
  4. Add port injection and ethanol tuning – $1,500. Total build: $8,000. Horsepower: 580-600 whp on E50.

This progression respects the car’s limits and avoids unnecessary part swaps. Each stage can be driven comfortably before moving to the next.

Key Considerations Before You Start

Power mods are not just about the price tag. Here are critical factors to protect your investment.

Warranty and Reliability

Any engine modification will void the powertrain warranty for related failures. Some dealers may not flag a Stage 1 tune even if detected, but a Stage 2 or 3 tune is obvious. If your car is under warranty, consider a piggyback module like the JB4 that offers removable tuning, but it does not match the smoothness of a proper flash tune.

Reliability is generally excellent up to 500 whp. Past that, consider upgrading the crank hub, oil pump chain, and PCV system. B9 S4 engines have a known issue with the PCV valve failing under high boost, which can cause oil consumption and smoke. A catch can or upgraded PCV kit from 034Motorsport is a smart preventive measure.

Professional Installation vs. DIY

Many of these mods (intake, intercooler, downpipe) can be done with basic tools. The ECU tune is entirely DIY upload. The transmission tune, however, requires access to a laptop and a stable internet connection. For turbo swaps and fuel system upgrades, professional installation is strongly recommended to avoid labor costs from damage.

Emissions and Legalities

In many states (California, New York, Colorado), removing catalytic converters or installing a catless downpipe will fail emissions testing. Look for catted downpipes with high-flow GESI cats that are OBD2-and-carb-legal in some areas. Always check local laws before purchase.

To help you navigate the aftermarket, here are three trusted sources for B9 S4 parts and information:

  • 034Motorsport – Offers ECU/TCU tunes, intercoolers, intake systems, and comprehensive build guides for the B9 platform.
  • APR Performance – Known for their Stage 1 and Stage 2 tunes, intercoolers, and exhaust systems. Reliable and well-tested.
  • Audizine Forums – A massive community of B9 S4 owners sharing dyno sheets, installation tips, and group buys. Use the search function for “B9 S4 build” to see real-world results.

Conclusion: Is the Investment Worth It?

The B9 S4 is one of the most satisfying platforms to modify because the gains come easily. For $500 you get a car that feels 50 horsepower stronger. For $3,500 you can build a 520+ whp sedan that embarrasses cars costing three times as much. The key is to plan your upgrades in stages, allocate budget for supporting modifications (fueling, cooling, transmission), and work with reputable tuners who understand the EA839’s quirks. With the right approach, your S4 will deliver years of thrilling performance and still serve as a daily driver. The cost breakdown above gives you a roadmap—now it’s up to you to choose your path.