Table of Contents
Understanding the FA24 Engine
The FA24 is a 2.4-liter turbocharged flat-four that powers the second-generation Subaru WRX. Compared to its predecessor, the FA20, the FA24 features a larger displacement, improved cylinder head design, and a stronger factory block. The engine uses a closed-deck semi-closed design (with reinforced main bearing caps) that offers better structural rigidity under high boost. The direct injection system (DIT) provides precise fuel control, but also introduces challenges with carbon buildup on intake valves. While the factory internals can handle around 400–450 wheel horsepower on a conservative tune, pushing beyond that toward the 500+ hp mark requires significant upgrades. The block itself can be retained, but pistons, rods, and valvetrain must be replaced. The FA24’s twin-scroll turbocharger is restrictive at higher airflow levels, so turbo and exhaust upgrades become essential. Understanding these strengths and limitations helps in planning a reliable build.
Engine Internals for 500+ HP
The stock pistons in the FA24 are cast aluminum and will crack under sustained high boost and engine knock. Connecting rods are powder-forged steel but still have a lower fatigue limit than dedicated aftermarket alternatives. To safely handle 500+ hp, internal components must be upgraded to forged or billet parts designed for high-stress, high-RPM use.
Pistons and Connecting Rods
Choose forged pistons made from 2618 or 4032 aluminum alloy. For a 500+ hp street or track car, 2618 offers higher fatigue strength at the cost of slightly more expansion, requiring careful cold-start warm-up. Compression ratio should remain near the factory 10.6:1 or be lowered to 9.5–10.0:1 if running higher boost levels or ethanol blends. Popular piston options from Manley Performance and JE Pistons include custom or off-the-shelf sets with coated skirts and wrist pins rated for 800+ hp. For connecting rods, go with a high-quality forged steel H-beam or I-beam design—Manley H-beam Turbo Tuff rods or Carrillo Pro-H rods are proven choices. These rods handle the higher cylinder pressures and allow safe engine speeds up to 7,500–8,000 RPM.
Cylinder Head and Valvetrain
The FA24 cylinder head flows well from the factory, but for 500+ hp, a port and polish job improves airflow and reduces restriction. The intake ports benefit from bowl blending and removal of casting flash. Upgrade the valves to stainless steel (e.g., Ferrea or Supertech) with larger diameter options if desired. Valve springs must be upgraded to retain control at higher lift and RPM—dual or beehive springs with titanium retainers are typical. Camshafts can be upgraded to Stage 2 or Stage 3 profiles (like those from GSC or Kelford) to improve mid-to-top-end power, though be aware of increased overlap affecting idle quality. The cylinder head gasket should be a multi-layer steel (MLS) unit from a company such as Cometic or Cosworth, slightly thicker if reducing compression. Upgrade head studs to ARP 2000 or L19 studs to prevent head lift under high boost
Fuel System Upgrades
Stock fuel system components on the FA24 are sufficient for around 400 hp on pump gas, but 500+ hp demands significantly more fuel delivery. Upgrades must ensure adequate volume and pressure under all conditions, especially if running ethanol blends.
Injectors and Fuel Pump
Direct injection (DI) limits the fuel system because the high-pressure pump cannot match the flow needed for 500+ hp. Most builds add port injection (PI) to supplement the DI system. A secondary fuel rail with port injectors (e.g., Injector Dynamics ID1050x or ID1300x) controlled by a standalone ECU or a controller like the Cobb Tuning Flex Fuel + Port Injection kit is common. The low-pressure fuel pump (in-tank) must be upgraded to a unit that supports higher flow—Walbro 450 or 525 pumps are standard. For E85, dual pump setups or a larger brushless pump from AEM or Radium Engineering maintain pressure. The high-pressure fuel pump (HPFP) can be upgraded with a lobe kit (from Nostrum or others) to increase DI injector delivery, but most rely on port injection to meet fuel demands. An adjustable fuel pressure regulator (e.g., Aeromotive) may be needed for the port injection circuit, along with larger fuel lines (-6 AN or -8 AN feed, -6 AN return).
Flex Fuel and Ethanol Tuning
To maximize power safely, running E85 or a high ethanol blend (E60) is highly recommended. Ethanol’s high octane rating and cooling effect allow higher boost and more aggressive timing without detonation. A flex fuel sensor will adjust tuning dynamically as ethanol content changes. This setup requires the port injection system and a tune that maps fuel tables and timing compensation. On a 500+ hp FA24, ethanol can add 30–50 wheel horsepower over pump gas alone, with a lower risk of knock.
Turbocharger Setup
The factory twin-scroll turbo is too small to support 500+ hp efficiently. At that power level, a larger aftermarket turbo is required, along with supporting hardware to get the most out of it.
Turbo Selection
For 500+ whp on the FA24, look for a turbo that flows 50–65 lb/min of airflow. Popular choices include:
- BorgWarner EFR 6758 or 7064 – fast spool, ball bearing, integrated recirculation.
- Garrett GTX3071R or GTX3576R – proven durability, excellent efficiency range.
- Precision Turbo 5862 or 6062 – budget-friendly billet wheel options.
- Subaru-specific kits from IAG Performance or Full-Race – bolt-on compatibility.
Exhaust Manifold and System
Replace the restrictive factory exhaust manifold with a high-flow twin-scroll header. Options include cast stainless manifolds from IAG or Tomei, or tubular headers from companies like Killer B Motorsport or Perrin. The header must maintain equal-length primaries (within reason) to preserve exhaust pulse separation for twin-scroll performance. Upgrade the uppipe and downpipe to at least 3-inch diameter with a high-flow catalytic converter or catless setup. A cat-back exhaust should be 3-inch or larger to minimize backpressure.
Intercooler and Charge Air Cooling
A larger front-mount intercooler (FMIC) is necessary to reduce intake air temperatures. Look for a core at least 4 inches thick with efficient bar-and-plate construction. Brands like Process West, ETS, and Cobb offer FMIC kits that retain the car’s crash bar. If staying with a top-mount, choose a thick high-flow core such as the Process West Verticooler, but be aware of heat soak limitations. Optional: water-methanol injection (e.g., Snow Performance or Aquamist) can suppress detonation and cool air below ambient when running higher boost on pump gas.
Boost Control and Wastegate
Use an external wastegate (such as Tial 44mm MVS or Turbosmart) to precisely control boost pressure and prevent spike. Run a dedicated boost reference line and use an electronic boost controller (e.g., Grimmspeed MAC valve or GFB G-Force). Map the wastegate duty to maintain stable boost across the RPM range.
Supporting Modifications
Oil and Cooling System
High power increases thermal load on the engine oil. Install an oil cooler (e.g., Setrab or Mocal) with a thermostatic sandwich plate that allows flow to the cooler at oil temperatures above 200°F. Upgrade the oil pan to one with increased capacity and baffling (from Killer B Motorsport or IAG) to prevent oil starvation during cornering. A lightweight damper pulley and a high-quality external surge tank (optional) further protect the oiling system.
Intake and Airflow
Replace the factory intake box with a high-flow intake system (e.g., Cobb SF Intake or GrimmSpeed CAI) to reduce restriction and allow the turbo to breathe. Ensure the intake uses a K&N or AEM dry filter for adequate filtration. Upgrade the throttle body and intercooler piping to match the new turbo outlet size (usually 3-inch).
Drivetrain
At 500+ hp, the stock WRX clutch and flywheel will slip. Choose a dual-disc clutch kit (e.g., South Bend Stage 3 Daily or ACT Heavy Duty) rated for 600+ ft-lbs. Street-driven cars should opt for a sprung hub organic disc to maintain some drivability. Consider a lightweight flywheel to improve rev-matching and response. The transmission itself (Tremec or STI six-speed swap for the manual, or upgraded valve body for the CVT) will require attention—many builders upgrade to a Subaru STI six-speed transmission to handle the torque. For the rear differential, a limited-slip unit (from OEM WRX or aftermarket) is highly recommended for traction.
Tuning and Calibration
Any 500+ hp build must be properly tuned by an experienced tuner using a dedicated ECU solution. The factory ECU can be reflashed with a Cobb AccessPort or Open ECU tools, but for DI+PI setups, a standalone ECU such as a Haltech Elite 2500 or Motec M130 may be required. The tune must include precise fuel and ignition mapping, closed-loop boost control, knock detection, and all safety limits. Dyno tuning is mandatory to verify the air/fuel ratio (target 11.5–12.0 under load on pump gas, 11.8–12.5 on ethanol), boost curve (target 22–28 psi), and timing advance (avoiding knock). Install a wideband O2 sensor (AEM or Innovate) and a boost pressure gauge to monitor parameters in real-time. Consider using Cobb’s Flex Fuel kit and tuning via AccessPort with a custom tune from an authorized pro tuner like Mikey Botti or Clark Turner. Before road tuning, perform a proper break-in cycle on new internals (500 miles, varying load, low boost).
Conclusion
Building a FA24 WRX to 500+ horsepower is a demanding but rewarding process when approached with the right parts and knowledge. The foundation: forged internals, a dual fuel system (DI+PI), a properly sized twin-scroll turbocharger, supporting cooling and drivetrain upgrades, and a professional tune using ethanol and boost control. Reliability hinges on not cutting corners—use high-quality parts, take the time for proper install and break-in, and monitor vital parameters. With these upgrades, your FA24 WRX can deliver usable, streetable power that dominates both the drag strip and the back road. Always consult with reputable builders and tuners to tailor the setup to your specific goals and driving conditions.