The EJ255 engine, found in Subaru's Forester XT, Legacy GT, and Outback XT among others, has earned a reputation for being a stout foundation for high-horsepower builds. For enthusiasts aiming for a reliable 450 wheel-horsepower (whp) target, combining a high-quality turbocharger system like APS with forged internal components such as Cosworth pistons represents a proven path. However, the total investment goes well beyond the parts themselves. This article provides a comprehensive cost breakdown of a 450 hp EJ255 performance build centered around an APS turbo kit and Cosworth pistons, covering every essential component, machining labor, tuning, and the often-overlooked supporting modifications needed to ensure the engine stays together under sustained boost.

Understanding the EJ255 Platform

The EJ255 is a 2.5-liter turbocharged flat-four from Subaru's EJ family. Introduced in the mid-2000s, it features a closed-deck block (on many variants), semi-closed or open deck on others—but all benefit from large displacement and a relatively short stroke that allows for high RPM potential. The engine’s Achilles' heel at elevated power levels is the factory pistons (hypereutectic cast aluminum) which are prone to cracking above 350–400 hp, and the rod bolts that can stretch under high cylinder pressure. For a target of 450 wheel horsepower, forged pistons, stronger rods, and upgraded fasteners are non-negotiable. The APS turbo kit, known for its high-quality cast manifolds and genuine Garrett or BorgWarner cores, delivers the airflow necessary to reach that power level without excessive backpressure.

Why 450 Horsepower?

A 450 whp goal sits in a sweet spot for the EJ255: it offers exhilarating acceleration (roughly 500–520 crank horsepower) while remaining streetable with a properly sized turbo. At this level, the engine still uses a relatively mild cam profile and stock-port heads, keeping costs manageable. Going much higher requires billet main supports, full head porting, and a standalone ECU, which can double the budget. Thus, this cost breakdown targets a realistic, durable, and fun street build.

Key Component Selection and Rationale

APS Turbo Kit

APS (Australian Performance Systems) is respected for its patented cast-divorced wastegate manifolds and high-flow downpipes. Their kits typically include a T3 or T4 flange, a genuine Garrett GT3582R or PT6262 journal-bearing turbo, and all necessary piping. The cast manifold reduces the risk of cracking common with tubular stainless steel manifolds on high-mileage builds.

  • APS Turbo Kit (incl. turbo, manifold, downpipe, lines): $2,500–$3,000
  • Installation labor: $500–$800
  • Note: Some kits require a custom inlet pipe; if not included, budget an extra $150–$200 for a silicone coupler and T-bolt clamps.

Cosworth Pistons

Cosworth is a motorsport powerhouse. Their forged pistons for the EJ255 use a 2618 alloy, which withstands higher thermal and mechanical stress than standard 4032 forgings. They feature advanced skirt coatings and a precise ring package to reduce blow-by. For a 450 hp build, choose a 9.0:1 or 9.5:1 compression ratio to keep cylinder pressure manageable while maintaining off-boost responsiveness.

  • Cosworth Piston Set (4 pistons + pins + rings): $800–$950
  • Machine work (bore & hone, deck block, balance rotating assembly): $600–$1,000
  • Note: Always replace wrist pins and clips at the same time; included in most piston sets.

Forged Connecting Rods

The stock cast rods may survive 450 hp with careful tuning, but upgrading to a forged H-beam or I-beam rod (e.g., Manley, Eagle, or Carillo) adds crucial safety margin. The added strength also allows a higher rev limit (7,500+ RPM) if desired later.

  • Forged Rods: $500–$700
  • Installation (included in machine work above, or separate): $200–$300 if charged separately

Main and Rod Bearings

OEM Subaru bearings are adequate for stock power, but a 450 hp build benefits from performance bearings such as ACL Race series or King XP. These have higher load capacity and better oil clearance control.

  • Bearing set (main + rod): $150–$250
  • Installation (part of short block assembly): already accounted in machine work

Head Studs

Factory head bolts stretch and may lift under high boost. ARP head studs are a must for 450 hp to maintain consistent clamp load and prevent head gasket failure.

  • ARP Head Studs: $200–$300
  • Installation: $100–$200 (if charged separately)

Upgraded Oil Pump

At higher RPM and boost, the stock oil pump can cavitate, leading to bearing failure. A 12mm or 11mm oil pump from an STI or a billet aftermarket pump (e.g., IAG, Kelford) ensures consistent oil pressure.

  • Upgraded Oil Pump: $200–$400
  • Installation (included in engine assembly): $0–$100

Fuel System Upgrades

To support 450 whp on pump gasoline or E85, the fuel system must deliver adequate volume and pressure. Stock injectors and pump are inadequate.

  • High-Impedance Fuel Injectors (1000–1300cc): $400–$600
  • Fuel Pump (AEM 340, Walbro 525, or similar): $200–$300
  • Fuel pressure regulator (if return-style system): $100–$200
  • Installation labor (including wiring for pump): $200–$400

Intercooling System

The TMIC (top-mount) may be sufficient with an upgraded core (e.g., Process West, Spearco) or switch to a front-mount (FMIC) for lower intake temps at high boost. A FMIC adds complexity but supports higher power levels with consistency.

  • Upgraded TMIC: $500–$1,000
  • FMIC Kit: $800–$1,500
  • Installation (TMIC: bolt-in; FMIC: custom piping): $200–$500

Exhaust System

A free-flowing downpipe (preferably 3-inch with high-flow cat) and a cat-back exhaust reduce backpressure and let the turbo spool efficiently.

  • Downpipe: $250–$500
  • Cat-Back Exhaust: $500–$1,200
  • Installation: $200–$400

ECU Tuning

Professional tuning is the most critical part. A bad tune can destroy forged pistons as fast as stock ones. Use a reputable tuner who specializes in Subaru OpenECU (Cobb Accessport) or standalone systems (Haltech, Link, Motec).

  • Protune / E-tune: $500–$800
  • Accessport or tuning device (if not already owned): $650
  • Note: For the APS turbo, a custom tune is absolutely required; no off-the-shelf map will be safe.

Additional Components and Miscellaneous Costs

Beyond the major items, several smaller but essential parts add up:

  • Gaskets and seals (full engine gasket set): $200–$300
  • Belts (timing belt, accessory belts): $100–$200
  • Water pump (while engine is apart): $100
  • Thermostat: $30
  • Spark plugs (colder step): $50
  • Oil and filters (break-in and first change): $100
  • Coolant: $40
  • Miscellaneous hardware (studs, nuts, seals): $100–$200
  • Contingency (unexpected machining, shipping, core charge): $300–$500

Total Cost Summary

Below is a consolidated list of estimated costs for a full 450 hp EJ255 build with an APS turbo, Cosworth pistons, and all necessary supporting modifications. Prices vary by location, shop rates, and part availability.

  • APS Turbo Kit (complete): $3,000
  • Cosworth Pistons + machine work: $1,600
  • Forged Connecting Rods: $700
  • Bearings: $200
  • ARP Head Studs: $250
  • Upgraded Oil Pump: $350
  • Fuel System (injectors + pump + regulator): $800
  • Intercooler (FMIC kit): $1,200
  • Exhaust System (downpipe + cat-back): $1,000
  • ECU Tuning: $700
  • Gaskets, belts, water pump, thermostat, plugs, fluids: $600
  • Labor (engine removal, disassembly, assembly, installation): $2,000–$3,000
  • Miscellaneous / contingency: $500

Estimated Total Range: $12,000 – $14,000

This total assumes a shop performs all labor. A DIY enthusiast with engine-building tools could save $2,000–$3,000 in labor but must factor in rental costs for specialty tools (e.g., torque plates, piston ring compressor, bearing bore gauge).

Final Thoughts

Building a reliable 450 hp EJ255 with an APS turbo and Cosworth pistons is an achievable project, but it requires a serious financial commitment. Spending less than the figures above often means cutting corners on machine work, fuel system quality, or tuning, which risks engine failure. The single most important takeaway is to prioritize professional machine work (especially squaring the block, honing with a torque plate, and balancing the rotating assembly) and a custom tune on a dyno or through a reputable remote tuner.

For further reading, consult resources such as the Cosworth piston product page for exact specifications, APS’s official site for turbo kit details, and the LegacyGT forums for real-world build logs. With careful planning and a solid budget, this build can deliver years of high-performance driving without constant repairs.