Understanding the EJ255 Engine Platform

The Subaru EJ255 is a 2.5-liter turbocharged flat-four engine that served as the powerhouse for models such as the 2004–2014 Forester XT, 2005–2009 Legacy GT, and 2004–2008 Outback XT. Its semi-closed deck block, forged connecting rods (in later revisions), and a twin-scroll capable IHI VF40 or VF46 turbocharger make it a robust foundation for building reliable power. The EJ255 shares much of its architecture with the legendary EJ257 found in the STI, but with a smaller turbo and slightly different heads. Despite these differences, the engine responds exceptionally well to modifications and can comfortably support 450 wheel horsepower with proper fueling, cooling, and engine management.

To hit 450 horsepower, you will need to address four key areas: airflow into the engine, fuel delivery, exhaust flow, and engine management. The APS Cold Air Intake and DW300 Fuel Pump are two critical upgrades, but they must be part of a complete system. Below we break down each component, its cost, and the supporting modifications necessary to achieve a reliable 450 HP EJ255 setup.

Key Components for a Reliable 450 HP Build

APS Cold Air Intake – Maximizing Airflow

The APS cold air intake system relocates the air filter to a cooler area within the fender well, away from engine heat. It uses a large-diameter aluminum tube and a high-flow dry or oiled filter element to reduce intake restriction. This design allows the turbo to ingest denser, cooler air, which directly contributes to higher horsepower. For the EJ255, an intake like the APS can add 15–25 horsepower when paired with a proper tune.

Cost: The APS Cold Air Intake typically retails between $300 and $400. Some vendors offer it for around $320, while others may list closer to $400. You can occasionally find used units for $200–$250, but be cautious of contamination.

Alternatives: If the APS intake is out of stock or over budget, consider the Cobb SF Intake + Airbox ($280–$350) or the K&N Typhoon for the same chassis ($250–$300). These also improve airflow, though the APS design has a proven track record for cooler intake temperatures on the EJ255.

Installation Note: The intake replaces the factory airbox and part of the intake tract. Expect installation time of 1–2 hours for a DIY mechanic. You will need to remove the front bumper fascia to access the fender well on most models. A tune is required after installation to optimize air/fuel ratios.

DW300 Fuel Pump – Meeting Fuel Demands

At 450 horsepower, the factory fuel pump will not keep up with fuel demand. The DW300 is a 320 LPH (liters per hour) high-pressure in-tank pump with an integrated controller. It is designed as a direct drop-in for most Subaru fuel hangers, including the EJ255’s saddle-style tank on the Forester XT and Legacy GT. The DW300 delivers consistent fuel pressure even under high boost and heavy load, which is essential to prevent dangerous lean conditions.

Cost: A new DW300c (with controller) runs between $150 and $200. Without the controller, prices can drop to $120–$150, but we recommend purchasing the version with the controller for easier installation and to avoid electrical issues.

Alternatives: The AEM 50-1215 ($180) and Walbro 525 ($220–$250) are both capable of supporting 450 HP. However, the DW300 is quieter and requires less wiring modification on the EJ255. For a budget build, a used factory STI pump from an EJ257 (flow-rated at about 230 LPH) can support around 400 HP, but for 450 HP you need the higher volume of the DW300.

Installation: The pump installs inside the fuel tank. Dropping the tank or accessing it through the rear seat requires 2–3 hours of labor. If you are not comfortable working with fuel, expect $150–$250 shop labor for the pump swap. Do not forget a new fuel filter and O-ring kit during the install.

Supporting Modifications for 450 HP

To safely and reliably make 450 HP, the EJ255 needs help on the boost, exhaust, and cooling sides. The following upgrades are nearly mandatory once you cross the 400+ HP threshold.

Upgraded Intercooler – Keeping Intake Temps Down

The factory top-mounted intercooler (TMIC) on the EJ255 is small and prone to heat soak. At 450 HP, you will generate substantial boost pressure (20–24 PSI), which heats the intake charge significantly. An aftermarket intercooler with a larger core, bar-and-plate construction, and better end tanks will drop intake air temperatures by 30–50°F on a hot day.

Options:

  • Process West Verticooler TMIC ($900–$1,100) – direct fit, excellent thermal management.
  • Cobb TMIC ($750–$850) – works well with their intake and tune.
  • Mishimoto TMIC ($500–$600) – budget pick, still an improvement over stock.
  • Front-mount intercooler (FMIC) kit ($800–$1,500) – more volume, but requires cutting the bumper beam and relocating the fog lights.

Best for 450 HP: A quality TMIC is sufficient for 450 HP if you live in a mild climate. If you track the car or live in a hot area, step up to a FMIC. Count on $600–$1,000 for the intercooler itself.

Performance Exhaust System – Reducing Backpressure

The stock exhaust system on the EJ255 necks down to 2.25″ in places and includes restrictive catalytic converters. For 450 HP you need a full turbo-back exhaust (downpipe + midpipe + catback) with a 3″ diameter. Free-flowing exhaust reduces backpressure, spools the turbo faster, and can add 20–30 horsepower.

Cost Breakdown:

  • Downpipe (catted or catless): $250–$500
  • Midpipe with resonator: $150–$300
  • Catback exhaust system: $400–$900
  • Full turbo-back kit (single purchase): $800–$1,500

Popular Brands: Invidia, Cobb, Tomei, A’PEXi. Check local laws for catalytic converters – many states require a catted downpipe for emissions compliance. A catless downpipe is louder and may produce a Check Engine Light for catalyst efficiency; this can be tuned out.

ECU Tuning – The Brain of the Operation

No 450 HP build is complete without professional tuning. The factory ECU will not understand the larger injectors, higher fuel flow, lower intake restriction, and increased boost. You have two main paths:

  • Accessport + Off-the-shelf Map: Cobb Accessport V3 ($650 new, $500 used) with a stage 2+ map. These maps are conservative and designed for common mods (intake, downpipe, intercooler, fuel pump). They work well but leave a little power on the table.
  • Custom Protune: Either via an opensource tuning platform (e.g., RomRaider, ECUFlash – free software, but you need a Tactrix cable for $180) or a dyno tune at a shop. A professional pro tune costs $400–$800 for a road tune up to $1,200 for a dyno tune with wideband logging.

For 450 HP, a custom protune is strongly recommended. Off-the-shelf maps are not adjustable enough for the specific turbo and intercooler combination you choose. Budget $500–$1,200 for tuning. Expect a protune to add 40–60 HP over an off-the-shelf map for the same hardware.

Turbocharger Upgrade – The HP Gatekeeper

While the original article focused only on intake and fuel pump, realistically achieving 450 wheel horsepower on an EJ255 requires a turbo upgrade. The stock VF40 or VF46 max out around 320–350 HP on pump gas. To reach 450 HP you need a larger turbo that can flow more air.

Turbo Options for 450 HP:

  • VF48 (STI take-off) – good for 380–400 HP, used price $300–$500. With aggressive tuning it might squeak to 420 HP, but 450 is a stretch.
  • Blouch Dominator 1.5 XTR ($1,200–$1,400) – rated for 450 HP, spools quickly, uses factory location.
  • BNR Supercars EFR 6758 or 7163 kit ($2,000–$2,500) – newer technology, excellent spool and top end.
  • Garrett GTX3071R with custom rotated kit ($1,500–$3,000) – supports 450–550 HP, but requires more fabrication.

Budget Recommendation: The Blouch Dominator 1.5 XTR is a popular choice for the EJ255 because it bolts on with minimal modifications. It includes install components and supports the 450 HP target without pushing the turbo to its limit. Cost for turbo alone: $1,200–$1,500.

Fuel Injectors – Don’t Starve the Engine

The stock side-feed injectors in the EJ255 are 560 cc/min – insufficient for 450 HP. You will need upgraded injectors in the 850–1,000 cc/min range.

  • DeatschWerks DW850cc side-feed injectors: $350–$400
  • Injector Dynamics ID1000 pigtails: $500–$600 (require harness adapters)
  • Five-O 900cc side-feed: $300–$350

With a proper tune, 850 cc injectors can support 450 HP at moderate duty cycles. We recommend 1000 cc to stay safe. Budget $350–$600 for this upgrade.

Boost Control and Gauges

An electronic boost control solenoid (EBCS) is required for stable boost levels. The factory boost control solenoid is unreliable at higher pressures.

  • GrimmSpeed EBCS: $140–$170
  • Aftermarket boost gauge and wideband A/F sensor: $200–$400

Detailed Cost Breakdown Table

Below is a comprehensive list of the parts needed for a reliable 450 HP EJ255 build, with low and high cost estimates. Prices are approximate and may vary by vendor.

  • APS Cold Air Intake: $300 – $400
  • DW300 Fuel Pump: $150 – $200
  • Upgraded Fuel Injectors (850-1000cc): $300 – $600
  • Upgraded Intercooler (TMIC or FMIC): $600 – $1,200
  • Turbocharger (e.g., Blouch Dominator 1.5 XTR): $1,200 – $1,500
  • Turbo-back Exhaust (3″): $800 – $1,500
  • ECU Tuner (Cobb Accessport): $550 – $650
  • Professional Custom Tune: $400 – $1,200
  • Electronic Boost Control Solenoid: $140 – $170
  • Gauges (boost, wideband): $200 – $400
  • Optional: Catch can/AOS: $150 – $400
  • Optional: Upgraded clutch (suggested for 450 HP): $400 – $800

Total Estimated Investment

Summing the low-end estimates: $300 + $150 + $300 + $600 + $1,200 + $800 + $550 + $400 + $140 + $200 = $4,640

Summing the high-end estimates: $400 + $200 + $600 + $1,200 + $1,500 + $1,500 + $650 + $1,200 + $170 + $400 = $7,820

So the realistic cost to achieve 450 HP with the APS cold air intake and DW300 fuel pump, including all supporting modifications, lies in the range of $4,600 to $7,800 in parts alone. If you pay for labor (installation $1,000–$2,500), add $1,000–$2,500 more.

This is significantly higher than the original article’s estimate of $1,750 – $3,300 because the original article omitted the necessary turbo upgrade, fuel injectors, boost control, gauges, and clutch. You cannot reach 450 HP on the stock turbo or factory injectors. The intake and fuel pump alone are only the beginning.

Reliability Considerations at 450 HP

Making 450 HP on an EJ255 is achievable, but the engine’s longevity depends on several factors beyond the parts list:

  • Transmission: The 5-speed manual in the Forester XT/Legacy GT is the weak point. At 450 HP, it may break under hard launches. Budget $2,000–$3,500 for an STI 6-speed swap if you plan to track or drag race. For street driving with conservative pilot, the 5-speed can survive, but it is a ticking bomb.
  • Clutch: Stock clutch will slip at 350 HP. An upgraded disc and pressure plate are mandatory. Expect $400–$800 for a stage 2/3 clutch and flywheel combo, plus $300–$500 labor if not DIY.
  • Cooling: The EJ255 runs hot. Consider a larger radiator, oil cooler, and possibly a Killer B oil pickup and baffle for $200–$600.
  • Maintenance: Before modding, ensure timing belt, water pump, head gaskets, and oil seals are in good condition. A tired EJ255 will not last at 450 HP.
  • Fuel Quality: 450 HP on 91 octane is risky. Use 93 octane or ethanol. Ethanol (E85) is ideal for higher power and lower knock, but requires larger injectors and a dedicated tune.

External Resources

For further reading and community wisdom, check these sites:

Conclusion

Achieving 450 horsepower from the Subaru EJ255 is a rewarding goal that requires careful planning and investment. The APS Cold Air Intake and DW300 Fuel Pump are excellent starting points, but they cannot deliver the power by themselves. You must also upgrade the turbocharger, fuel injectors, intercooler, exhaust, and engine management, plus address the transmission and clutch for reliability.

By budgeting $4,600–$7,800 for parts and another $1,500–$3,000 for labor and tuning, you can build a 450 HP EJ255 that is both fast and durable. Skip a necessary item, and you risk damaging a good engine. Use the detailed cost breakdown above as a checklist to ensure you include every component. With the right parts and a professional tune, your Subaru will deliver astonishing performance for years to come.