Subaru EJ25: Everything You Need To Know

They were brought to us by the automotive company that once a division of Fuji Heavy Industries became internationally recognized for their boxer engines. Subaru made their way onto the streets introducing EJ engines.

In the succeeding years of its development, EJ engines became the mainstay engine of Subaru on most of its vehicles.

And today, we will talk about the largest engine in the EJ family, the EJ25 engine.

Join me to discuss the engine’s design, applications, issues, reliability, and many more. Let’s get right to it!

What are Subaru EJ25 Engines?

The Subaru EJ25 is a 2.5 Liter engine manufactured by Subaru in Gunma Oizumi Plant and one of the members of the EJ series. It is the most widely used and largest engine among other EJ family members – EJ15, EJ16, EJ18, EJ20, and EJ22, having a displacement of 2.5 Liters.

If you’re confused about the jargon or engine codes, the first two letters are the engine family, and the succeeding numbers are the engine’s displacement. For this case, EJ family and 2.5 Liter displacement.

Anyway, to proceed, EJ25 is a flat-four boxer engine that started its production in 1989, but Subaru materialized it in the middle of 1996 with the naturally-aspirated version having been launched for the Subaru Legacy here in the United States.

This EJ25 engine that we encounter today started becoming famous since the first Impreza WRX STI (GDB Chassis) was sold in the United States came with these engines in the mid-2000s.

Up until this day, EJ25 engines are commonly swapped and tuned due to their potential and strong internals.

Engine Specifications and Design

  • Production Run: 1996 – Present
  • Cylinder Head Material: Aluminum
  • Cylinder Block Material: Aluminum
  • Configuration: Flat-4 (opposed)
  • Bore: 99.5 mm
  • Stroke: 79 mm
  • Valvetrain: SOHC two valves per cylinder and DOHC four valves per cylinder
  • Displacement: 2.5 L (2457 cc)
  • Compression Ratio(s): 9.5 (EJ25D 1996), 9.7 (EJ25D 1997 – 1999), and 10.1
  • Weight: 500 lbs.
  • Maximum HP: 341 HP at 6,400 RPM
  • Maximum Torque: 330 lb-ft at 3,600 RPM

The EJ25 engine used the same aluminum cylinder block with dry cast-iron sleeves applied to the EJ20 engine.T

he difference is evident between the EJ20 and EJ25 since the latter has a bigger cylinder bore from 92 mm to 99.5 mm, a longer piston stroke from 75 mm to 79 mm, and a lower compression height from 30.7 to 32. 7 mm.

Connecting rods length stands the same at 130.5 mm as well as the cylinder block deck height.


The earliest and first generation of EJ25 engine, EJ25D, has two variations sold in the United States market. It has DOHC aluminum heads with four valves per cylinder, two for both intake and exhaust valves.

The first-generation EJ25Ds were introduced in 1996 as part of the Legacy 2.5 GT, Legacy Outback, and LSi. This version used HLA heads has lower power and torque ratings than the later 1997 – 1999 EJ25D; it was recommended to run only on 91 octane fuel and was only made available with an automatic transmission.

In 1997, the first generation EJ25D was revised, and Subaru introduced the use of shimmed bucket lifters for the heads instead of the HLAs. The second-gen was designed to run on a lower 87 octane fuel and offered in either automatic or manual transmission.

Due to its DOHC valve design, the spark plugs are much more difficult to service than the SOHC variant. Therefore, the DOHC engines were installed with platinum spark plugs extending the spark plug’s service life of 80,000 miles.

The EJ25D uses smaller rod journals because of the adjustments made on the phase 1 EJ25 offset grind crankshaft design, compared to the EJ22 of the same year and the machines that use a rod journal.

To add, the EJ25D from the 1999 Outback is a factory hybrid made available for a year only, which uses the phase 2 case halves with an 8-bolt bell housing, contrary to the 4-bolt bell housing used until 1998.

It also uses a phase 2 crankshaft and connecting rods with rod journals but did not change the piston as the 1997 – 1998 variant to have the same compression rating.

The 1996 EJ25D pistons differ from those in 1997 – 1999, increasing the compression rating significantly if mated with the 1997 – 1999 EJ25D heads.

The EJ25D engines were advertised in the Japanese Domestic Market from January 1994 and made available from October 1994 in the 250T legacy and later on the Grand Wagon.

It is the same basic design used in the US market hydraulic EJ25D, with HLA, but has a compression rating of 9.5. These HLA heads are reminiscent of the Hemi-style combustion chamber design.

The engine was then facelifted as the BG9B was introduced in mid-1996 to have solid lifters and a higher compression rating of 10.7. The solid lifter head has a cloverleaf-style combustion chamber.

The power outputs of each EJ25D engine based on the model year:

  • 1994 – 1996 DOHC JDM 158 HP at 6,000 RPM and 155 lb-ft of torque at 2,800 RPM
  • 1996 DOHC USDM 155 HP at 5,600 RPM and 140 lb-ft of torque at 2,800 RPM
  • 1996 – 1998 DOHC JDM 173 HP at 6,000 RPM and 170 lb-ft of torque at 3,800 RPM
  • 1997 – 1999 DOHC USDM 165 HP at 5,600 RPM and 162 lb-ft of torque at 4,000 RPM

Applications of EJ25D Engine:

  • 1998 USDM Impreza 2.5 RS
  • 1998 USDM Forester
  • 1998 – 1999 Legacy Sedan
  • 1996 – 1997 Legacy GT
  • 1994 – 1998 JDM Legacy/Lancaster/Grand Wagon


Subaru developed a long list of SOHC-designed engines for the United States market, and the first version to come out from that franchise is the EJ251. This engine produces 165 HP at 5,600 RPM and 167 lb-ft of torque at 4,400 RPM.

Unfortunately, the EJ251 was soon replaced by the EJ253 in many models due to improved engine management, better cooling to aid in head gasket life, and sensors.

There are reports that EJ251 engines experience head gasket failures resulting in exterior fluid leaks or interior channel breaches due to their usage of a single layer coated gasket, which was first introduced in the early EJ25D.

The volume in the EJ251 is calculated via a MAP sensor, while the EJ253 uses a MAF sensor. The compression rating is 10.1.

Applications of EJ251 engine:

  • 1998 – 2003 except 1999 Impreza 2.5 RS
  • 1999 – 2004 USDM Forester
  • 1999 – 2001 USDM Legacy 4EAT
  • 1999 – 2004 USDM Legacy
  • 2000 – 2001 USDM 4EAT
  • 2002 – 2004 USDM Outback
  • 2003 – 2005 USDM Baja


The EJ252 engine is worthwhile, just like the EJ251 in the USDM Legacy Outback for the 2000 and 2001 model years.

They commonly appear in the Model Year 2000 Legacy Outbacks produced before the end of 199, while the Model Year 2000 Legacy Outbacks manufactured in 2000 rarely have the EJ252 code designation in the VIN.

Subaru did not issue a direct list of revisions between the two engines, EJ251 and the lesser-known EJ252. Some information suggests that the EJ252 engine is an alternate version of the EJ251, which was only made to meet California Emission Standards when these engines arrived in North America.

The power output of EJ252 was rated at 156 HP but is often contested, and some asserted that it holds the exact specification with the identical EJ251. In addition, the EJ252 shares the same cams, blocks, heads, pistons, connecting rods with the EJ252.

Their only differences are the unique intake manifold and throttle body designs to cater to the different IACV and MAP sensor locations. They also have different crank sprocket reluctor configurations and cam.


This engine was rated at 162 HP at 5,600 RPM and 167 lb-ft of torque at 4,400 RPM. Unlike the EJ251, the one it replaced, the EJ253 intake volume is measured using a MAF sensor. I-Active valves (VVL intake side) on 2006 models have 175 HP rating at 5,600 RPM and 169 lb-ft of torque available at 4,400 RPM.

PZEV-equipped 2007 and succeeding models have ISO 173 HP at 5,600 RPM and 166 lb-ft of torque at 4,000 RPM. The compression rating is 10.0.

Applications of EJ253 Engine:

  • 1999, 2004 – 2011 Impreza
  • 2005 – 2012 USDM Legacy, Outback
  • 2003 – 2009 Europe Legacy BL/BP
  • 2009 – 2012 Legacy Europe Variant 165 HP
  • 2003 – 2009 Europe Outback
  • 2005 – 2010 SG, SH Forester
  • 2005 Baja
  • 2005 – 2006 Saab 9-2x Linear


The EJ254 was the 2.5 liter DOHC AVCS (Active Valve Control System). This is the first appearance of variable valve timing AVCS on an EJ engine alongside the EJ204.

The 1998 – 2004 DOHC was rated 165 HP at 6,000 RPM and 173 lb-ft of torque at 2,800 RPM.

Applications of EJ254 Engine:

  • 1998 – 2002 JDM Forester T25
  • 2004 – Present Forester
  • 1998 – 2003 JDM Legacy Lancaster
  • 1998 – 2003 JDM Legacy 250T


There are three versions for the EJ255 engine.

But generally, EJ255 engines are DOHC 16-valve turbo with sodium-filled valves initially designed for the North American market, which were also used now in some European Impreza and Legacy models heading to South Africa as well as Liberty models for Australia.

The power production was rated at 261 HP at maximum.

The earliest EJ255 version was used in the 2005 and 2006 Legacy and the 2004 and 2005 Forester. This engine uses the short block and heads as the EJ257 in USDM 2004 – 2006 STI.

The next version is equipped with AIS, which uses a slightly different AB630 short block and AB820 heads. The only difference between the EJ255 and 2004 – 2006 EJ257 block is their pistons. Everything else is the same.

The latest version has dual AVCS, 9.5 compression rating pistons, and E25 heads with storage for a scavenge oil pump turbo. The oil pan was added with an additional mounting bolt, and one of the oil pan bolts has a change position compare to other EJ255 versions.

Applications of EJ255 engine:

North America:

  • 2006 – 2014 WRX Impreza
  • 2004 – 2013 Forester XT
  • 2005 – 2012 Legacy GT
  • 2005 – 2009 Outback XT
  • 2004 – 2006 Baja Turbo
  • 2006 SAAB 9-2X

Other Countries:

  • 2007 – Present Legacy
  • 2005 – Present Impreza
  • 2005 – 2010 Forester


This engine version is a DOHC with four valves per cylinder. Fuel feed through a Sequential Multipoint Fuel Injection turbo (SMPFI). EJ257 is initially designed for the North American 2004 Impreza STI with single AVCS and Drive by Wire.

The 2004 – 2007 STI models used the same block, valvetrain, and B25 heads as the EJ255 in the model year 2005 – 2006 Legacy GT and the same engine block as the 2004 – 2005 Forester XT.

In the following years, EJ255 used a revised block, heads, and pistons.

They also integrated the much improved Dual-AVCS W25 cylinder heads and a better cylinder block design. 2019 EJ255 variants feature a revised piston design for increased rigidity, additional improvements to the valvetrain, new ECU and engine programming.

The engine has a redline of 6,700 RPM.

Applications of EJ257 Engine:

  • 2004 – 2007 USDM Impreza WRX STI 300 HP at 6,000 RPM and 300 lb-ft of torque at 4,000 RPM
  • 2008 – 2019 USDM Impreza WRX STI 305 HP at 6,000 RPM and 290 lb-ft of torque at 4,000 RPM
  • 2010 – Present USDM WRX STI 310 HP at 6,000 RPM and 290 lb-ft of torque at 4,000 – 5,200 RPM
  • 1019 USDM Sti S209 341 HP at 6,400 RPM and 330 lb-ft at 3,600 RPM
  • 2005 – 2006 USDM Legacy GT 250 HP
  • 2004 – 2005 USDM Forester 210 HP
  • 2005 – Present Asian and European Market Impreza WRX STI 276 HP and 289 lb-ft torque

EJ259 appears in 2004 Legacy, Legacy GT, and Legacy Outback which was only sold in New England and California

Engine Tuning, Upgrades, and Modifications

EJ25 engines are good engine swaps as well as upgrades and tunes. However, it is not recommended to improve the naturally aspirated EJ25 because it’s just a waste of time and effort. But you can increase the turbocharged EJ25 engine power by purchasing a downpipe and adjusting the ECU.

It can give you an additional 30 – 40 HP.

If you own an EJ255 engine, you need to modify your machine, such as aluminum two-row Mishimoto radiator, EJ257 intake system, FMIC, Blow-off valves, new spark plugs, WRX STI oil pump, WRX STI injectors, ACL bearings, and Walbro 255 lph fuel pump.

It would be best to replace the stock turbocharger with IHI VF48 or, whatever your choice, buy a boost controller and a 3-inch performance exhaust system. These upgrades can give you a little more than 300 HP.

If you still feel that your EJ257 seems slow, we need to put a TD05-18G turbocharger or the likes of that.

Buy some HKS blow-off valve, FMIC, 800 cc/min fuel injectors, Cosworth fuel rail, heavy-duty timing belt, 12 mm oil pump, oil cooler, cold air intake, NGK iridium spark plugs, 3-inch performance exhaust system, and equal length headers.

After installing these performance parts, you need to configure the ECU, and this will result in a 350 HP gain.

To increase the power, you need to add an STI Group N engine mount set, sump baffle plate 3-row Mishimoto radiator, 1000 cc fuel injectors, 44 mm wastegate tial, Garrett GTX 3076R turbocharger, Aeromotive fuel regulator, and fuel lines.

It is also vital to change your internals to forged ones, H-beam rods, ACL rod and main bearings, new valves, valve springs, retainer kits, ARP cylinder head stud kit, and Cometic MLS head gaskets. Port and polish for the heads and remove the TGV valves in the intake manifold.

After installing these aftermarket parts and configuring ECU using Ecutek, you can gain 450 – 500 HP.

Problems Surrounding EJ25 Engine

EJ25 engines are outstanding and reliable throughout; many regard them as a solid entity of craftsmanship. However good as it may sound, EJ25 machines are not perfect and have weaknesses and issues that might arise later in their service life.

The first issue is EJ25’s high oil consumption. The primary cause of this problem might be coming from a faulty piston ring. High oil consumption can also be a result or derivative of a leakage that you might leave unattended.

You can smell burnt oil or drops if you haven’t noticed them yet.

Engine knocking is also EJ25’s issue as well as the EJ20. Knocking sound comes from the fourth cylinder, which is the hottest but the worst cooled among the cylinders, resulting in a knocking caused by the piston.

At first, it will pop up intermittently, then all the time, which makes it annoying.

Furthermore, due to the larger cylinder diameter of the EJ25 engine, the sidewalls between the cylinder became thinner, leading to a more concentrated heat to that region. If that happens, there is a high chance that the cylinder heads will be deformed, leading to a greater risk for the engine, such as head gasket leaks.

Lastly, if you have EJ257 and EJ255, you might experience connecting rod bearing failure, especially after increasing the power.


The Subaru EJ25’s compact profile and rally-bred type history mix well to add some spice in your everyday driving with a little bit of adrenaline to put you in a grumpy state to an exhilarating one.

Its strength is firmly attached to its responsiveness, great at mid-range torque, and can pick up the pace when needed.

Being a flat-four also gives this engine an edge over other inline fours due to its lower center of gravity and ground clearance which is ideal for cornering, feels more stable and grounded.

However, machines of this caliber have a price to pay, especially in the maintenance department, but it is just giving your engine a therapy for its service.

Overall, this engine is a good city driving experience that adds more flare to the usual Monday morning drive. It does not shy away from an increased power but entices you to reach its potential.

I hope that this simple discussion helped you understand EJ25’s engine design, issues, applications, reliability, and overall impact on the automotive industry.

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