Introduction

The Subaru EJ255 has long been a favorite among enthusiasts for its turbocharged capability and broad aftermarket support. Found in models like the 2004–2014 Forester XT, 2005–2006 Legacy GT, and 2008–2014 WRX, this 2.5-liter flat‑four offers a strong foundation for power builds. However, achieving long‑term reliability at elevated power levels requires addressing the engine’s known weaknesses—specifically its cast pistons and oil pump design. Many experienced builders and tuners have turned to two proven upgrades: forged Cosworth pistons and the OEM STI oil pump. This article takes a deep dive into the technical merits of these components, their installation considerations, and the real‑world long‑term results that make them a winning combination for the EJ255.

The EJ255 Engine: Strengths and Common Failure Points

The EJ255 shares its basic architecture with other Subaru flat‑fours, but its open‑deck block and turbocharged intake produce around 224–265 hp from the factory. While the engine responds well to tuning, the factory cast pistons are prone to ringland failure under sustained high boost or knock events. The OE oil pump, a 10 mm unit, can also struggle to maintain consistent pressure during high‑RPM operation, especially when bearing clearances are opened for aftermarket rods and pistons. Understanding these failure modes is essential to appreciating why the Cosworth pistons and STI oil pump are so effective.

Factory Cast Pistons

The stock EJ255 pistons are hypereutectic castings. They are lightweight and do a decent job at stock power levels, but they become a liability above roughly 350 whp on pump gas. The ringlands—particularly the top and second compression ring grooves—can crack under detonation or excessive cylinder pressure. Once a ringland breaks, the engine suffers blowby, oil consumption, and a rapid loss of compression. This is the most common internal failure on modified EJ255s.

Oil Pump Limitations

The factory EJ255 oil pump (part number 15010AA310) has a 10 mm gear width. While adequate for stock bearing clearances and normal driving, it can produce less than ideal oil pressure at high RPM when the oil is hot or when thicker bearing clearances are used. Many builders report that the stock pump’s pressure relief valve also opens prematurely, limiting flow to the cylinder heads and turbocharger. The STI oil pump (11 mm gear width, part number 15010AA280) is a direct swap that addresses these gaps.

Cosworth Pistons: Forged Strength for Serious Power

Cosworth is a name synonymous with high‑performance engine components across motorsport. Their pistons for the EJ engine series are forged from 2618 aluminum alloy—a material chosen for its high fatigue strength and ability to withstand extreme thermal cycling. Unlike hypereutectic cast pistons, forged units handle significant abuse before cracking.

Design Features

  • 2618 Alloy: Provides superior tensile strength and ductility compared to common 4032 or cast alloys. This reduces the risk of ringland fracture even under detonation events.
  • Optimized Ring Package: Cosworth pistons use a 1.2 mm top ring and 1.2 mm second ring with low tension to reduce friction while still sealing well. The oil ring is a three‑piece design for excellent oil control.
  • Accumulator Grooves: Small grooves on the skirt help retain oil for better lubrication and reduced scuffing during cold starts.
  • Weight Reduction: Forged pistons are often lighter than factory cast units, reducing reciprocating mass and allowing the engine to rev more freely.
  • Precision Machining: Each piston is CNC‑machined for consistent weight and dimensions, simplifying balancing during engine assembly.

Installation Considerations

Upgrading to Cosworth pistons requires a careful bore preparation. The block must be honed to the correct piston‑to‑wall clearance (typically 0.0035–0.0040 inches for 2618 alloy, depending on intended use). Because forged aluminum expands more than cast alloy, cold clearances are slightly larger—this means oiling is critical during warm‑up. Many builders also upgrade the wrist pins to a high‑strength tool steel pin and use full‑floating pin bushings in the connecting rods. It is strongly recommended to pair these pistons with quality forged rods (e.g., Manley or IAG) to match the strength of the rotating assembly.

Performance Gains

  • Higher Boost Threshold: With forged pistons, enthusiasts can safely run 25–30 psi of boost on pump gas or E85, resulting in 400–500 whp ranges when combined with appropriate turbo and fueling.
  • Improved Compression Ratio: Cosworth offers pistons in various compression ratios (typically 8.5:1 to 9.5:1 for turbo applications). A slightly higher compression can improve off‑boost response without sacrificing detonation tolerance if the tune is correct.
  • Better Heat Dissipation: The forged material and thermal design help extract heat from the combustion chamber, reducing peak cylinder head temperatures.

Read more about Cosworth’s piston technology on their official site: Cosworth EJ Forged Pistons.

STI Oil Pump: 11 mm for Consistent Pressure and Flow

The STI 11 mm oil pump was originally used on the EJ207 and some EJ257 engines. It has a wider gear set that provides approximately 10–15% more oil flow at a given RPM compared to the 10 mm pump. This extra capacity becomes crucial when the engine has looser bearing clearances (common with forged internals) or when the turbocharger oiling passages demand more volume.

Key Advantages

  • Higher Volumetric Flow: The 11 mm gears move more oil per revolution, ensuring the rod and main bearings, cylinder heads, and turbocharger receive adequate lubrication at all times.
  • Improved Pressure Regulation: The STI pump’s pressure relief valve opens at a slightly higher setpoint (around 85–90 psi) versus the stock pump (75–80 psi). This means oil pressure stays higher during high‑load, high‑RPM passes.
  • Durability: The pump housing and gears are designed for the rigors of high‑RPM operation. The STI unit uses a cast iron outer gear and hardened steel inner gear for long service life.
  • Direct Fit: No modifications are required to install the 11 mm pump onto an EJ255 block. The oil pickup tube and pan remain the same.

Potential Downsides

  • Parasitic Loss: A larger oil pump consumes slightly more power (typically 1–3 hp). This is negligible for a built engine.
  • Oil Windage at High RPM: Some builders recommend upgrading to a better oil pan baffle or a windage tray to prevent oil starvation during hard cornering, as the higher flow can aerate the oil if the pan lacks control.
  • Oil Cooler Recommended: With increased flow, oil temperatures can rise. A quality oil cooler (e.g., Setrab or Mocal) is strongly advised for track use.

For factory specifications, refer to the Subaru service manual or parts catalog: STI Oil Pump Part 15010AA280.

Synergy: How Cosworth Pistons and the STI Oil Pump Work Together

Individually, each upgrade solves a specific weakness. Together, they create a virtuous circle of reliability:

  1. The forged pistons allow higher cylinder pressures and boost without ringland failure.
  2. Higher horsepower generates more heat, requiring better oil flow to the rings and bearings.
  3. The 11 mm oil pump delivers the extra volume and pressure needed to keep the pistons’ oil jets (which are standard on EJ255) working effectively at higher RPM.
  4. Consistent oil pressure also helps cool the underside of the piston crown through the oil squirters, reducing peak piston temperatures by up to 50°F in some cases.
  5. With proper tuning and supporting mods, engines equipped with both upgrades have demonstrated tens of thousands of miles of street and track driving without internal failures.

Numerous forum threads on NASIOC and IWSTI document owners running 400+ whp for over 30,000 miles with this exact piston/pump combination. One prominent builder, Outfront Motorsports, recommends this pairing for all EJ25 engines aiming for 400–500 whp. See their build guide: Outfront Motorsports EJ25 Builds.

Long‑Term Results: Performance and Reliability Data

While every build is unique, a growing body of owner reports and dyno logs provides strong evidence that the Cosworth/STI pump combination delivers dependable long‑term performance. Below we summarize typical outcomes.

Horsepower and Torque Improvements

  • 400–500 whp common: With a 20G‑size turbo, E85, and proper tuning, many builds produce 420–480 whp on a Dynojet. Torque often reaches 400–450 lb‑ft.
  • Spool response: The lighter rotating assembly (forged pistons + often lighter rods) improves throttle response, with full boost arriving 200–400 RPM earlier than on a stock long‑block.
  • Consistent power output: Oil pressure stability means the timing retard due to knock is reduced; owners report fewer pulls with knock correction, especially in hot weather.

Reliability Enhancements Over Stock

  • No ringland failures: In a survey of 20+ built EJ255 engines on the Subaru enthusiast forum, none reported ringland fractures after 3+ years of mixed driving and autocross.
  • Lower oil consumption: Even with looser piston‑to‑wall clearance, the low‑tension ring package and excellent oil control lead to typical consumption of less than 1 quart per 3,000 miles—better than many stock EJ255s.
  • Extended rebuild intervals: Many builders report going 60,000–80,000 miles before needing a refresh (rings, bearings, seals), compared to 30,000–50,000 for stock‑spec builds pushing similar power.
  • Reduced bearing wear: The STI pump’s increased flow helps cool the rod and main bearings, reducing wear even under sustained high‑RPM operation.

Real‑World Example: A 2006 Legacy GT Build

One verified example from the LegacyGT.com forums: an owner installed Cosworth pistons (9.5:1 compression) and the STI 11 mm oil pump during a block rebuild at 80,000 miles. Supporting mods included a VF52 turbo, 750 cc injectors, and a Cobb AccessPort tune targeting 22 psi. After 50,000 miles of daily driving plus track days, the engine consumed only 0.5 quarts between oil changes, compression tests showed 155–160 psi across all cylinders, and a leak‑down test indicated less than 5% leakage. The owner reported zero mechanical issues aside from a worn turbo coolant line.

Supporting Modifications for Maximum Longevity

Pistons and oil pump alone don’t guarantee reliability. To fully realize the potential of the EJ255 upgrade, consider these essential companions:

Fuel System Upgrades

  • High‑flow fuel pump: A Walbro 450 or AEM 340 ensures enough fuel volume for higher horsepower.
  • Larger injectors: 1,000 cc/min or larger for E85 applications; 750–850 cc for pump gas.
  • Fuel pressure regulator: Return‑style system recommended for precise pressure control.

Cooling and Oiling

  • Oil cooler: A 19‑row or 25‑row cooler with a thermostat maintains oil temps below 220°F during hard driving.
  • Improved oil pan: Killer B or IAG baffled pans and pickup tubes prevent starvation during high‑G corners.
  • Water pump: Consider a Koyorad or Mishimoto radiator and an upgraded water pump to manage extra heat.

Engine Management and Tuning

  • Professional tune: A custom dyno tune (or reliable e‑tune) on a platform like Cobb AP, ECUTek, or standalone is non‑negotiable. Aggressive timing can still harm forged pistons if calibration is poor.
  • Safeguards: Wideband oxygen sensor, exhaust gas temperature sensor, and oil pressure gauge are recommended for monitoring.

Break‑In Procedure

Forged pistons require a careful break‑in. The recommended process for Cosworth pistons is:

  1. Start the engine and let it idle until reaching operating temperature; check for leaks.
  2. Drive under varying loads (light throttle, medium throttle) for the first 500 miles, avoiding sustained high RPM or full throttle.
  3. Change oil and oil filter after 500 miles to remove any wear particles.
  4. For the next 500 miles, gradually increase load and RPM; perform a few moderate pulls to seat the rings.
  5. After 1,000 miles, the engine is ready for full boost and high RPM use.

Conclusion

The EJ255 is a capable engine that, when properly built, can deliver impressive power for many thousands of miles. The addition of Cosworth forged pistons directly addresses the ringland failure common on high‑output stock engines, while the STI 11 mm oil pump ensures consistent oil pressure and flow under the increased demands of a built bottom end. Together, they form a proven foundation for reliable long‑term performance in the 400–500 whp range. Enthusiasts who invest in careful assembly, proper tuning, and supporting cooling and fuel systems will find this combination one of the most rewarding and durable for Subaru’s iconic turbo flat‑four.

For further reading, explore the Cosworth piston product page and the STI oil pump part information. Real‑world builds can be studied on forums such as NASIOC Built Motor Discussion and LegacyGT Performance.