Understanding the FA24 Boxer Engine

The FA24 is a 2.4-liter turbocharged flat-four engine produced by Subaru, a direct successor to the earlier FA20 and the EJ series. It features a closed-deck block, direct injection, and a twin-scroll turbocharger in most applications. Factory output ranges from 260 horsepower in the Ascent and Outback XT to 271 horsepower in the 2022+ WRX. The engine's low center of gravity, inherent to the boxer layout, improves handling and weight distribution. For enthusiasts, the FA24 offers a robust platform capable of significantly more than its stock power level. Reaching 300 wheel horsepower (whp) is a realistic and popular goal with the right combination of parts and calibration.

The Path to 300 Horsepower: Key Upgrades

To achieve a reliable 300 whp on the FA24, you need to address air flow, fuel delivery, and engine management. Below are the critical modifications, each with specific considerations and part options.

Turbocharger Upgrades

The stock twin-scroll turbocharger on the FA24 is relatively small and becomes a bottleneck above 300 whp. For the 300 hp target, you have two main options:

  • Stock turbo with boost controller: With a custom tune and boost increase to around 18-20 psi, the stock turbo can produce 290-310 whp on pump gas. This is the most cost-effective route but leaves little headroom.
  • Upgraded turbocharger: Aftermarket options such as the Garrett G25-550, BorgWarner EFR 6758, or OEM+ units like the Subaru STI turbo (slightly larger) can support 350-400 whp and run more efficiently at the 300 whp level. A larger turbo reduces boost pressure for the same power, lowering intake temperatures and improving reliability.

Turbo choice influences spool time and power delivery. A stock-size replacement (e.g., VF52 or upgraded stock-frame) spools quickly, while a larger unit like the G25 may have slight lag but stronger top-end pull.

ECU Tuning

No 300 whp build is complete without professional calibration. The FA24's ECU is sophisticated and requires proper fueling, timing, and boost control maps. Popular tuning platforms include:

  • Cobb Accessport – Cobblestoned tuning via off-the-shelf (OTS) maps or custom dyno/road tuning. The Accessport provides monitoring, data logging, and the ability to switch maps (e.g., pump gas, E85).
  • EcuTek – Another powerful platform used by many Subaru tuners; offers fine-grained control and features like flex-fuel support, launch control, and flat-foot shifting.
  • OpenECU/OpenSource – For advanced users who want to tune without proprietary hardware, using software like RomRaider and Tactrix cable. This requires significant expertise.

A custom tune is strongly recommended over OTS maps for safety and consistency. A good tuner will dial in fuel trims, ignition advance, boost targets, and knock control to suit your particular parts, fuel quality, and driving conditions.

Intercooler and Charge Air Cooling

The stock top-mount intercooler (TMIC) on the FA24 is adequate for stock power but heat-soaks quickly during spirited driving. Upgrading to a larger, more efficient intercooler is essential for maintaining power and preventing knock. Options include:

  • Performance TMIC – Units from Process West, COBB, or Grimmspeed feature thicker cores, better fin density, and improved airflow ducting. They fit in the stock location and are a direct upgrade.
  • Front-Mount Intercooler (FMIC) – Moving the intercooler to the front bumper offers lower intake temperatures and greater total thermal capacity. However, FMICs require piping modifications, increased cost, and sometimes trimming of bumper reinforcement. For 300 whp, a quality TMIC is usually sufficient and simpler.

An upgraded intercooler is particularly important if you run higher boost or use E85, as ethanol produces more heat during combustion.

Exhaust System

Reducing backpressure and improving exhaust flow helps the turbo spool more quickly and allows the engine to breathe at higher RPM. Key components:

  • Downpipe: The downpipe replaces the restrictive factory catalytic converter section. A catted downpipe (high-flow cat) is legal for street use in many areas and still flows significantly better than stock. A catless downpipe is lighter and flows best but is not emissions-legal. Expect a 10-15 whp gain with a tune.
  • Cat-back exhaust: A 3-inch or 3.5-inch cat-back system reduces restriction after the downpipe and alters the exhaust note. Gains alone are modest (5-10 whp), but necessary for overall system flow.

Most 300 whp builds pair a catted downpipe with a cat-back exhaust. If you are on a tight budget, a downpipe and stock cat-back may still reach the goal with sufficient tuning, but the full system is recommended.

Intake System

The FA24's airbox is fairly restrictive. A cold air intake (CAI) opens up the intake path and reduces restriction upstream of the turbo. Popular brands include COBB, Injen, AEM, and Perrin. Gains are typically 10-15 whp with calibration. Be aware that some intakes can cause issues with MAF sensor scaling; always ensure your tuner can compensate.

Fuel System Upgrades

To safely deliver enough fuel for 300 whp on pump gas (93 octane or similar), the stock injectors and fuel pump are marginal. Upgrades:

  • High-flow fuel injectors: 1000-1300cc injectors are common for 300-400 whp builds. Direct-injection injectors for the FA24 are limited options; some tuners use port injection (PI) as a supplement, but for 300 whp, upgraded high-pressure direct injectors (HPFP) can suffice.
  • High-pressure fuel pump (HPFP): The stock HPFP may not sustain pressure at higher loads. Upgraded units, such as from Nostrum or Fuel-It, provide the necessary volume and pressure.
  • Flex fuel (E85): If you want to run E85 for added knock resistance and power, you will need larger injectors, an upgraded HPFP, and a flex-fuel sensor. E85 allows higher boost and timing, often yielding 320-340 whp with the same turbo setup as a pump-gas 300 whp build.

For a pure pump-gas 300 whp target, upgraded injectors and HPFP are often enough. Some tuners have reached 300 whp with stock fuel system on a conservative tune, but it is riskier. Spend the extra for peace of mind.

Cost Breakdown of Performance Parts

The total cost depends on part quality, whether you buy new or used, and whether you install yourself. Below are realistic estimates for a complete bolt-on and tune package achieving 300 whp on pump gas.

  • Turbocharger upgrade: $1,200 – $2,500 (stock turbo with boost controller: $200-400 if just a boost solenoid, but that often requires tuning hardware anyway; a proper upgrade like a Garrett G25 starts at ~$1,800 including oil/coolant lines)
  • ECU tuning: $500 – $1,200 (Accessport ~$650 + custom tune $400-600; or EcuTek kit with license ~$1,000)
  • Intercooler: $600 – $1,500 (quality TMIC: $800-1,200; FMIC kits: $1,200-2,000)
  • Exhaust system: $800 – $1,800 (catted downpipe: $400-700; cat-back: $600-1,000)
  • Intake: $300 – $600
  • Fuel system upgrades: $800 – $2,500 (injectors: $400-800; HPFP: $500-1,200; flex-fuel kit: $400-700 additional)
  • Miscellaneous (gaskets, fluids, clamps, boost gauge): $200 – $500

Total part cost: $4,500 – $10,000. A budget build using a stock turbo with downpipe, intake, intercooler, and tune might come in around $3,500. A top-shelf build with upgraded turbo, FMIC, full exhaust, and flex fuel can exceed $8,000.

Installation and DIY vs Professional

Installation labor adds $800 – $2,500 depending on the shop hourly rate (typically $100-150/hr) and the complexity of the turbo and intercooler work. Replacing a turbocharger on the FA24 is not trivial; the engine bay is tight, and the boxer orientation makes access to the turbo slightly easier than on inline engines but still requires removing the downpipe, heat shields, and sometimes the intake manifold. A professional install ensures correct torque, proper alignment, and no leaks.

Experienced DIYers can save $1,000-2,000 by doing the work themselves. Essential tools include torque wrenches, a lift or jack stands, and an OBD-II scanner to check for codes. Remember that tuning must be done by a professional regardless, unless you are an expert calibrator.

Performance Expectations and Reliability

With the upgrades described, a properly tuned FA24 will produce 300 whp (approximately 340-360 crank horsepower) and around 330-360 lb-ft of torque. The powerband will be noticeably stronger from 3,000 RPM to redline, with faster spool and sustained pull. Acceleration times improve dramatically: a stock WRX runs 0-60 in ~5.5 seconds; a 300 whp version can dip into the 4.5-5.0 second range.

Reliability considerations:

  • Engine internals: The FA24 stock rods and pistons are strong enough for 300 whp on pump gas. However, continuous high boost and knock can cause failure. Use high-octane fuel and conservative timing.
  • Cooling: An upgraded intercooler and possibly an oil cooler are wise to maintain consistent performance on hot days or track use.
  • Clutch: The stock clutch may slip around 300 whp. A stage 2 or 3 clutch is recommended if you drive aggressively.
  • Maintenance: More power increases heat and stress. Shorten oil change intervals, use high-quality synthetic oil (e.g., 5W-30 or 5W-40), and inspect spark plugs regularly.

Overall, 300 whp is a well-established safe limit for the FA24 when using quality parts and a reputable tune. Many owners report tens of thousands of trouble-free miles at this power level.

Tuning Strategies: Pump Gas vs E85

For 300 whp, pump gas (93 octane) is sufficient, but it requires careful tuning to avoid knock. Running E85 (or blends up to E60) allows for higher boost, earlier timing, and lower exhaust gas temperatures, making 300 whp easier and safer. However, E85 reduces fuel economy by ~30% and requires the fuel system upgrades noted earlier. If you want maximum power on pump gas, consider a methanol injection kit as an alternative to E85, but it adds complexity.

Flex fuel capability is the most versatile solution: you can run pump gas day-to-day and switch to E85 for track days or extra performance. Many modern tuners integrate flex-fuel with Accessport or EcuTek, automatically adjusting fuel and timing based on ethanol content.

Real-World Examples and Community Feedback

The Subaru FA24 community on forums like NASIOC, WRX subreddit, and Facebook groups has documented numerous 300 whp builds. A typical setup for a 2022+ WRX includes a COBB Accessport, a Process West Verticooler TMIC, a COBB catted downpipe, a Perrin intake, and a custom tune. Owner reports often show 290-310 whp and 330-350 ft-lb on 93 octane. Those adding an upgraded turbo (e.g., BNR Supercars or Forced Performance) report 330-360 whp easily.

One common recommendation is to prioritize the intercooler and tune ahead of the turbo for the 300 whp target. The stock turbo can deliver the power, but the intercooler keeps the engine safe.

Conclusion

Achieving 300 horsepower with the FA24 Boxer engine is not just a possibility—it's a straightforward goal with well-understood parts and proven results. By focusing on smart upgrades: a quality intercooler, a downpipe, a proper intake, a reliable tune, and necessary fueling improvements, you can transform your Subaru into a significantly more potent machine. Budget accordingly, invest in professional tuning, and respect the engine's limits to enjoy a fast, reliable, and exhilarating driving experience.

For further reading, consult leading parts suppliers such as COBB Tuning, MAPerformance, and the official Subaru site for specifications and part compatibility. Always verify part fitment for your specific FA24 application.