The Subaru FA24 engine family has rapidly become the backbone of the brand’s performance and utility lineup. Found in the 2022+ WRX, Ascent, Outback XT, and Legacy XT, this 2.4-liter direct-injected turbocharged boxer engine replaces the older FA20 and EJ25 with a significantly stronger foundation. While Subaru’s factory calibration prioritizes emissions and fuel economy, the aftermarket has quickly discovered the FA24’s immense tuning headroom. This guide provides a structured, safe roadmap for unlocking substantial power gains while maintaining the long-term reliability that makes the FA24 an engineering highlight.

Anatomy of the FA24: Why It’s a Tuner’s Dream

The FA24F (or FA24DIT) is not just a larger FA20. Subaru made several critical changes that directly support high horsepower applications. The most significant upgrade is the closed-deck block design. Unlike the earlier FA20’s open-deck configuration, where the cylinders are only supported at the bottom and top, the FA24’s cylinders are fully supported by the block casting along their entire length. This drastically reduces cylinder wall flex under high combustion pressures, allowing the FA24 to safely handle 450-500 wheel horsepower on stock internals with proper tuning.

Supporting this robust bottom end is a fully forged steel crankshaft and forged connecting rods. The pistons are hypereutectic cast aluminum, which are strong but become a limiting factor around the 500-550 whp mark. The bore and stroke measure 94.0 mm x 86.0 mm, giving the engine a square-ish character that balances high-rpm capability with strong low-end torque production.

The cylinder heads feature high-tumble intake ports and large valves, fed by a twin-scroll TDO5 turbocharger. The twin-scroll design separates the exhaust pulses from cylinders 1 and 2 from cylinders 3 and 4, drastically reducing turbo lag and improving spool time. The factory direct injection system operates at high pressure (up to 3,000 psi) and provides excellent fuel atomization for power and efficiency. As iAG Performance details in their teardowns, the FA24’s oiling system also received improvements, including a higher-capacity oil pump and revised oil pan baffling to handle sustained cornering loads. iAG’s FA24 FAQ offers an excellent deep dive into the specific casting differences.

Foundations of a Safe Tune: Protocols and Monitoring

Modifying any modern turbocharged engine requires a methodical approach. The FA24 is robust, but it operates with high compression (10.1:1) and direct injection, which presents unique tuning challenges. Skipping steps or using aggressive off-the-shelf (OTS) maps without verifying your specific engine’s condition is a common path to failure.

Datalogs Are Non-Negotiable

Before adding any parts, you need a baseline. Acquiring a tuning platform like the Cobb Accessport or an open-source tuning solution is the first and most important purchase. You must log and understand key parameters:

  • Dynamic Advance Multiplier (DAM): This is your engine’s overall timing health. A stable DAM of 1.000 means the ECU trusts the fuel and conditions. A drop below 0.800 indicates significant knock or poor fuel quality.
  • Fine Knock Learn (FKL) and Feedback Knock Correction (FBKC): These indicate real-time knock events. Occasional -1.4 corrections on a hot day can be normal, but consistent -5.6 or higher under boost is dangerous.
  • AF Learning 1 and AF Correction 1: These measure how hard the ECU is working to maintain the target air-fuel ratio. Values beyond +/- 8% indicate a potential fuel system limitation or vacuum leak.
  • Boost Target vs. Actual: The FA24’s ECU is highly active in controlling boost. The actual boost should closely track the target boost defined in the calibration.

Fuel Quality Is King

The FA24 thrives on quality fuel. Stock maps are calibrated for 91 octane, but real performance begins with 93 octane or ethanol blends. Aggressive timing and boost increases directly depend on the fuel’s knock resistance. Never run a high-octane tune on low-octane fuel. Doing so can melt pistons in a single pull. If you have access to E85, an ethanol sensor and flex fuel tune allow the ECU to dynamically adjust for the optimal blend (usually E60), providing massive knock resistance and cooling properties. Cobb Tuning’s FA24 tech blog regularly discusses the synergistic benefits of flex fuel with their staged tuning packages.

Stage-by-Stage Modifications for Power

Building a tuned FA24 is best approached in stages. This allows you to isolate each modification’s effect and ensures the supporting systems are in place before adding stress.

Stage 1: Software Only

A Stage 1 tune on a completely stock car is the safest and highest-return modification. By recalibrating the boost target, throttle mapping, and ignition timing, a Stage 1 93-octane tune typically yields 260-280 whp (up from the factory ~240 whp). The real benefit is improved throttle response and a much flatter torque curve. No hardware is required, but a monitoring device like the Accessport is essential for verifying the tune’s safety.

Stage 2: Bolt-Ons and Fueling

Once you are comfortable with a Stage 1 tune, adding free-flowing intake and exhaust components allows the turbo to spool more efficiently and reduces backpressure. A typical Stage 2 package includes:

  • High-flow intake: Replaces the restrictive factory airbox and silencer.
  • J-pipe (downpipe): Replaces the restrictive factory catalytic converter. A catted J-pipe is recommended for daily drivers to avoid excessive exhaust odor and drone.
  • Charge pipe and intercooler: The stock intercooler becomes a heat sink on back-to-back pulls. A front-mount (FMIC) or larger top-mount (TMIC) is essential for maintaining consistent intake air temperatures.

Stage 2 power levels (330-360 whp) push the factory fuel system to its limits. The high-pressure fuel pump (HPFP) can struggle to maintain rail pressure at high rpm. It is highly recommended to upgrade the in-tank lift pump and consider a fuel pressure sensor to monitor pump health.

Stage 3: Turbo Upgrade and Full Fuel System

The stock TDO5 turbocharger runs out of steam around 350-380 whp. To reliably exceed this, a larger turbocharger is required. Options range from hybrid stock-frame units (which bolt directly to the factory location) to full-frame rotated mounts. Any turbo upgrade at this level demands a full fueling solution:

  • Upgraded HPFP: Necessary to maintain 3,000+ psi rail pressure.
  • High-flow fuel injectors: Direct injection injectors or supplemental port injection (if tuning for very high power).
  • Flex fuel kit: Provides the octane and cooling needed to run higher boost (22-26 psi) on a larger turbo.

With a well-matched turbo (like a Gen 2 VB or Full-Race 6060) and ethanol fuel, Stage 3 FA24s comfortably produce 450-500 whp. This is broadly considered the reliability ceiling for stock pistons. Cylinder pressures at this level are immense, and detonation must be avoided entirely.

Stage 4: Built Internals

If your goal is 550+ whp, closed-deck block rigidity is no longer enough. The stock hypereutectic pistons will eventually crack under extreme cylinder pressure or sustained spark knock. A built short block with forged pistons (often 100mm bore for extra displacement) and forged connecting rods is mandatory. At this power level, every component must be upgraded:

  • Forged pistons and rods
  • ARP head studs
  • Upgraded valve train and springs
  • Full billet fuel system (port injection or upgraded DI)
  • Massive intercooler and oil cooling systems

600+ whp FA24s are not uncommon, but they are not daily drivers. They demand meticulous tuning and a race fuel or dedicated ethanol mixture.

Cooling and Support Systems

Power is nothing without control. A tuned FA24 generates significantly more heat than a stock one. Neglecting cooling will lead to performance degradation and long-term damage through oil breakdown and pre-ignition.

Oil Cooling

The factory oil cooler is adequate for stock power levels on standard roads. Under track conditions or sustained highway pulls, oil temperatures can soar past 260°F. At this point, oil viscosity breaks down and bearing wear accelerates. An aftermarket oil cooler with a thermostatic plate should be considered mandatory for any Stage 2 or higher vehicle used for performance driving.

Charge Air Cooling

As mentioned in the stage guide, intercooler upgrades are critical. The stock top-mount intercooler (TMIC) becomes heat-soaked within a single 3rd gear pull on a warm day. A front-mount intercooler (FMIC) provides superior thermal mass and consistent airflow. Water-methanol injection is an excellent addition for high-boost applications, providing evaporative cooling directly into the intake charge to suppress knock.

Post-Tuning Maintenance and Longevity Protocols

Once you have increased the power output of your FA24, the maintenance schedule must become more aggressive. The engine is working harder, generating more heat, and placing greater stress on every rotating component. Following these protocols will extend the life of your tuned engine:

  • Shorten Oil Change Intervals: Subaru’s standard 6,000-mile interval is not appropriate for a tuned vehicle. Change oil and filter every 3,000 to 3,500 miles.
  • Use High-Quality Synthetic Oil: A heavy-duty 5W-30 or 5W-40 synthetic (such as Motul 8100 X-Clean or Amsoil Signature Series) provides superior film strength and high-temperature shear stability compared to standard off-the-shelf oils.
  • Conduct Oil Analysis: Using a laboratory like Blackstone Labs at every other oil change allows you to track wear metals, fuel dilution, and coolant contamination. This provides an early warning system for bearing wear or ring seal issues. Blackstone Labs offers specific Subaru FA24 analysis kits that help you monitor engine health over time.
  • Inspect Spark Plugs Annually: High boost and rich fuel mixtures can foul plugs faster. Check the gap and condition of your spark plugs every 10,000-15,000 miles.
  • Monitor Boost Leaks: The FA24 uses a complex system of rubber and plastic charge pipes. A boost leak will cause poor throttle response, rich AFRs, and potential engine damage. Perform a pressure test annually.

Building for the Long Haul

The Subaru FA24 engine represents a high water mark for OEM turbocharged four-cylinder production. Its closed-deck block and robust rotating assembly provide a stable foundation for substantial power increases. The difference between an engine that lasts 100,000 tuned miles and one that fails early often comes down to three factors: fuel quality, cooling capacity, and calibration quality. By following a staged modification approach, investing in proper monitoring and support systems, and adhering to a strict maintenance regimen, you can safely double the output of your FA24 while retaining the daily drivability and reliability that the engine is known for. Always work with a professional tuner who has specific experience with the FA24 platform, and never cut corners on fuel delivery or cooling.