Introduction

The Subaru FA20 engine, found in the BRZ, Toyota GT86, and Subaru WRX (2015+), has become a favorite among tuners for its balance of affordability and performance potential. While the naturally aspirated version in the BRZ/86 delivers modest output, the turbocharged FA20 in the WRX and Levorg offers a robust foundation for substantial power gains. With a well-planned Stage 2 setup—typically involving a downpipe, tune, and supporting modifications—owners can unlock up to 50 additional horsepower at the wheels, pushing the engine well past the 300 whp mark on pump gas. This article breaks down the five most impactful power gains from Stage 2 modifications, explaining not just the parts but the engineering principles behind each upgrade. Whether you are a weekend enthusiast or a dedicated builder, understanding these components will help you build a reliable, powerful FA20 that delivers real-world driving thrills.

1. Upgraded Turbocharger

The factory turbocharger on the WRX FA20—the TD04L-16T—is designed for quick spool and daily drivability, but it runs out of steam at higher RPMs. Swapping to a larger turbocharger is the single most effective way to increase airflow and boost pressure, directly translating into significant horsepower gains. Stage 2 builders often choose a turbo like the Garrett GTX2860R or the BorgWarner EFR 6258 for their balance of response and top-end flow. These units can support 320–380 whp on 93 octane with proper tuning.

Boost Pressure and Cylinder Filling
Larger compressor wheels move more air per revolution, allowing higher boost pressure without exceeding the turbo's efficiency range. On the FA20, a conservative upgrade to 22–24 psi (versus factory ~16 psi) can yield 30–40 whp. However, the real gain comes from improved air mass flow; the engine's volumetric efficiency increases, and the fuel system (with upgraded pump and injectors) can deliver the extra fuel needed for safe combustion.

  • Spool Characteristics: A larger turbo may shift the torque curve slightly higher in the RPM range. Pairing it with a twin-scroll turbine housing (like the EFR 6758) helps retain low-end response.
  • Heat Management: Larger turbos generate more heat. Upgraded wastegates and boost controllers are essential to maintain precise boost control and avoid knock.

For those staying on the stock block, the FA20's closed-deck design can handle up to about 400 whp. Beyond that, forged internals are recommended. Always consult a professional tuner to dial in the boost curve and fuel mapping for your specific turbo choice.

2. High-Performance Intercooler

After increasing turbo output, the stock top-mount intercooler (TMIC) quickly becomes a bottleneck. Heat soak sets in after a few hard pulls, causing inlet air temps (IAT) to spike, which reduces air density and triggers knock detection. Upgrading to a larger front-mount intercooler (FMIC) or a high-efficiency bar-and-plate TMIC is critical for sustaining power gains.

FMIC vs. TMIC for the FA20

Front-mount intercoolers offer superior heat dissipation thanks to a larger core area and direct airflow. A typical FMIC kit from Mishimoto or Grimmspeed can reduce IAT by 30–50°F compared to stock. The trade-off is increased complexity and potential pressure drop if the piping is too large. High-quality cores maintain low pressure drop (under 1 psi at rated airflow).

  • Preventing Knock: Cooler intake charge allows more aggressive ignition timing and richer fuel mixtures safely, often recovering 10–15 whp that is lost to heat soak on the stock TMIC.
  • Consistency: On a warm day, an FMIC-equipped FA20 will continue pulling hard lap after lap, while the stock car will pull timing. This is valuable for track days or autocross.

If you prefer a stealthy look, a high-performance TMIC (Perrin Big Mount is a popular choice) offers a direct bolt-on with significantly more volume and fin density. Either way, the intercooler should be paired with a blow-off valve or bypass valve designed for higher boost to maintain throttle response.

3. Performance Exhaust System

Restrictive exhaust components are a major source of lost power on the FA20. The factory downpipe has a catalytic converter that is both small and highly restrictive, while the cat-back section muffles flow. A full turbo-back exhaust (downpipe, midpipe, axle-back) dramatically reduces backpressure, allowing the turbo to spool faster and the engine to expel exhaust gases more freely.

Downpipe and Catted vs. Catless

The downpipe is the single most important exhaust component for power. Replacing the stock downpipe with a 3-inch version (catted or catless) typically yields 15–25 whp on its own when paired with a tune. Many tuners recommend a high-flow catted downpipe to stay legal while still getting notable gains. Catless designs flow better but are louder and produce a stronger smell.

  • Spool Improvement: A free-flowing downpipe can reduce spool time by 200–400 RPM, meaning full boost arrives earlier—a huge benefit for daily driving and exiting corners.
  • Peak Power: Combined with a straight-through cat-back and an Axle-back muffler delete or performance muffler, total exhaust system gains can reach 25–35 whp.

Materials matter: stainless steel systems (like Cobb or Invidia) resist corrosion and offer consistent sound. Titanium systems save weight but cost more. Ensure the exhaust system matches your turbo upgrade—a 3-inch downpipe is sufficient for up to ~400 whp; beyond that, consider 3.5-inch piping.

4. Engine Tuning

No matter how many upgraded parts you install, the engine needs a custom tune to unlock their potential. The FA20's factory Engine Control Unit (ECU) retards timing and enriches fuel to protect the engine under heavy load, sacrificing power. Professional tuning—either through a plug-and-play AccessPort from Cobb Tuning or a custom ECU flash—reconfigures fuel maps, ignition timing, boost targets, and variable valve timing (AVCS) to optimize performance.

What a Stage 2 Tune Changes

  • Fuel Tables: Tuners lean out the air-fuel ratio from the factory's rich 10:1 to a safer 11.5:1 under boost for more power while still avoiding knock.
  • Ignition Timing: Timing is advanced until knock onset, maximizing torque. The FA20's direct injection allows precise combustion control, making gains of 15–25 whp typical from tuning alone.
  • Boost Targeting: The wastegate duty cycle is adjusted to hold target boost through the RPM range. Many Stage 2 tunes target 18–20 psi tapering to 15–16 psi at redline.

Tuning also enables features like flat-foot shifting, launch control, and improved throttle mapping. Always use a reputable tuner with experience on the FA20—improper tuning can lead to detonation and engine failure. Consider a flex-fuel tune if E85 is available; ethanol can add another 40–60 whp thanks to its higher octane and cooling effect.

5. Cold Air Intake System

A cold air intake (CAI) is often the first mod enthusiasts install, but it is also one where quality matters. The FA20's stock airbox flows adequately for Stage 1 levels, but as you increase boost and airflow with Stage 2 parts, the factory intake becomes a restriction. A well-designed CAI provides a less restrictive path for air and positions the filter in a cooler location.

Gains and Considerations

On a Stage 2 FA20, a quality CAI from Grimmspeed or ETS can add 8–15 whp, mostly in the midrange. The key is the air filter design and heat shield. A silicone intake tract reduces turbulence and turbulence-based pressure drops. Combined with a sealed heat shield, IATs remain close to ambient, preserving power during summer drives.

  • Flow vs. Filtration: Dry filters (like AEM or AFE Pro Dry) flow well without oil contamination risks. Oiled filters (K&N) flow slightly better but require careful maintenance to avoid coating the MAF sensor.
  • MAF Sensor Scaling: After installing a CAI, the ECU must relearn airflow readings. Tuning is necessary to recalibrate the MAF transfer function—a professional tune will do this automatically, or you can use a programmable AccessPort map.

Pro tip: On the FA20, avoid intakes that place the filter inside the engine bay without a proper heat shield. Hot air intake will reduce power and increase knock count. A true CAI that routes the filter behind the front bumper or into the fender is ideal for Stage 2 builds.

Bringing It All Together: Supporting Mods and Reliability

The five modifications above are the pillars of a Stage 2 FA20 build, but they cannot work in isolation. To safely handle the extra 50+ horsepower, you must address supporting systems:

Fuel System Upgrades

At 300–350 whp, the stock fuel pump runs near its limit. Upgrading to a Walbro 525 or equivalent high-flow pump, along with larger injectors (such as 700–850 cc/min), ensures adequate fuel delivery. On E85, a flex fuel sensor is strongly recommended to adjust timing and fueling automatically.

Clutch and Transmission

The stock clutch may start slipping with increased torque. A Stage 1 or Stage 2 clutch kit (e.g., ACT or Exedy) rated for 350–400 ft-lbs is a wise investment. The FA20's six-speed manual (WRX) is robust, but longer-term builds may benefit from a transmission cooler if you track the car.

Engine Monitoring

Fit an oil pressure gauge, wideband O2 sensor, and boost gauge. These tools help you spot problems early—low oil pressure, lean mixtures, or boost spikes—preventing catastrophic failure. Many tuners include logging parameters in the AccessPort, so reviewing data logs after each tune revision is standard practice.

Conclusion

Subaru's FA20 engine responds exceptionally well to Stage 2 modifications, giving enthusiasts the ability to extract up to 50 additional wheel horsepower while maintaining excellent drivability. The path to that power is straightforward: upgrade the turbocharger for airflow, the intercooler for temperature control, the exhaust for flow, the tune for optimization, and the intake for better air delivery. Each component contributes a piece of the puzzle, and when they work together, the result is a transformed driving experience—stronger, faster, and more rewarding than the stock car.

Remember: Always research parts compatibility, invest in professional tuning, and never compromise on reliability for a few extra horsepower. A well-executed Stage 2 FA20 build will provide years of smiles and track passes without leaving you stranded. For further reading, consult Subaru-specific forums like NASIOC or engage with a certified tuner who has extensive FA20 dyno experience. Choose your upgrades wisely, and enjoy the climb from stock to 50 whp stronger.