Subaru EJ25 Engine

1. Specifications
2. Overview, problems
3. Performance tuning

Subaru EJ25 engine specs

Subaru EJ25 engine reliability, problems and repair

This engine was produced since 1995 and was the biggest engine of the EJ family. It was widely applied in all main car models .
Subaru EJ25 engine used the aluminum cylinder block with dry sleeves. It is similar to the EJ20 cylinder block, but diameter of cylinders was increased to 99.5 mm, cylinder block deck height remained the same (201 mm). It contained the crankshaft with a piston stroke 79 mm. The rod length remained the same 130.5 mm, and piston compression height decreased to 30.7 mm. Due to all this it was possible to get 2.5 liters capacity.
The first variant of EJ25 Phase 1 was called EJ25D and it used DOHC cylinder heads with 4 valves on the cylinder. The timing system uses timing belt. The timing belt replacement is necessary for each 50,000-60,000 miles of mileage (90,000-100,000 km).

The EJ25D power is 155 HP at 5,600 rpm. In 1997 pistons were modified, and power increased to 165 HP.
This motor was used for Subaru till 1998, then it was replaced by EJ251 engine.

EJ251 belongs to Phase 2 and it used new SOHC heads, one camshaft for each. Also new moly coated pistons were used, and compression ration was increased to 10.1.

In the State of California you can find EJ252 engines, meeting the advanced environmental standards. EJ252 differs in intake system, throttle body and the placement of idle air control valve and MAP sensor.

Since 1999 Subaru started producing EJ253 engine. It differs in MAF sensor, and also intake manifold which uses Tumble Generator Valves. It improved its ecological index.
Since 2006, they began to apply variable valve lift system i-AVLS, on the intake side.
In 2009 this motor was slightly modified: it received lightweight pistons, changed intake ports, new plastic intake manifold, new spark plugs, improved i-AVLS system and lightweight exhaust system.
Now we will examine engines with DOHC heads.

In 1998 they started production of EJ254 engine which used DOHC heads. It is a direct successor of EJ25D and refers to Phase 2. EJ254 motor used variable valve timing system AVCS on intake camshafts.

Subaru EJ255 engine was applied for Forester, Impreza WRX and Legacy (2004-2005). This turbocharged motor had semi-closed deck block, DOHC heads and variable valve timing system AVCS on intake cams. The compression ratio was reduced to 8.4 on EJ255 and TD04L turbocharger was applied. Maximum boost pressure is 11.5 psi (0.8 bar). Due to all this, Subaru EJ255 shows 210 HP at 5,600 rpm.
Also there is EJ255 version with the increased intercooler and with maximum boost pressure 13.5 psi (0.93 bar). It allowed to receive the power 230 HP at 5,600 rpm.
VF41 turbocharger is applied to EJ255 which is installed on JDM Forester STI. Subaru Impreza WRX III has VF52 turbocharger, with the maximum boost pressure 13.3 psi (0.92 bar).
Subaru Legacy GT (till 2009) had VF46 turbocharger. Its maximum boost pressure 13.8 psi (0.95 bar) that gave the chance to get 250 HP at 6,000 rpm, and torque 339 Nm at 3,600 rpm.
2009+ Legacy GT had VF45 turbocharger, its boost pressure was 12.5 psi (0.87 bar). Such Legacy GT developed 265 HP at 5,600 rpm, and engine torque 350 Nm at 2,400 rpm.

Subaru Impreza WRX STI had EJ257 engine with semi-closed deck block. The main differences between EJ255 and EJ257 that EJ257 uses new low compression pistons (compression ratio is 8.0) modified heads with changed combustion chambers and with AVCS system. This engine uses IHI VF48 turbocharger, its maximum boost pressure is 14.5 psi (1 bar). This is enough to get 280 HP at 5,600 rpm.
2007+ Subarus Impreza WRX STI III received changed EJ257 engine: compression ratio is increased to 8.2, and it also uses variable valve timing system Dual AVCS on intake and exhaust camshafts.
Because of this, power increased to 300 HP at 6,000 rpm, and torque 407 Nm at 4,000 rpm.
USDM Impreza WRX STI used IHI VF39 turbocharger, and boost pressure is 14.5 psi (1 bar).

EJ25 firing order is 1-3-2-4.
Except well- known EJ20 and EJ25, the Subaru EJ family also includes: EJ15, EJ16, EJ18 and EJ22.
In 2011 naturally aspirated EJ25 was replaced with FB25, and turbocharged EJ25 with FA20DIT.

Subaru EJ25 engine problems and malfunctions

The main problems of EJ25 are high oil consumption, oil leak, knocking. EJ20 engine has similar problems. HERE you can learn in details about them their reliability.
Besides, due to the increased diameter of cylinders, sidewalls between cylinders became thinner. It may cause EJ25 overheating. If it happens, then there is a high risk that heads of the cylinder block will be deformed and in its turn it will cause leaks through the head gaskets.
Also, having EJ257 and EJ255, you may face connecting rod bearing failure, especially after power increase.

Subaru EJ25 engine tuning

Tuning guide

Probably, you want to know more about how to turbo EJ25? There is no sense to waste time for improvement of naturally aspirated EJ25, you will just waste your money. It is even more useless to build NA EJ25. Sell it and buy turbocharged EJ255 (WRX) or EJ257 (WRX STI).
To increase turbocharged EJ25 power, you need to buy downpipe and to adjust ECU more aggressively. It will give additional 30-40 horsepower.
If you are the owner of EJ255, then you need some modifications such as aluminum 2-row Mishimoto radiator, FMIC, EJ257 intake system, BOV valve, WRX STI oil pump, new spark plugs, WRX STI injectors, Walbro 255 lph fuel pump, ACL bearings, the boost controller, 3″ performance exhaust system. But all this makes no sense if you use stock turbocharger, therefore you should buy IHI VF48.
Such engine will show 300+ horsepower.
Your Subaru WRX STI with EJ257 engine seems to be slow? Then let’s make it go faster. For building EJ257, you need to buy FMIC, HKS BOV, Cosworth fuel rail, 800 cc fuel injectors, heavy duty timing belt, cold air intake, 12 mm oil pump, oil cooler, NGK iridium 8 spark plugs, equal length headers and 3″ performance exhaust system. You also need TD05-18G turbocharger and after installation of all these performance parts, you need the ECU control. All this will allow to receive 350+ horsepower.
To increase power even more, you will need to buy additionally to the above mentioned STi Group N engine mount set, a 3-row Mishimoto radiator, sump baffle plate, 1000 cc fuel injectors, Garrett GTX 3076R turbocharger, wastegate tial 44 mm, Aeromotive fuel regulator, fuel lines. Still it is necessary to buy H-beam rods, forged pistons, ACL rod and main bearings, 278/274 cams, Manley valves, valve spring and retainer kits, Cometic MLS head gaskets, ARP cylinder head stud kit. It will be very good if you make port and polish for heads and remove TGV valves in intake manifold.
Using these aftermarket parts and ECU Ecutek, you receive 450-500 horsepower and it will be fast low 11 sec 1/4 mile car.

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