Introduction: The Pursuit of More From the B9 Audi S4

When I took delivery of my B9 Audi S4, I knew I was getting one of the best-balanced sport sedans on the market. The stock 3.0-liter turbocharged V6 (EA839) delivers 349 horsepower and 369 lb-ft of torque on paper (333 hp / 325 lb-ft in some markets), offering effortless daily performance. Yet after six months of ownership, I found myself wanting more – sharper throttle response, stronger mid-range punch, and a soundtrack that matched the chassis’ capability. The logical next step was a Stage 2 tune, and the name that kept surfacing in forums and owner groups was APR (Audi, Porsche, Volkswagen specialists based in Opelika, Alabama).

After extensive research and a frank conversation with my local APR dealer, I committed to the APR Stage 2 ECU upgrade along with the required hardware. The total outlay came to approximately $2,500, including labor. In this article I will detail what that investment delivers – not just the dyno numbers, but how it fundamentally changes the character of the S4. Whether you are considering a similar upgrade or simply curious about modern ECU tuning, this account provides an honest, data-backed look at the transformation.

What Is an APR Stage 2 Tune? A Technical Overview

APR’s Stage 2 calibration is a software-only modification (the ECU is reflashed via the OBD-II port), but it requires a specific set of supporting hardware to safely handle the increased boost pressure and airflow. Unlike Stage 1, which works on an otherwise stock engine, Stage 2 demands a high-flow intake system and an upgraded downpipe (or a full turbo-back exhaust). The tune then optimizes ignition timing, air-fuel ratios, boost targets, and throttle mapping to extract the maximum safe output from the EA839 engine.

APR engineers have spent hundreds of hours on the dynamometer and in real-world testing to ensure that the calibration respects component limits. The ECU flash itself is fully reversible, and APR offers up to six different program slots (e.g., 91 octane, 93 octane, 100 octane, valet mode, stock mode) that can be selected via the cruise control stalk. This flexibility means the car remains daily-driver friendly while delivering race-ready power on demand.

Hardware Requirements for Stage 2

Before the tune can be applied, the following mechanical upgrades are mandatory:

  • High-flow intake system: APR recommends their open-element intake or an equivalent that reduces restriction and draws cooler air from outside the engine bay. The stock airbox becomes a bottleneck once boost exceeds 18 psi.
  • Downpipe (catalytic converter replacement): The stock downpipe has a dense catalyst that creates backpressure. A 200-cell or 100-cell sport cat (or a catless option) reduces exhaust restriction by up to 40% and allows the turbocharger to spool faster. This is the single most impactful hardware change.
  • Intercooler upgrade (recommended but not mandatory): In warm climates or for track use, the stock intercooler heat-soaks quickly. APR’s Stage 2 intercooler system reduces charge air temperatures by up to 50°F, maintaining power on repeated pulls.
  • Spark plugs (one step colder): Higher cylinder pressures demand a colder heat-range plug to prevent pre-ignition. APR specifies NGK or equivalent plugs gapped to 0.024 inches.

My installation included the APR intake, a 200-cell downpipe from AWE Tuning, and fresh spark plugs – totaling about $1,200 in parts plus $600 in labor. The tune itself cost $899 (including the cable for DIY flashing if desired), and I paid $400 for dealer-level flashing and final diagnostics. The grand total was $2,499.

Installation Process: What to Expect

If you are handy with tools, you can tackle the hardware installation yourself and then drive to an APR-authorized facility for the ECU flash. I opted for a one-stop shop where the technician performed everything in a single day. Here is the step-by-step sequence:

  1. Base-line diagnostics: The vehicle is scanned with a factory-level tool (VCDS or ODIS) to confirm there are no fault codes, and the engine oil and coolant levels are checked.
  2. Intake and downpipe installation: The stock airbox is removed and replaced with the APR intake (25 minutes). The downpipe requires lifting the car, disconnecting the exhaust system, and removing the heat shields – about 1.5 hours. The new downpipe is bolted in place with new gaskets and hardware.
  3. Spark plug replacement: Accessing the plugs on the EA839 is straightforward; the technician carefully removes the coils, replaces the plugs to the correct gap, and torques them to spec.
  4. ECU flash: After the hardware is verified, the APR tool is connected to the OBD-II port. The stock file is read and saved, then the Stage 2 calibration (93 octane file) is written. The process takes about 20 minutes.
  5. Test drive and data log: The technician performs a 15-minute road test, monitoring boost levels, intake air temperatures, and lambda values to confirm everything is within safe parameters. A final inspection for exhaust leaks completes the process.

The entire job took 5.5 hours at a shop that specializes in German performance. The customer experience was professional, and APR’s customer support hotline answered a quick question about the intercooler upgrade within ten minutes.

Performance Gains: From Fast to Seriously Quick

The difference from stock is immediately perceptible. The S4’s engine now feels effortlessly potent from 2,500 rpm all the way to redline. APR’s official dyno charts show peak gains of 70–85 wheel horsepower and 80–100 lb-ft of torque over stock, depending on fuel quality and ambient temperature. My car, running on 93 octane pump gas, put down 398 whp and 402 lb-ft on a Mustang Dyno (stock is typically 320–330 whp). That is an increase of roughly 23% at the wheels.

Real-World Acceleration Metrics

While dyno numbers tell part of the story, the stopwatch reveals the true value. Equipped with the seven-speed DSG transmission, my S4 recorded these times using a Racelogic VBOX Sport:

  • 0–60 mph: 3.6 seconds (stock was 4.3 seconds).
  • 0–100 mph: 8.1 seconds (stock was 10.0 seconds).
  • 60–100 mph passing: 4.9 seconds (stock 6.8 seconds).
  • 1/4-mile: 11.7 seconds at 118 mph (stock 12.6 at 110 mph).

These numbers put the tuned S4 in the same league as the RS5, the BMW M3, and even the Porsche 718 Cayman GTS. The car simply feels like a different animal – the turbo spools noticeably faster, there is no hesitation when you mat the throttle, and the exhaust crackles and pops on downshifts thanks to the less restrictive downpipe.

Driveability Enhancements Beyond Raw Power

What surprised me most about the Stage 2 tune was how much it improved day-to-day driving. Many tuners focus on peak horsepower at the expense of low-speed manners, but APR’s calibration preserves smoothness. The throttle mapping is now linear rather than the stock Audi “lazy at light tip-in” characteristic. In city traffic, I no longer need to breathe on the pedal to get moving; there is immediate torque from 1,800 rpm, making stop-and-go feel effortless.

The DSG transmission also benefits. While the gearbox was not reflashed (APR offers a separate DSG tune), the increased torque means the factory clutch packs engage with more authority. Shifts are felt more distinctly, and the transmission kicks down faster when passing because the engine reaches its power band sooner. Fuel economy on the highway actually improved by 1–2 mpg because the engine can maintain cruising speed with less throttle angle – 70 mph now requires only about 10% throttle instead of 15%. I average 27 mpg on the interstate compared to 25 mpg stock.

Sound and Character Transformation

The high-flow downpipe changes the exhaust note from a refined murmur to a deeper, more aggressive growl. At idle, there is a subtle lope. Under full throttle, the V6 emits a musical wail with metallic overtones. The factory valves in the exhaust remain functional, so the car is still subdued in Comfort mode and properly vocal in Dynamic mode. There is no drone at highway speeds, and cold starts are only slightly louder than stock.

Maintenance and Long-Term Considerations

Tuning inevitably raises questions about reliability and warranty. APR’s track record with the EA839 engine is excellent – there are no widespread reports of catastrophic failures from Stage 2. Still, it is a more stressed engine. I now change oil every 5,000 miles instead of 10,000, and I use a premium 5W-40 full synthetic. Spark plugs are replaced every 15,000 miles (the colder plugs wear faster).

The downpipe’s 200-cell cat has a one-year warranty, but high-quality units can last the life of the car.

Warranty coverage is a legitimate concern. My local Audi dealer is mod-friendly, but most dealers will flag a tune if they connect to the factory scan tool and see the flash count or calibration ID. APR’s EMCS tool can revert the ECU to stock in about 10 minutes, but the TD1 marker may still be detected. I accepted this risk, and after a year of ownership, I have not had any warranty-related issues. If you are leasing or have a factory warranty you wish to preserve, Stage 1 (which requires no hardware) may be a safer bet.

Cost Breakdown and Is It Worth It?

Let’s examine the full financial picture:

  • APR Stage 2 ECU tune: $899 (includes cable or shop flash).
  • APR high-flow intake: $429.
  • AWE downpipe with 200-cell cat: $895.
  • NGK spark plugs (set of 6): $72.
  • Labor (5.5 hours at $110/hr): $605.
  • Total hardware + labor: $2,001 + $899 tune = $2,900 (if you include intercooler, add another $1,200).

Because I shopped during an APR sale, I saved $200 on the tune, bringing my out-of-door cost to $2,499. Is that good value? For a car that now accelerates harder than many $80,000 cars, absolutely. The experience of a mid-11-second quarter-mile in a four-door sedan with heated seats and a 360-degree camera is surreal. If you are a driving enthusiast and plan to keep the car for several years, the cost-per-smile is very low.

Community Feedback and Comparisons

Online forums (Audizine, AudiWorld, Reddit’s r/Audi) are filled with positive Stage 2 experiences. Many owners report hitting 3.4–3.5 seconds to 60 mph with the DSG launch control and sticky tires. The main complaint is the need for upgraded intercoolers in hot climates – the car pulls timing when intake temps exceed 130°F on repeated hard pulls. I plan to add the APR intercooler as a future upgrade. Competitors like Unitronic and 034Motorsport offer similar Stage 2 products, but APR’s extensive testing and dealer network are strong selling points.

Compared to a stock Audi RS5 (which has a naturally aspirated 4.2L V8 in older generations or a 2.9L twin-turbo V6 in newer), the tuned S4 is faster in a straight line and nearly as engaging in corners. The RS5 may hold a slight edge in terms of exhaust note and prestige, but the S4 with Stage 2 offers better daily comfort and lower running costs.

Potential Drawbacks and Trade-Offs

No modification is perfect. The Stage 2 tune causes the check engine light to illuminate if the car has OBD readiness monitors for the secondary air injection system – a common issue that can be cleared with a scan tool. The downpipe also makes the car smell slightly richer at idle, especially if you opt for a catless unit. Additionally, the additional torque stresses the DSG clutches; some owners have reported premature wear on heavily tracked cars. The APR DSG tune ($799) can optimize shift pressures and clutches, which I will consider in the future.

Final Verdict: A Transformative Investment

The $2,500 APR Stage 2 tune is the single best modification I have made to any car. It has elevated the Audi S4 from a refined, capable sport sedan to a true performance machine that can hang with dedicated sports cars. The improvements in throttle response, mid-range torque, and overall attitude are felt every single drive – whether you are commuting to work, carving back roads, or letting it rip on an on-ramp.

If you are on the fence, I suggest you find a local owner with a Stage 2 S4 and ask for a ride. The visceral punch will make the decision easy. Just be prepared to budget for better tires and more frequent fuel stops – the temptation to use that extra power is irresistible.

For further reading, check out APR’s official product page for the B9 S4 Stage 2, owner experiences on Audizine, and Motortrend’s independent dyno test of a Stage 2 S4. That last source provides an objective third-party validation of the performance claims.