Introduction: The FA24 Era for Subaru Enthusiasts

The Subaru WRX and STI have long defined the affordable all-wheel-drive performance segment. With the introduction of the FA24 engine in the latest generation, both platforms have received a significant leap in displacement and turbocharging potential. For owners pushing beyond stock numbers, two aftermarket brands consistently stand out: APS (Advanced Power Solutions) and TurboSmart. This article provides a comprehensive comparison of the power gains achievable on the Subaru WRX and STI when using upgrades from both manufacturers. We will examine stock specifications, kit components, dyno-verified results, and real-world driving characteristics to help you choose the right path for your build.

Deep Dive: The FA24 Powerplant

Specifications and Design

The FA24 is a 2.4-liter turbocharged boxer four-cylinder engine that replaced the earlier FA20 and EJ25 engines across the Subaru lineup. Its closed-deck design and larger displacement provide a stronger foundation for boost increases compared to its predecessors. Key stock specifications include:

  • Displacement: 2,387 cc
  • Compression Ratio: 10.6:1 (WRX), 9.5:1 (STI)
  • Turbocharger: Twin-scroll (WRX), larger twin-scroll (STI)
  • Direct Injection: Yes, with dual injection on some variants

The WRX and STI share the same engine block but differ in turbocharger sizing, intercooler capacity, and ECU mapping. The STI also includes a mechanical center differential and strengthened transmission, which influences how power is delivered to the wheels.

Stock Power Output Differences

Factory-rated outputs differ between the two models:

  • Subaru WRX: 271 hp at 5,600 RPM, 258 lb-ft torque at 2,000 RPM
  • Subaru STI: 310 hp at 6,000 RPM, 290 lb-ft torque at 2,000 RPM

These numbers come from different turbo and tuning strategies. The WRX spools quicker due to a smaller turbo, while the STI pulls harder in the mid-range and top end. Understanding these baseline differences is crucial when evaluating aftermarket upgrades.

APS Upgrades: Engineering for Max Torque

APS has been a trusted name in Subaru performance for over two decades. Their FA24 packages focus on high-flow components and aggressive calibration to exploit the engine’s thermal capacity.

Key APS Components for FA24

APS offers a complete stage kit that typically includes:

  • High-Performance Turbocharger: A billet wheel, ball-bearing unit with a larger compressor housing and optimized turbine. The WRX kit uses a 56 mm inducer wheel; the STI kit uses a 58 mm wheel.
  • Upgraded Front-Mount Intercooler: Bar-and-plate core with 3-inch inlet/outlet, designed to reduce intake air temperatures by up to 40°F under sustained boost.
  • ECU Tune: Custom calibration via Cobb Accessport or ECM via EcuTek, adjusting fuel, timing, and boost targets.
  • High-Flow Intake and Downpipe: Mandrel-bent 3-inch piping with high-flow catalytic converter (optional catless).

Power Gains from APS on the WRX

On a well-tuned FA24 WRX with Stage 2 APS components (turbo, intercooler, intake, downpipe, and ECU tune), experienced tuners report the following peak gains on 93 octane pump fuel:

  • Horsepower: +65-70 hp (reaching approximately 340-345 whp depending on dyno)
  • Torque: +80-90 lb-ft (peak torque near 370 lb-ft at the wheels)

Spool characteristics: The APS turbo reaches full boost around 3,400 RPM, only 200 RPM later than stock, but holds boost to redline. The increased torque mid-range makes the WRX feel significantly stronger during daily driving and on tight courses.

Power Gains from APS on the STI

The FA24 STI, with its slight compression reduction and larger stock turbo, responds even more aggressively to APS upgrades. Stage 2 results on 93 octane show:

  • Horsepower: +70-80 hp (around 390-400 whp)
  • Torque: +85-95 lb-ft (torque peaks at 405 lb-ft at the wheels)

The STI's mechanical drivetrain handles the added torque well, though clutch upgrades are recommended above 400 lb-ft. The APS intercooler effectively combats heat soak, allowing repeatable power on track days.

TurboSmart Upgrades: Precision Boost Control

TurboSmart focuses on fine-tuning the turbocharger's response through its iconic blow-off valves, wastegates, and boost controllers. Their FA24 kit is designed to maximize spool and transient response while maintaining safe EGTs.

Key TurboSmart Components

TurboSmart’s recommended FA24 upgrade path includes:

  • Adjustable Wastegate Actuator: Includes a linear, high-flow actuator with adjustable spring preload (typically 12 PSI base) to improve boost threshold and prevent creep.
  • High-Flow Blow-Off Valve: Dual-port design that vents compressor surge while maintaining idle stability. Suitable for both recirculating and atmospheric applications.
  • Boost Control Solenoid: A three-port, high-frequency solenoid that replaces the factory unit for more precise boost targeting and quicker spool.
  • Turbo Inlet Pipe & Intercooler Charge Pipes: Smooth transitions to reduce turbulence and pressure drop.
  • ECU Tune: Optional tune file from TurboSmart’s partners (Cobb or EcuTek) tailored to the specific components.

Power Gains from TurboSmart on the WRX

With TurboSmart’s component kit and a appropriate tune (using a Cobb Accessport or open-source), the WRX gains are described as smooth and linear:

  • Horsepower: +55-60 hp (reaching approximately 330-335 whp)
  • Torque: +70-75 lb-ft (torque around 355 lb-ft)

The adjustable wastegate actuator allows customizing boost ramp. Many users report that the TurboSmart combination improves part-throttle response and reduces turbo lag noticeable during gear changes.

Power Gains from TurboSmart on the STI

On the STI, TurboSmart components complement the larger factory turbo. Gains are slightly higher due to the bigger turbine:

  • Horsepower: +60-65 hp (around 375-380 whp)
  • Torque: +75-80 lb-ft (torque about 380 lb-ft)

The STI setup benefits from the high-flow blow-off valve, which helps maintain compressor speed during closed-throttle shifts, resulting in faster recovery. Boost control precision allows the STI to hit and hold 18-20 PSI smoothly without spiking.

Comparative Analysis: APS vs. TurboSmart

Performance Metrics

When comparing the two brands, differences emerge in power delivery and component philosophy:

  • Peak Power: APS consistently delivers higher peak horsepower and torque numbers, especially on the STI. The larger turbo and intercooler sacrifice a little spool time for top-end pull.
  • Spool and Response: TurboSmart components improve spool through better boost control and reduced pressure losses. The response gains are more noticeable in daily driving and autocross where quick power onset matters.
  • Heat Management: APS’s intercooler upgrade is superior for sustained track use. TurboSmart relies on the stock intercooler (unless paired with an aftermarket unit), which can heat soak faster.
  • Cost: APS Stage 2 kits retail higher (approximately $3,500-$4,000) due to the turbo and intercooler. TurboSmart component-only kits are around $800-$1,200, but require additional supporting parts like a downpipe and tune to maximize gains.

Real-World Driving Feel

Drivers who prioritize low-end grunt for street driving often prefer the APS setup because of the fat torque curve. Those who enjoy precise throttle modulation and autocross may lean toward TurboSmart for its immediate response and lighter mod weight.

External Resources and Further Reading

For more detailed dyno graphs and community experiences, visit these trusted sources:

Conclusion: Choosing Your Path

Both APS and TurboSmart offer proven gains on the FA24 Subaru WRX and STI, but they cater to different priorities. The APS route is for enthusiasts who want maximum horsepower and torque, especially in the STI, and are willing to invest in a comprehensive parts package. The TurboSmart route is more modular, focusing on boost response and control, making it ideal for those who already have supporting mods or prefer a more budget-friendly upgrade path. Regardless of choice, proper tuning is non-negotiable to ensure reliability and safety. With either brand, the FA24 engine proves itself as a robust platform that rewards careful modification with a thrilling driving experience.