Why the 350 Horsepower Goal Makes Sense for Your WRX

The Subaru WRX, especially the fifth-generation (2022+) and earlier models with the FA20 or EJ series engines, has a robust aftermarket. Setting a target of 350 wheel horsepower (whp) on a flex fuel setup is a sweet spot: it’s aggressive enough to transform the car’s character without requiring a full engine rebuild or a built block. At this power level, the stock internal components typically hold up well (provided the tune is safe and maintenance is up to date), and the car remains reliable for daily driving. Ethanol’s knock resistance and cooling properties also lower the risk of detonation, making 350 whp a realistic, repeatable goal.

What Is Flex Fuel and Why It Matters for Subaru Tuners

Flex fuel systems allow the ECU to automatically adjust the air-fuel ratio and ignition timing based on the ethanol content detected in the fuel tank. Ethanol (E85, typically a blend of 51% to 83% ethanol by volume) offers two key advantages over pump gas: a higher octane rating (around 100–105 RON) and a greater latent heat of vaporization. The higher octane suppresses knock, enabling more boost and timing, while the cooling effect reduces intake charge temperatures. For a WRX, switching from 93 octane to E85 often yields a 10–15% horsepower gain on the same turbo setup. A dedicated flex fuel sensor communicates ethanol percentage to the ECU, allowing seamless transition between gasoline and ethanol blends.

Parts Required for a 350 WHP WRX Flex Fuel Build

Below is an expanded look at each major component, with pricing ranges based on mid-line to premium aftermarket offerings. Prices are approximate and may vary by region, vendor, and current market conditions (2025 estimates).

Turbocharger Upgrade

To reach 350 whp, the stock turbo (often a TD04 or a smaller twin-scroll unit) will be a bottleneck. A popular choice is a Garrett GTX2867R Gen II, a BorgWarner EFR 6758, or a hybrid stock-location unit like the Blouch 18G-XT or 20G-XT. These turbos spool quickly and flow enough to hit the target with ethanol.
Cost range: $1,500 – $2,500 (including actuator and gaskets).

Fuel Injectors

Ethanol requires roughly 30% more fuel volume than gasoline for the same air mass. High-impedance 1000–1300cc injectors are the minimum. Brands like Injector Dynamics, Fuel Injector Clinic, or DeatschWerks are proven choices. Use new or professionally cleaned/flow-matched injectors.
Cost range: $600 – $1,000.

Fuel Pump

An in-tank 340 lph or 450 lph pump (e.g., AEM 340, Walbro 535) is sufficient for 350 whp on E85. For peace of mind, some builders add a secondary inline pump, but a single high-flow unit works well.
Cost range: $200 – $400.

Intercooler Upgrade

The factory top-mount intercooler (TMIC) can handle moderate power, but ethanol’s cooling effect is best leveraged with a larger bar-and-plate intercooler. Options include a larger STI TMIC (dual-core) or a front-mount (FMIC) kit. FMICs add weight and complexity but offer superior charge air cooling.
Cost range: $600 – $1,200 (TMIC) or $800 – $1,500 (FMIC).

Exhaust System

A free-flowing turbo-back exhaust (downpipe, mid-pipe, and cat-back) reduces backpressure and helps spool. A 3-inch diameter is the standard. Catted downpipes are recommended for an inspection-friendly build, while a catless downpipe offers lower cost and slightly more flow.
Cost range: $800 – $1,500.

ECU Tuning & Software

You’ll need an access port (Cobb) with an open-source tune, or a standalone ECU like a Haltech or Motec. For stock ECU tuning, Cobb’s Accessport is the most common tool; the cost includes the device and the license for some custom tuning platforms. E-tunes or remote sessions are cheaper than dyno time.
Cost range: $500 – $1,000 for hardware and software.

Flex Fuel Sensor

A GM-style flex fuel sensor (e.g., Continental 13577429) or a kit built for Subarus converts ethanol content to a signal the ECU can read. You’ll need a wiring harness adapter if using an Accessport or a standalone ECU.
Cost range: $150 – $300.

Intake System

A cold-air intake or high-flow intake with a larger MAF housing allows the engine to breathe properly. Some kits include a pre-filter and heat shield. Avoid oiled filters if you have a MAF sensor that is sensitive to contamination.
Cost range: $300 – $600.

Miscellaneous Parts

Brass or aluminum fuel line adapters, PTFE hoses for the fuel system, gaskets, bolts, a boost controller (for manual boost adjustment), and possibly a catch can or AOS (air-oil separator) to reduce oil blow-by when running high boost.
Cost range: $200 – $500.

  • Fuel pressure regulator (FPR) – around $150
  • Gauges (AFR, boost, ethanol content) – $200 – $500
  • Clutch upgrade – at 350 whp, the stock clutch may slip; expect $400 – $900 for an aftermarket unit
  • Engine oil cooler – $300 – $600 for track use

Estimated Total Parts Cost (Detailed Breakdown)

To give you a clearer picture, here are two scenarios: a budget-conscious build using proven mid-range parts, and a premium build with top-tier components and optional upgrades.

Budget Build (Mid-Range Parts)

  • Turbo: Blouch 18G-XT – $1,600
  • Injectors: DeatschWerks 1000cc – $600
  • Fuel pump: AEM 340 – $200
  • Intercooler: STI TMIC (used or aftermarket) – $700
  • Exhaust: Invidia 3″ catless downpipe + Nvidia Q300 catback – $1,000
  • ECU: Cobb Accessport (used) – $600
  • Flex fuel kit: JDL or iWire flex fuel kit – $250
  • Intake: Cobb SF intake – $350
  • Misc. (gaskets, lines, clamps) – $300

Total parts (budget): approx. $5,600

Premium Build (Name-Brand, New Parts)

  • Turbo: Garrett GTX2867R Gen II – $2,200
  • Injectors: Injector Dynamics 1050x – $900
  • Fuel pump: Walbro 535 – $400
  • Intercooler: Process West Verticooler (TMIC) – $1,200
  • Exhaust: AWE Tuning Touring edition turbo-back – $1,500
  • ECU: Cobb Accessport + flex fuel license – $800
  • Flex fuel sensor + harness – $250
  • Intake: Eventuri carbon intake – $600
  • Misc. (PTFE lines, AOS, catch can) – $600
  • Clutch: South Bend Stage 3 – $800

Total parts (premium): approx. $9,250

Tuning Costs: Ethanol-Specific Considerations

Tuning a flex fuel WRX is more complex than a pure gasoline tune. The tuner must create multiple tables (ethanol content vs. timing, boost, and fuel) that interpolate between E0 (pure gas) and E85. A dyno tune is the gold standard because it allows real-time adjustments under load, but remote E-tunes (using data logs) are cheaper and work well for experienced owners who can monitor knock and AFR.

  • Dyno tune session (2–4 hours): $500 – $900
  • E-tune (remote, includes revisions): $300 – $600
  • ECU reflash (if using open-source like RomRaider + Tactrix cable): $100 – $300 (labor/tuning fee)

Most reputable Subaru tuners (e.g., Panda Motorworks, Boosted Performance, or Cobb Tuning authorized pro tuners) charge $500–800 for a dyno tune on a flex fuel setup. Some offer discounts if you purchased their parts bundle.

Installation Costs: DIY vs. Professional Shop

Labor costs vary widely by location and shop rates ($100–$200 per hour). A full flex fuel build (excluding clutch install) typically takes 10–20 hours for an experienced Subaru shop.

  • Professional install: $1,200 – $2,500 (includes fuel system, turbo, intercooler, exhaust, and tuning session coordination)
  • DIY: $0 (but factor in at least two weekends of work, specialty tools like a fuel line disconnect tool, and a lift or jack stands)

If you do the installation yourself, you’ll still need a professional to finalize the tune on a dyno. Some shops offer a “tune only” rate if you bring the car mechanically ready.

Total Build Cost: Low-End vs. High-End

Combining parts, tuning, and installation (professional) yields the following ranges. Fuel, unintended expenses (broken bolts, adapters), and shipping are not included.

  • Low-end (budget parts, DIY install, remote tune): $5,600 (parts) + $400 (tune) + $0 (install) = $6,000
  • Mid-range (mid-priced parts, shop install, dyno tune): $7,000 (parts) + $700 (tune) + $1,500 (install) = $9,200
  • High-end (premium parts, shop install, dyno tune, clutch): $9,250 (parts) + $800 (tune) + $2,500 (install) + $800 (clutch install labor maybe included) = $13,350

Most owners targeting 350 whp on a WRX will land in the $8,000–$10,500 range if they shop sales and do part of the labor themselves.

Ethanol Supply & Maintenance Considerations

Running flex fuel requires a few lifestyle adjustments. E85 is not available at every gas station, especially in certain regions. Use apps like E85Finder to locate pumps. Ethanol absorbs moisture, so avoid leaving the car parked with E85 for more than a month; switch to gasoline or use a stabilizer. You’ll need to change your oil more frequently (every 3,000–4,000 miles) because ethanol can contaminate the oil with moisture and fuel. The fuel system components (rubber lines, seals) should be ethanol-compatible; PTFE hoses are mandatory for long-term reliability.

Performance Results: What to Expect from a 350 WHP Flex Fuel WRX

On an all-wheel-drive dyno (DynoJet or Mustang), a FA20 WRX with a GTX2867R, 1000cc injectors, and E70 fuel typically lays down 340–370 whp and 340–370 lb-ft of torque. That translates to roughly 4.2–4.5 seconds to 60 mph (with a good launch) and high-11-second quarter-mile times. The powerband is fat and early, with full spool by 3,500–4,000 RPM. Compared to a stock WRX (about 225–240 whp on 93 octane), the difference is night and day.

Conclusion

A 350 horsepower flex fuel WRX build is one of the most rewarding upgrades you can make. The cost is moderate compared to a full engine swap, and the parts are well-documented. By following a clear budget and relying on experienced tuners, you can achieve a reliable, streetable, and exhilarating car. Always prioritize safety (good clutch, proper fuel lines, and a conservative tune) and consider purchasing a flex fuel kit from a vendor like Motorsport Engineering or a Cobb-licensed shop to simplify the wiring. With careful planning, you’ll be enjoying the flexibility of ethanol and the rush of 350 whp before you know it.