Understanding the VA WRX Platform

The 2015–2021 Subaru WRX (VA chassis) uses a turbocharged 2.0-liter FA20F engine, a departure from the older EJ series. The FA20F features a twin-scroll turbocharger, direct injection, and a high 10.6:1 compression ratio. While the factory tune delivers 268 hp and 258 lb-ft of torque, the engine’s closed-deck block and robust bottom end can reliably support 400 wheel horsepower with the correct upgrades. However, reaching that number requires a coordinated combination of modifications—simply bolting on a larger turbo without addressing fuel, cooling, and engine management will lead to disappointment or mechanical failure.

This guide walks through each critical system, from intake to exhaust, along with supporting modifications that ensure your 400 hp VA WRX remains drivable and durable. Expect to invest time in tuning and fabrication; a 400 hp WRX is no bolt-on weekend project.

Core Power Modifications

1. Intake System

The stock intake box and air filter are restrictive once you exceed 300 hp. A performance intake both reduces restriction and, with a proper heat shield, lowers intake air temperatures. Two common paths exist:

  • Cold Air Intake (CAI) – A long-tube intake that pulls air from the front bumper or inner fender. Models from COBB Tuning and Grimmspeed relocate the mass air flow (MAF) sensor to a larger housing, requiring recalibration via a custom tune. CAIs reduce intake temperatures by 10–20°F compared to the stock box.
  • Short Ram Intake – Mounts directly to the throttle body. Easier to install but draws hot engine bay air, often reducing performance in warm climates. Best used with a good cold-air duct or a larger intercooler.

Key consideration: On the FA20, the MAF scaling is highly sensitive. A poorly designed intake can cause erratic air-fuel ratios, leading to knock or lean conditions. Always pair an intake with a professional tune.

2. Turbocharger Upgrade

The stock twin-scroll TD04L turbo runs out of steam around 300–320 whp. For 400 whp, you need a larger unit that flows more air at higher boost levels while retaining quick spool. Top choices include:

  • Garrett GTX3071R or GTX3576R – Ball-bearing turbos with excellent efficiency. The GTX3071R spools like a stock turbo and supports up to 450 whp on E85. Requires an aftermarket turbo inlet, downpipe, and a boost controller.
  • BorgWarner EFR 6758 or 7163 – Features integrated recirculation and a lightweight turbine wheel. The EFR 7163 can hit 400 whp with outstanding transient response.
  • IAG Performance Stage 2 or Stage 3 Turbo Kits – Complete packages including a billet compressor wheel, reinforced wastegate actuator, and necessary hardware. These are popular among WRX owners targeting 400–450 whp.

Supporting mods: Upgraded turbo blanket (retains exhaust heat, reducing spool time), electronic boost control solenoid (EBCS) for precise boost targeting, and a blow-off or recirculating bypass valve compatible with higher boost levels.

3. Intercooler and Charge Air System

At 400 hp, the stock top-mount intercooler (TMIC) becomes a heat sink, causing intake air temperatures to spike after a couple of pulls. Two avenues exist:

  • Upgraded Top-Mount Intercooler – Units from Process West or Grimmspeed feature larger core volume, bar-and-plate construction, and cast end tanks. They fit under the stock hood and retain low-pressure drop. Good for daily drivers who don't track the car hard.
  • Front-Mount Intercooler (FMIC) – For sustained high-power driving, an FMIC is superior. Kits from ETS, Cobb, or TurboXS include polished piping, a large core, and necessary couplers. Expect a slight increase in lag compared to a TMIC, but much lower charge temperatures allow higher boost without knock.

Important: Upgrade the intercooler charge pipes to at least 2.5-inch diameter. The stock plastic pipes can crack under boost above 20 psi, and rubber couplers can blow off.

4. Fuel System

The FA20F’s direct injection system can supply enough fuel for about 350–380 whp on pump gas. To reach 400 whp reliably, you must upgrade several components:

  • High-Pressure Fuel Pump (HPFP) – The stock pump cannot maintain adequate rail pressure at high fuel flow. Upgraded units from Nostrum or IAG are rated for 500+ whp. Installation requires careful assembly and calibration.
  • Low-Pressure Fuel Pump – An in-tank pump like the AEM 340 LPH or Walbro 525 ensures consistent delivery to the HPFP. Wiring upgrade (hardwire kit) may be needed to avoid voltage drop.
  • Fuel Injectors – Stock direct injectors flow ~1000 cc/min. For 400 whp, consider upgraded injectors from FIC or Injector Dynamics (1000–1300 cc/min). On port injection setups, aftermarket port injectors and a secondary fuel controller are required.
  • Flex Fuel Capability – Running E85 is the most cost-effective way to gain power. Flex fuel kits from Cobb allow the ECU to automatically adjust for ethanol content, enabling 400 whp on E85 with the same turbo and supporting mods that would only yield 360 whp on 93 octane.

5. Engine Management and Tuning

No engine reaches 400 hp without precise control. The stock ECU can be reflashed using a Cobb Accessport V3 or a standalone like a Haltech Elite 1500. However, a custom tune is non-negotiable.

  • Accessport with ProTune – The Cobb platform is the most popular. A professional tuner (via e-Tune or dyno) modifies fuel, timing, boost, and VVT maps to match your hardware. Expect to pay $500–$900 for a custom calibration.
  • Ethanol Tune – If using E85, you need a separate tune file (or a flex fuel sensor to adjust in real time). Ethanol’s cooling effect and octane rating allow higher boost and earlier ignition timing.
  • Safety Features – A quality tune includes knock control, intake temperature compensation, and boost cut. Additionally, logging tools like Accessport Track or a wideband O2 sensor are recommended to monitor air-fuel ratios.

Note: Never use an off-the-shelf (OTS) tune from Cobb or a generic base map. Every car responds differently to mods, and a custom tune is the only way to avoid detonation and component damage.

6. Exhaust System

A free-flowing exhaust reduces backpressure and helps the turbo spool faster. For 400 hp, you need a full turbo-back exhaust:

  • Downpipe – Replaces the restrictive catalytic converter. A catted downpipe (e.g., Cobb Gesi or Grimmspeed) flows well and keeps the car street-legal in many areas. A catless downpipe adds 5–10 whp but may cause emissions issues and a strong noxious smell.
  • Intermediate Pipe / Midpipe – 3-inch diameter from the downpipe through the cat-back. Many aftermarket systems include a high-flow resonator to tame drone.
  • Cat-Back Exhaust – Gains minimal power on a 400 hp build, but improves sound and reduces weight. Choose a single exit (lighter) or dual exit (looks) from brands like Injen, Borla, or AWE Tuning. Avoid overly small mufflers that create deafening cabin noise.

Extra consideration: The FA20F uses a twin-scroll manifold from the factory. Aftermarket downpipes must maintain divided gas paths to preserve twin-scroll benefits. Many aftermarket options combine into a single 3-inch outlet soon after the turbo—while this sacrifices some low-end torque, it simplifies installation and is acceptable for a 400 hp goal.

7. Supporting Induction Mods

Several other air-path upgrades help the engine breathe more freely:

  • Turbo Inlet Pipe – The stock plastic inlet is squishy and restricts flow. A hard aluminum or silicone inlet (Perrin, Grimmspeed) smooths the path from the MAF to the turbo compressor.
  • Intercooler Piping – Upgrade to aluminum 2.5-inch or 3-inch pipes to reduce turbulence. Include a blow-off valve flange if using an atmospheric valve.
  • Intake Manifold Spacers – Phenolic spacers (e.g., Killer B) insulate the manifold from engine heat, reducing intake air temperatures by 5–10°F. They also improve torque slightly in the mid-range.

Drivetrain and Handling Considerations

Building 400 hp is pointless if you can’t put the power down or your transmission fails on the first launch. The VA WRX’s 6-speed manual (TR690) is notoriously weak—it may break at 350–400 whp when driven aggressively.

  • Clutch Upgrade – The stock clutch slips around 350 lb-ft. An organic disc with a heavy-duty pressure plate (Exedy Stage 1 or 2) handles 400–450 lb-ft. Avoid ceramic or puck clutches for daily driving; they chatter and wear quickly.
  • Transmission – For drag racing or repeated hard pulls, consider an STI 6-speed swap. The STI gearbox (JDM or USDM) is significantly stronger. The swap requires a tailshaft, driveshaft, and matching rear differential. Budget $4,000–$6,000 for a complete swap.
  • Rear Differential – The stock R160 can handle 400 hp with a VLSD (viscous limited-slip), but an STI R180 is more robust. If keeping the stock diff, change the fluid to a high-quality 75W90.

Suspension and Tires

Increasing power by 50% demands better grip and control. The VA WRX’s suspension is soft from the factory; upgrading provides sharper turn-in and reduces body roll during hard acceleration.

  • Coilovers – Adjustable dampers from Ohlins, KW, or Bilstein improve ride quality and allow corner-weighting. Entry-level options like BC Racing offer good value for street use.
  • Lowering Springs – A cheaper alternative that drops ride height 1–1.5 inches. Pair with uprated sway bars (22–25 mm front, 22 mm rear) to tame understeer.
  • Wheels and Tires – A 400 hp WRX needs 245–265 section width tires on 17- or 18-inch wheels. For street use, Michelin Pilot Sport 4S or Continental ExtremeContact Sport provide superb dry and wet grip. For the drag strip, a set of drag radials (Mickey Thompson ET Street S/S) helps avoid wheelspin.

Cooling and Reliability Upgrades

Higher power generates more heat. Managing oil and coolant temperatures is critical for longevity.

  • Oil Cooler – A thermostatically controlled oil cooler (e.g., Setrab, Mishimoto) with a 18–25 row core keeps oil temps below 230°F during hard driving. This protects the turbocharger and bearings.
  • Radiator – An all-aluminum radiator (Koyo, CSF) with dual electric fans increases coolant capacity. Stock plastic end tanks can fail under high boost and heat cycling.
  • Air-Oil Separator (AOS) – The FA20 accumulates blow-by under boost. An AOS from IAG or Crawford returns oil to the pan instead of venting to the intake. This prevents detonation from oil ingestion and keeps the intake valves cleaner.
  • Spark Plugs – Upgrade to one-step colder plugs (NGK LFR7AIX or equivalent) gapped to 0.024–0.026 inches. Cold plugs reduce the risk of pre-ignition at high boost levels.

Charge Air Cooling: Water/Meth Injection

If you cannot run E85 or a larger intercooler, a water-methanol injection kit (Aquamist, Snow Performance) can suppress intake temperatures and raise the octane of the incoming charge. This allows higher boost without knock. However, it requires a separate reservoir and consistent refilling.

Building in Stages

A realistic approach to 400 hp involves three phases:

  1. Stage 1 (350–370 whp): Intake, catless jpipe, EBCS, and a custom tune on 93 octane. Replace the intercooler with an upgraded TMIC. This is safe and reliable for daily driving.
  2. Stage 2 (400–420 whp): Add a larger turbo (GTX3071R or equivalent), larger downpipe, fuel system upgrade (HPFP + injectors + pump), and a front-mount intercooler. Tune on E85 or with flex fuel. This requires strong engine internals—the FA20 rods and pistons are forged but rated to about 450 whp. Beyond that, closed-deck block prep is needed.
  3. Stage 3 (450+ whp): Built short block, port injection, larger turbo, 6-speed STI swap. This is beyond the scope of this 400 hp guide but illustrates the ceiling of the FA20 platform.

Always dyno test after each stage to verify results and air-fuel ratios. Many tuners recommend leaving a safety margin: target 380–400 whp rather than squeezing every last horsepower.

Common Pitfalls and Mistakes

  • Skipping the fuel system: A larger turbo without upgraded injectors and HPFP leads to dangerously lean conditions. Invest in fuel mods before more exotic turbo kits.
  • Ignoring the drivetrain: A stock WRX transmission will eventually fail if you launch often at 400 hp. Budget for a clutch and consider the transmission swap.
  • Using generic OTS tunes: No two cars behave identically. A custom tune from a reputable tuner (e.g., Dr. Gain or Brentuning) is worth every dollar.
  • Overlooking heat management: Heat soak is the enemy of the FA20. If you cannot fit a large intercooler, upgrade the oil cooler and radiator first.

Final Thoughts

A properly built 400 hp VA WRX delivers exhilarating acceleration, a throaty exhaust note, and surprising agility. This is not a budget build—expect to spend $8,000–$15,000 on the modifications listed here, plus labor and tuning. However, if executed correctly, the result is a well-balanced performance car that can hang with modern sports cars and remain daily-drivable. Plan each step, choose quality components, and do not cut corners on calibration. Your clutch, your bank account, and your engine will thank you.