IAG 550 EJ25 2.5L Subaru Short Block for WRX, STI, Legacy GT, Forester XT – 550 BHP

Part Details:

Built for high-boost Subaru EJ25 applications, the IAG Performance 550 Short Block is a reinforced bottom-end designed for builds where factory internals are no longer reliable. As boost levels increase and tuning becomes more aggressive, OEM pistons and rods can become a limiting factor—especially in modified WRX, STI, Legacy GT, and Forester XT platforms.

The IAG 550 Short Block addresses these limitations by replacing known factory weak points with upgraded forged internals, creating a dependable foundation for performance-focused Subaru builds targeting up to 550 BHP.

Engine Internals & Construction

At the core of the assembly are IAG-spec H-Beam forged connecting rods secured with ARP2000 rod bolts, paired with ACL or King race bearings. This combination is designed to handle higher RPM operation and increased engine loads while maintaining durability under demanding conditions.

The short block also features IAG-spec JE forged pistons (99.75mm), developed specifically for high-pressure turbocharged applications. Manufactured from 2618-T6 forged aluminum and designed with an asymmetrical skirt, these pistons reduce weight while improving stability, flame travel, and detonation resistance. Heavy-duty wrist pins and Tru-Arc locks complete the piston assembly for added strength.

Reliability Benefits

One of the most common failure points in OEM EJ engines under increased boost is ringland failure. The IAG 550 Short Block significantly improves durability in these conditions by utilizing forged pistons with thicker, reinforced ringlands—making it better suited for higher cylinder pressures and sustained hard driving.

Each short block is assembled using 100% brand-new components, including new EJ25 case halves and a nitride-treated STI crankshaft, ensuring consistent tolerances and long-term reliability.

Vehicle Fitment Overview

This short block supports a wide range of turbocharged Subaru EJ applications, including WRX, STI, Legacy GT, and Forester XT models. Full year-by-year compatibility is listed in the Vehicle Fitment tab above.

This is a performance short block, not a stock replacement. Vehicle configuration, cylinder head selection, and build goals should always be verified prior to purchase.

What’s Included

• Brand-new Subaru OEM EJ25 case halves (99.75mm stock bore)
• IAG-spec JE 99.75mm forged pistons with ring set, chromoly wrist pins, and clips
• IAG-spec H-Beam forged connecting rods with ARP2000 rod bolts
• ACL or King race rod and main bearings
• New Subaru nitride-treated STI crankshaft (2017+)
• OEM Subaru case and main bolts

Compression Ratios

• 9.3:1 – EJ255 (D heads) with OEM head gasket
• 8.8:1 – EJ257 (B, V, W heads) with OEM head gasket
• 9.4:1 – S20 heads (no combustion mod) with OEM head gasket
• 9.0:1 – EJ255 (D heads) with .039″ JE ProSeal head gasket
• 8.5:1 – EJ257 (B, V, W heads) with .039″ JE ProSeal head gasket
• 9.1:1 – S20 heads (no combustion mod) with .039″ JE ProSeal head gasket

Installation & Build Notes

• Professional installation is strongly recommended
• ECU tuning is required
• Supporting modifications (fueling, turbo, cooling, oiling) are necessary to achieve higher power safely
• Final power capability depends on the complete build and tuning strategy

Recommended Supporting Upgrades

• Air / Oil Separator (AOS) – helps reduce oil vapor ingestion and protect the intake system
• Oil pickup and baffle upgrades – improves oil control and reduces bearing failure risk
• Long block installation kits – simplifies installation with required gaskets and hardware
• Engine installation fluid kits – includes break-in oil and coolant for first startup

Performance Airflow Upgrades

• Corsa Veloce Stradale Rear-Facing Intake Manifold– increases airflow and supports high-horsepower EJ engine builds

Learn more about IAG Performance engine and reliability solutions