The Audi B9 S4, produced from model year 2017 onward, occupies a sweet spot in the performance sedan segment: it blends everyday luxury with genuine sporting capability. But for enthusiasts, the stock power output—while satisfying—is just a starting point. The APR Stage 3 upgrade transforms the B9 S4 into a genuine powerhouse, promising 420 horsepower and 470 lb-ft of torque. This article provides a detailed comparison between the stock B9 S4 and the APR Stage 3 configuration, covering engine architecture, real-world performance, hardware changes, and critical considerations for anyone thinking about the jump.

B9 S4 Engine Architecture: The 3.0T V6

At the heart of the B9 S4 lies the 3.0L TFSI V6 (EA839 engine family), a direct-injected, supercharged six-cylinder that replaced the previous generation’s supercharged 3.0T. This engine features a compact Roots-type supercharger mounted in the V of the block, an intercooled charge air system, and dual variable valve timing. In stock form, the engine is rated at 349 horsepower and 369 lb-ft of torque. However, many dyno tests have shown that the B9 S4 often underrates its output at the crank—real-world wheel figures are typically close to 330 whp. That reserve means the engine has proven robust enough to handle substantial additional power when properly supported.

The EA839 block is made of a cast aluminum alloy with centrifugally cast iron cylinder liners. The intake and exhaust manifolds are integrated into the cylinder head, reducing weight and improving packaging. Fuel delivery uses a combination of port injection and direct injection to help manage carbon buildup and ensure proper air-fuel mixing under high load. The stock supercharger spins at up to 23,000 rpm and produces peak boost around 11–12 psi. The engine management system, from Bosch, is highly capable and can be reflashed by tuners such as APR to increase boost and optimize timing.

Stock B9 S4 Performance Numbers

In factory trim, the B9 S4 delivers a balanced, predictable driving experience. The supercharger provides linear power delivery without the lag of a larger turbocharger, though the top-end rush is less dramatic. Key factory specifications:

  • Horsepower: 349 hp @ 5,400–6,400 rpm
  • Torque: 369 lb-ft @ 1,370–4,500 rpm
  • 0–60 mph: ~4.4 seconds (Audi claim, independent tests often show 4.2 seconds)
  • Quarter-mile: ~12.8 seconds @ 108 mph
  • Top speed: Electronically limited to 155 mph (limited by software; chassis can go higher)
  • Transmission: 8-speed ZF 8HP automatic with launch control
  • Drivetrain: Quattro with self-locking center differential

The stock car already offers strong mid-range torque, making it easy to pass at highway speeds. The 8-speed automatic shifts crisply, and the Quattro system delivers prodigious traction. However, the engine clearly has headroom: the same 3.0T V6 in Audi’s RS models (e.g., the RS5) with a twin-turbo V6 layout produces around 444 hp. For the B9 S4, the aftermarket has stepped in to close that gap—and then some.

APR Stage 3 Upgrade: What Changes

APR’s Stage 3 kit for the B9 S4 is a comprehensive hardware and software package. It is not a simple flash—it involves swapping the supercharger, intercoolers, and exhaust, plus a custom ECU calibration. The core components are:

  • Larger supercharger: APR uses a proprietary oversized Roots-type unit (often sourced from a Vortech or similar supplier), with an increased housing and rotor set that flows significantly more air than stock. Maximum boost rises to ~18–20 psi.
  • Upgraded intercoolers: A pair of front-mounted intercoolers replace the factory side-mount units. Larger cores and improved airflow keep charge air temperatures under control during repeated hard pulls.
  • High-flow exhaust system: APR includes a cat-back exhaust (with optional downpipes) to reduce backpressure and enhance the V6’s note.
  • ECU reflash: A specially calibrated tune for the Stage 3 hardware adjusts fuel mapping, ignition timing, boost targets, and transmission shift parameters. APR offers both 93-octane and 100-octane files.
  • Optional cooling and intake upgrades: A high-flow air intake and an auxiliary heat exchanger can be added for maximum performance.

The kit is engineered to bolt directly to the EA839 engine with no permanent modifications required, making it reversible if needed. APR claims the Stage 3 setup delivers 420 horsepower at the crank and 470 lb-ft of torque, though independent dyno runs often show 380–400 whp on 93 octane.

APR Stage 3 Power Specifications (Claimed)

  • Horsepower: 420 hp @ 6,200 rpm
  • Torque: 470 lb-ft @ 3,200–5,500 rpm
  • 0–60 mph: ~3.7 seconds (with launch control and sticky tires)
  • Quarter-mile: ~11.8 seconds @ 117–119 mph
  • Top speed: Electronically limited to 155 mph (limiter can be raised to ~180+ mph)
  • Boost: 18–20 psi peak

Performance Comparison: Stock vs. APR Stage 3

The difference between stock and Stage 3 is night and day. Below is a head-to-head comparison of key metrics:

Metric Stock APR Stage 3
Peak HP (crank) 349 hp 420 hp
Peak Torque (crank) 369 lb-ft 470 lb-ft
0–60 mph 4.4 sec 3.7–3.9 sec
0–100 mph ~10.5 sec ~8.2 sec
Quarter-mile ~12.8 sec @ 108 mph ~11.8 sec @ 118 mph
Peak Boost ~11 psi ~19 psi
Power Delivery Linear, smooth Aggressive, strong mid-range and top end

Acceleration and Throttle Response

The most immediate sensation when driving an APR Stage 3 B9 S4 is the torque surge. The stock car already pulls hard from 2,000 rpm, but the Stage 3 setup multiplies that feeling. The larger supercharger builds boost more quickly, and the improved intercoolers keep intake air temperatures stable, allowing consistent power during repeated runs. The 0–60 mph drop of 0.5–0.7 seconds may not sound huge, but it shaves nearly a full second off the quarter-mile time. In real-world terms, a Stage 3 B9 S4 will pull away from a stock car from any speed, on any road.

Throttle response also sharpens. APR’s ECU calibration includes faster accelerator pedal mapping and reduced torque management intervention. The transmission shift points are raised and shifts are firmer, matching the higher power output. The car feels more alive, more willing to rev, and more urgent when you lean on the gas.

Driving Experience on the Road

On public roads, the stock B9 S4 already has more power than most drivers can use legally. Stage 3 makes it a genuinely fast car by any standard. The extra torque makes highway passing effortless: merging from a 45 mph on-ramp to 80 mph takes just a few seconds. The supercharger whine becomes more pronounced with the high-flow intake, adding a mechanical soundtrack that enthusiasts appreciate. However, the aggressive throttle and boost can make low-speed traffic driving a bit jumpy if you’re not careful. The car demands more attention during daily commuting—it is no longer a docile cruiser.

For track days or autocross, the Stage 3 upgrade provides the power to hang with cars like the BMW M3 (F80) and Mercedes-AMG C63 S. The Quattro system gives immense traction out of corners, and the torque curve is wide enough that you can leave it in a higher gear and still accelerate hard. But the added weight of the supercharger system and the need for better brakes and tires become more apparent. Many owners upgrade to ceramic brakes (or at least high-performance pads) and fit wider, stickier tires to put the power down properly.

Cost Breakdown of APR Stage 3

The APR Stage 3 kit is not cheap. As of 2025, a rough breakdown:

  • Supercharger kit: $4,500–$5,500 (including pulley, brackets, and installation hardware)
  • Intercooler kit (upgraded): $1,200–$1,500
  • Cat-back exhaust: $1,500–$2,000 (downpipes add another $500–$800)
  • ECU tune and installation: $1,200–$1,800
  • Optional: intake, heat exchanger, hoses: $500–$1,000
  • Labor for full installation (8–12 hours): $1,200–$2,400
  • Total estimated cost: $10,000–$14,000 (depending on options and labor rates)

That sum is more than the price of many used cars, but for a B9 S4 owner seeking track-level performance without buying a new vehicle, it makes financial sense compared to trading up to an RS5 or a later RS3. Additionally, APR parts come with a limited warranty and transferable registration, which helps resale value.

Reliability and Long-Term Considerations

Increasing power by 70 hp and 100 lb-ft places extra stress on the engine and drivetrain. The EA839 engine is stout, but the following points deserve attention:

  • Heat management: The supercharger generates more heat at higher boost. The upgraded intercoolers are essential. Some owners add an oil cooler or water-methanol injection for extended track sessions.
  • Transmission: The ZF 8HP is rated for up to 600 lb-ft in some applications, so 470 lb-ft is within its capacity. However, the torque converter and valve body may benefit from a high-performance transmission tune (APR includes a TCU tune with Stage 3).
  • Fuel quality: To run the maximum 93-octane tune, you must use premium fuel. Using lower octane can cause knock and reduce power. APR also offers a 100-octane file for track days.
  • Warranty: Any aftermarket upgrade will void the factory powertrain warranty. APR offers a limited warranty on its parts (typically 1 year), but Audi will not cover damage attributed to modifications.
  • Maintenance: Shortened service intervals are recommended—oil changes every 3,000–5,000 miles, spark plugs every 15,000 miles, and supercharger belt replacement at 30,000 miles.

Alternatives and Competitive Upgrades

The APR Stage 3 is not the only path to big power. Competitors such as Unitronic and 034Motorsport offer Stage 2+ or Stage 3 kits with similar output. Unitronic’s Stage 2+ for the B9 S4 (without a supercharger swap) uses a smaller pulley, intercooler, and exhaust to reach 400 hp, at a lower cost ($3,000–$5,000). However, the APR Stage 3’s larger supercharger provides superior top-end power and more consistent cooling. For owners seeking the absolute highest numbers, a hybrid or full turbo conversion exists, but those cost double and require extensive fabrication.

Another alternative is the B9 RS5 (2.9L twin-turbo V6), which comes with 444 hp stock and can be tuned to over 500 hp. The RS5 has a different engine and transmission, but it is a natural upgrade path. The B9 S4 with APR Stage 3, however, offers similar performance for less money upfront—if you already own the S4.

Real-World Owner Perspectives

We spoke with two owners who have made the jump. Jake, a track enthusiast in Arizona, says: “The Stage 3 turns the S4 into a monster. On the track, I can keep up with GT350s. The biggest surprise was how drivable it is on the street—yes, it’s loud and aggressive, but I can still cruise to work in comfort.” Another owner, Maria from Colorado, emphasized the need for supporting mods: “I did the brakes and tires first. With Stage 3, the stock Pirellis are useless. I upgraded to Michelin Pilot Sport 4S and added an oil cooler. The car now feels bulletproof after 30,000 miles.”

Both owners noted that the installation is not trivial. It took a specialized Audi shop two days to complete the work, and the initial tune required a few revisions to eliminate a hesitation at part-throttle. APR’s customer support is generally considered good, but they require the car to be tuned at an authorized dealer, not via a remote flash.

Conclusion: Is APR Stage 3 Worth It?

The stock B9 S4 is a refined, quick sports sedan that satisfies most drivers. The APR Stage 3 upgrade elevates it to a different performance tier—comparable to factory RS models or even older supercars. With 420 hp and 470 lb-ft, it becomes seriously quick, with excellent tractability and a thrilling power delivery. However, the cost—over $10,000—and the potential for increased wear and warranty loss mean it is not for everyone. For the dedicated enthusiast who values performance and is willing to invest in supporting upgrades, the APR Stage 3 kit is a proven, well-engineered solution that transforms the B9 S4 into a memorable driver’s car. The decision ultimately hinges on your budget, your tolerance for maintenance, and your desire for a truly elevated driving experience.

For more details, visit APR’s official B9 S4 Stage 3 page, or check out real-world dyno charts on Audizine forums. For an independent review, Eurotuned’s Stage 3 review offers additional data.