Understanding the FA24 Engine in the 2024 WRX

The 2024 Subaru WRX marks a significant leap forward for the iconic platform, powered by the FA24F engine. This 2.4-liter turbocharged flat-four is a direct evolution from the previous FA20, offering a larger displacement and a host of modern technologies. From the factory, the FA24 WRX produces around 271 horsepower and 258 lb-ft of torque, but the engine’s robust design—featuring a closed-deck block, reinforced internals, and direct injection—provides a substantial foundation for power upgrades. Enthusiasts are finding that with the right modifications, this engine can reliably surpass 400 wheel horsepower, making it one of the most promising platforms for performance builds today. Before diving into specific mods, it’s crucial to understand the engine’s strengths and limitations, particularly its fuel system capacity and turbocharger efficiency, which are key bottlenecks at higher power levels.

Top Power Mods for the FA24 WRX: From 300 to 400+ HP

Achieving a 100+ horsepower increase requires a strategic approach. Simply bolting on parts without a plan can lead to diminished returns or reliability issues. Below are the most effective modifications, ranked by impact, to help you reach and exceed 400 wheel horsepower.

1. Upgraded Turbocharger: The Heart of High Power

The stock FA24 turbocharger is a twin-scroll unit that provides excellent spool and low-end response, but it quickly runs out of steam above 350 whp. To push past 400 horsepower, an upgraded turbo is non-negotiable. Options like the Garrett GTX3071R Gen II or the BorgWarner EFR 7163 are popular choices. These units offer significantly higher flow capacity while maintaining reasonable spool characteristics. A larger turbo will deliver a substantial mid-range and top-end power increase, but it will also shift the power band higher in the RPM range. Pairing an upgraded turbo with a suitable wastegate and boost controller is essential for precise boost management.

When selecting a turbo, consider the intended use. For a street-driven car that sees occasional track days, a GTX3071R-based kit provides an excellent balance. For all-out drag or high-speed circuit work, a larger frame turbo like the EFR 8374 can push power well beyond 500 whp, though spool will be slower. Be prepared to also upgrade the turbo inlet pipe and intercooler piping to accommodate the larger compressor outlet.

2. ECU Tune: Unlocking the Potential

Without a proper tune, no other modification will reach its full potential. The FA24’s ECU is highly adaptive and will actively work against aggressive modifications if left uncalibrated. A custom ECU tune, typically performed via an accessport like Cobb’s Accessport or an Ecutek license, allows you to adjust fuel maps, ignition timing, boost targets, and throttle response. For the 2024 WRX, Cobb has released its Accessport with native support for the FA24, while Ecutek offers advanced features like flex-fuel capabilities and knock control.

A protune from a reputable shop is strongly recommended over off-the-shelf (OTS) maps, as OTS tunes cannot account for variations in fuel quality, altitude, or individual engine health. A solid stage 2 tune (exhaust + intake) on a stock turbo can net around 330-350 whp. With a full bolt-on setup and an upgraded turbo, a custom tune is what enables the 400+ whp marks. Be sure to work with a tuner experienced with the FA24 platform to ensure knock-free, reliable power.

3. High-Performance Intercooler: Managing Heat Soak

As boost pressures increase, so does intake air temperature. The stock top-mount intercooler (TMIC) on the FA24 WRX is adequate for factory power but becomes a restriction under sustained acceleration or in hot weather. A larger aftermarket TMIC or a front-mount intercooler (FMIC) greatly reduces intake temperatures and pressure drop, allowing for denser air and more consistent power output. Mishimoto and ETS are leading manufacturers offering well-tested options. For builds aiming for 400+ whp, a FMIC is often the better choice due to its larger core volume and ability to dissipate heat more effectively. However, FMIC kits require cutting the front bumper beam and relocating the crash bar, which may affect street legality or crash safety in some regions.

Regardless of the type, upgrading the intercooler is one of the most impactful mods for maintaining power on repeated pulls. It also reduces the risk of pre-ignition or knock caused by high charge temperatures, which is critical when running aggressive timing on pump gas or ethanol blends.

4. Performance Exhaust System: Letting the Engine Breathe

The stock exhaust system on the FA24 WRX is restrictive, particularly the downpipe (the section between the turbo and the rest of the exhaust). Replacing the downpipe with a high-flow catted or catless unit dramatically reduces backpressure and allows the turbo to spool more freely. This is often the single most effective bolt-on mod for power gain after a tune. A complete cat-back exhaust will further improve flow and provide a more aggressive tone. Brands like Cobb, Invidia, and Borla offer systems specifically designed for the 2024 WRX. Note that catless downpipes are not street-legal in many areas due to emissions regulations, so check local laws before purchasing.

When upgrading the exhaust, consider the diameter. A 3-inch system is the standard for 400+ whp builds. Coupling the exhaust with a properly sized wastegate dump tube can also help reduce exhaust backpressure and improve boost control. A well-designed exhaust system will not only add power but also decrease spool time, enhancing throttle response.

5. Upgraded Fuel System: Feeding the Monster

The FA24 uses direct injection (DI) which, while efficient, has limitations at high power. The stock high-pressure fuel pump (HPFP) and injectors can only supply enough fuel for approximately 400-420 whp on pump gas. To go beyond that, or to run full E85, you will need a supplemental port injection system or upgraded DI components. A popular solution is to add a port injection kit that uses additional fuel injectors in the intake runners, supplementing the DI system. Companies like Injector Dynamics and Fuel Injector Clinic offer reliable solutions. Additionally, upgrading the low-pressure fuel pump in the tank to a Walbro 525 or similar is necessary to maintain fuel pressure under high demand.

A proper fuel system upgrade also includes larger fuel lines and a surge tank for cars seeing sustained high-G driving. For most 400-450 whp street builds, a combination of a high-flow LPFP and a port injection kit with a dedicated fuel controller is sufficient. Running E85 requires roughly 30% more fuel flow than pump gas, so plan accordingly. Flex-fuel capability (tuning that automatically adjusts for ethanol content) is highly recommended for daily drivers who want to use both pump gas and E85.

6. Cold Air Intake: Improving Airflow

While a full intake system may only add 5-10 horsepower on its own, it becomes critical when paired with a larger turbo and aggressive tune. A high-flow cold air intake reduces restriction at the filter and smooths out the airflow path to the turbo. The FA24 OEM airbox is decent, but aftermarket intakes from brands like AEM, K&N, or Cobb draw cooler air from outside the engine bay and use larger, less restrictive filter elements. For cars running upgraded turbos, a 4-inch or larger intake is required to feed the compressor inlet. Be cautious of intakes that are too large for your setup, as they can reduce air velocity and hurt low-end throttle response.

Look for a sealed intake box to keep heat out, especially if you are still using a top-mount intercooler. Some intakes also come with a crankcase ventilation kit or require modifications to the factory PCV system, so ensure compatibility with your WRX model year.

7. Lightweight Flywheel: Quicker Revs

Reducing rotating mass frees up horsepower that would otherwise be used to spin the flywheel. A lightweight flywheel, typically made from chromoly or billet steel, can weigh half as much as the stock unit. This allows the engine to rev faster on acceleration and decelerate more quickly, improving shift response and overall drivability. ACT and Exedy offer popular options for the FA24. The trade-off is a slight decrease in low-speed idle smoothness and potentially increased gear rattle. For cars primarily used for street driving or drag racing, a flywheel of around 13-15 lbs offers the best balance. For track cars, even lighter options exist, but they may be too aggressive for daily use.

Supporting Modifications for Reliability

Reaching 400+ horsepower puts considerable stress on the drivetrain and cooling systems. To ensure your build stands the test of time, consider these supporting upgrades:

  • Oil cooling: An aftermarket oil cooler helps maintain safe oil temperatures during hard driving. The FA24 runs hot, and cooler oil prevents bearing wear and viscosity breakdown.
  • Drivetrain upgrades: The stock clutch will slip at higher torque levels. A stage 2 or stage 3 clutch kit from companies like South Bend Clutch is a must. Additionally, upgrading the transmission mount and differential bushings reduces wheel hop and improves power delivery.
  • Suspension and brakes: More power demands better stopping power and handling. Upgraded brake pads, rotors, and a brake master cylinder brace improve pedal feel. Coilovers or upgraded sway bars keep the car planted during acceleration.
  • IAT and knock monitoring: A real-time monitoring system via Accessport or a separate gauge allows you to watch intake air temperature and knock correction. This proactive approach can save your engine from damage if something goes wrong.

Conclusion: Building Your 400+ HP FA24 WRX

Boosting your 2024 WRX from its stock output to over 400 horsepower is an exciting and thoroughly achievable goal. The FA24 engine is a robust platform that responds well to modifications, but success lies in a balanced approach. Start with a solid foundation: a high-quality ECU tune, an upgraded turbo, and an adequate fuel system form the core of any 400+ whp build. Supplement these with breathing mods like intercooler, intake, and exhaust, and don’t overlook supporting components that ensure reliability. Whether you’re building a street beast or a track weapon, careful planning and quality parts will let you enjoy the thrill of 400 horsepower without sacrificing daily drivability. Always consult with experienced tuners and mechanics, and remember that power without control is just noise—invest in suspension and braking upgrades to match your newfound speed.