Subaru EJ25 Tuning

"All you need to know about performance parts and tuning the Subaru EJ25 engine!"

In this article, we delve into the heart of Subaru's performance legacy – the revered EJ25 engine. As the powerhouse behind various Subaru models, the EJ25 has become a canvas for modification, offering a playground for those looking to elevate their driving experience.

At the core of this customization journey lies the EJ25's distinctive 2.5-liter boxer engine. Its horizontally opposed configuration provides not only a unique mechanical symphony but also a foundation for exceptional balance and stability on the road.

Equipped with multi-point fuel injection and available in turbocharged versions, the EJ25 boasts a versatile platform for those seeking both efficiency and exhilarating performance.

From the versatile Subaru Outback to the spirited Impreza and the adventurous Forester, the EJ25 has left its mark across the Subaru lineup. We'll explore which models had the privilege of housing this powerhouse and how its characteristics complemented each vehicle's identity.

The EJ25 engines were 2.5-litre variants and several Subaru models were offered with this engine that included but were not limited to the 2006 to 2014 Subaru WRX, 1998 to 2011 Impreza and 1998 to 2010 Forester.

The production timeline of the EJ25 engine spans over several decades, weaving its way through the late 1990s into the 2010s. Discover the evolution of this iconic engine and the impact it had on Subaru's commitment to performance and reliability.

Our journey takes us into the world of modifications, where enthusiasts explore the untapped potential of the EJ25.

Please use the comments below to help us build this into the most comprehensive EJ25 tuning guide and help other like minded owners get the best from their engine.

This pages aim is review and look at EJ25 tuning and show the optimum upgrades.

History, Power & Specs of the Engine

There were regional variations and different versions of this engine had alternative intake and throttle body layouts which can make swapping parts around a bit of a challenge.

EJ25D

• DOHC (JDM)(1994-1996) - SAE - 157.8 hp (118 kW; 160 PS) at 6000 rpm and 210.8 Nm (155 lbft) of torque at 2800 rpm
• DOHC (USA)(1996) - SAE - 155 hp (116 kW; 157 PS) at 5600 rpm and 190 Nm (140 lbft) of torque at 2800 rpm
• DOHC (JDM)(1996–1998) - SAE - 172.6 hp (129 kW; 175 PS) at 6000 rpm and 230.5 Nm (170 lbft) of torque at 3800 rpm
• DOHC (USA)(1997–1999) - SAE - 165 hp (123 kW; 167 PS) at 5600 rpm and 220 Nm (162 lbft) of torque at 4000 rpm

• USA Impreza 2.5 RS 98
• USALegacy/Outback 96~99
• JDM Legacy/Grand Wagon/Lancaster 94~98
• Forester 97~98

EJ251 165 hp @5600 rpm 167 lbft @4400 rpm

• Impreza 2.5RS, 2.5TS, 2.5OBS 98-03 (US) (excludes 99)
• Forester 99-04 (US)
• Legacy 99-01 (US, 4EAT)
• Legacy 99-04 (US)
• Outback 00-01 (US, 4EAT)
• Outback 02-04 (US)
• Baja 03-05 (US)

EJ252

• Legacy/Outback 00 - 02 (5MT only)

EJ253

The intake valves used variable valve timing from 2006 onwards which led to higher power outputs.

• 162 hp at 5600 rpm, 226 Nm (167 lbft) torque at 4400 rpm
• 175 hp at 5600 rpm, 229 Nm (169 lbft) torque at 4400 rpm
•  173 hp at 5600 rpm, 225 Nm (166 lbft) torque at 4000 rpm
• Impreza 99 04-11
• Legacy, Outback (North America) 05-12
• Legacy [BL/BP] 03-09 (Europe)
• Legacy [BM/BR] 09-12 (Europe, with 123 kW (165 hp; 167 PS))
• Outback 03-09 (Europe)
• Forester 99 (SF), 05-10 (SG, SH)
• Baja 05+
• Saab 9-2x Linear 05, 06

EJ254

Active Valve Control debuted on this engine but was primarily added for low end torque and fuel economy.

Variable Valve Timing (VVT):

AVCS is a type of Variable Valve Timing system, which means it can adjust the timing of the opening and closing of the engine's intake and exhaust valves.

Variable valve timing helps optimize engine performance under different operating conditions.

How AVCS Works:

AVCS adjusts the camshaft position, allowing for dynamic control of valve timing based on factors such as engine speed, load, and throttle position.

The system uses oil pressure to move the camshaft, altering the valve timing for improved efficiency across a range of driving conditions.

• Forester T25 '98-'02(JDM)
• Forester 04-present (certain countries)
• Legacy Lancaster '98-'03 (JDM)
• Legacy 250T '98-'03 (JDM)

EJ255

This is a turbo engine and was often fitted with super cool sodium filled valves. Later versions had Active Valve Control System AVCS on intake & exhaust and improved oil circulation.

North America:

• Impreza (WRX): '06–'14
• Forester XT: '04–'13
• Legacy GT: '05–'12
• Outback XT: '05-'09
• Baja Turbo: '04 - '06
• SAAB 9-2X: '06 only.

Rest of world:

• Legacy/Outback: '07–present
• Impreza: '05–present
• Forester: '05–'10.

EJ257

From 2007 these had improve cylinder heads and better optimized Dual AVCS. The 2019 revision boasts stronger pistons and heavily revised ECU tune & valvetrain and are considered by many to be the best incarnation of the EJ25.

• US  Impreza WRX STi MY 2004~2007
  300 bhp (304 PS; 224 kW) at 6000 rpm (New SAE standard) and 407 Nm (300 lbft) at 4000 rpm of torque.
• US Impreza WRX STI MY 2008~2018
  305 bhp (309 PS; 227 kW) at 6000 rpm (New SAE standard) and 393 Nm (290 lbft) at 4000 rpm of torque.
• US WRX STI MY MY 2019–current
  310 hp (314 PS; 231 kW) at 6000 rpm (New SAE standard) and 290 lbft (393 Nm) at 4000-5200 rpm of torque
• US STI S209 MY 2019
  341 hp (346 PS; 254 kW) at 6,400 rpm and 330 lbft (450 Nm) at 3,600 rpm of torque.
• US Legacy GT/Outback XT MY05-06
  250 bhp (253 PS; 186 kW) (New SAE standard)
• USForester XT MY04-05
  210 bhp (213 PS; 157 kW)
• Asian, European Impreza WRX STi 05-present
  280 PS (206 kW; 276 hp), 40 kg⋅m (392 Nm; 289 lbft)

Tuning the Subaru EJ25 and best EJ25 performance parts.

Best EJ25 parts

Just because particular mods are are common on EJ25 it doesn't mean you should fit it, we shall concentrate on the top mods that will give your EJ25 the biggest power gain return for your cash.

The naturally aspirated EJ25 engines can be tuned & upgrades, but you'll see much better gains on the turbo units, which have better designed and stronger parts already fitted. The difference is not just the turbo and tune, so going with a turbo upgrade can prove complex compared with the relative simplicity of doing a swap.

Instead of us falling into the common "if it's shiny and makes more noise it must be good" mentality of many car sites and tuning magazines let's look at each mod in detail and pass on the tips we've had already on this great engine.

EJ25 Performance Camshafts

Camshaft upgrades stand out as a key modification to unlock hidden power potential but is a complex mod and requires careful planning and understanding.

While the EJ25 engine, renowned for its boxer design, already boasts impressive capabilities, optimizing the camshaft can further enhance performance, especially for models without variable valve timing (VVT) like the AVCS (Active Valve Control System).

The camshaft plays a pivotal role in managing the opening and closing of the engine's intake and exhaust valves. Upgrading the camshaft involves altering the cam profile, determining when the valves open and close, and the duration of their operation.

For EJ25 engines without VVT, the fixed camshaft design can be fine-tuned to maximize top-end power, making it a sought-after modification for performance enthusiasts.

EJ25 engines lacking variable valve timing, such as the AVCS, can particularly benefit from camshaft upgrades focused on improving top-end power.

By optimizing the cam profile, these upgrades aim to enhance airflow at higher RPMs, where fixed camshafts traditionally face limitations. The result is a broader power band, allowing drivers to tap into increased horsepower and torque at higher engine speeds.

The modification is especially advantageous for those who enjoy spirited driving and pushing their Subaru to its limits.

The adjusted camshaft profile helps the engine breathe more efficiently at higher RPMs, delivering a noticeable surge in top-end power. This can be particularly advantageous on the track or during aggressive driving scenarios where access to higher RPM ranges becomes crucial.

It's essential to approach camshaft upgrades with a comprehensive understanding of the engine's characteristics and the desired performance goals.

Working with reputable tuners and ensuring compatibility with other engine modifications are key factors for a successful upgrade. Additionally, proper engine tuning post-installation is crucial to optimizing the new camshaft profile for the best results.

Performance or "Fast road" cams tend to increase the bhp throughout the rpm band, you may sacrifice a little bottom end bhp but the high end rpm power will be higher.

These work well at 4000-6000rpms where you want the power to be peaking.

Different EJ25 engines respond better to different cam durations so set your engine up on a rolling road.

The map and injectors and fuel pump also will make differences on the bhp gains you'll make.

Longer valve durations can alter the bhp band and on most engines the exhaust and intake durations do not need to match, although most cams and tuners use matched pairs there are some advantages to extending the intake or exhaust durations.

Please watch our video which covers the 5 principles of tuning your car. Be sure to keep up with our latest YouTube content and subscribe.

Best Engine Mods for your EJ25

1. Engine Tunes - engine tuning/remapping provides the most advantage in terms of cost savings,  aftermarket ECUs, and piggyback ECUs are all alternatives.
2. Fast road cams are one of the most significant mechanical changes, but they must be installed by someone who knows what they're doing and they are not always easy to source but you might find a local firm to regrind a stock camshaft.
3. Intake and Exhaust - Note that on their own these mods will NOT ADD POWER in most cases, but they can help enhance power after other mods by removing the restriction.
4. Upgrades to turbochargers and superchargers - forced induction is the most efficient approach to increase air supply, allowing you to burn more fuel and make more power. It is one of the most costly upgrades but provides the best gains.
5. Cooling upgrades - radiator & intercooler
6. Head work - The goals of porting and flowing the head are to get air flowing into the engine while removing flow restrictions and turbulence.
7. Internal engine mods - forged components (stronger rods, pins, pistons, crank etc) will retain reliability at higher power figures.

EJ25 Tuning stages

Typical stage 1 mods often include: Intake manifolds, drilled & smoothed airbox, Remaps/piggy back ECU, Sports exhaust header/manifold, Fast road camshaft, Panel air filters.

Typical stage 2 mods often include: fuel pump upgrades, Fast road cam, Ported and polished head, high flow fuel injectors, Sports catalyst & performance exhaust, induction kit.

Typical stage 3 mods often include: Twin charging conversions, Adding or Upgrading forced induction (turbo/supercharger), Internal engine upgrades (head flowing porting/bigger valves), Competition cam, Crank and Piston upgrades to alter compression, Engine balancing & blueprinting.

The EJ25 engines respond well to upgrades and we note that there are plenty of parts and performance parts around.

Conrods & Bearings are worth upgrading for larger power gains as these tend to be the weak spot on the EJ25. 400hp is generally the upper limit for most stock EJ25 blocks, but obviously the parts used varied between applications.

EJ25 Tunes & ECU Remaps

ECU mapping will help fully realize the full potential of all the tuning mods you've fitted to your EJ25.

(In some cases, as the factory ECU is locked flashing is not an option, so an aftermarket ECU is the route to take, and many of these will outperform factory ECU's but make sure it has knock protection and that you get it setup properly.)

It will usually give you around 30% more power on turbocharged vehicles, but the end result often rely on the tuning mods you've done and the condition of your engine.

One of the most common and cost-effective mods that are often performed by owners of cars with these engines is remapping or engine tuning.

Remapping not just gives the best results by ensuring power delivery in the desired RPM range but in some cases it also improves the fuel efficiency of the vehicle.

With remapping, you can expect a power increase of around 30hp whereas torque increments by 60Nm.

EJ25 Air Intake Improvements

Fitting big valve kits, doing a bit of port work and head flowing will also increase torque, and more importantly will raise potential for an improved torque increase on other tuning mods.

Most will focus on the intake and air filter, relatively low cost and easy to install, so are these good mods & upgrades on the EJ25 engine?

Air Intake Systems

An air intake system is one of the most commonly installed performance mods on any engine.

The same is the case with the EJ25 and that’s why no list of performance mods can be complete without mentioning the air intake systems.

Are Air Intake Systems On Stock Cars Worth The Cost?

In the case of a stock vehicle, the restriction in the air intake is negligible and thus the result of fitting an air intake system is also not that prominent. Most report around 5% top end power gain but a little less low end torque, it will vary if you have a turbo engine or not.

But if you have performed other mods on your car, and are now extracting much more power as a result, there might well be a restriction in the air intake system that did not exist in the first place.

So, in such a case it makes a lot of sense to fit an aftermarket air intake system to get higher power figures by getting rid of the restriction. You can't beat the induction roar on this lovely 4 cylinder engine.

Cobb Redline Carbon Fiber Intake is a commonly installed air intake option for vehicles with the EJ25 engine and it looks so nice on the engine.

Some other renowned names that offer air intake systems include:

• AEM Intakes;
• Mishimoto;
• HPS Silicone Hoses;
• K&N.

Turbo upgrades

NA (naturally aspirated) engines need quite a lot of work when you add a turbo, so we have a separate guide to help you take into account the pros and cons of going this route on your EJ25

The more air you can get into an engine, the more fuel it can burn and uprating the induction with a turbocharger upgrade makes superb power gains.

If an engine is fitted with a turbocharger tuning mods are going to net you a larger power gain and we find turbo charged engines are built with more solid components.

If you are looking for significant power gains, a turbocharger is one of the best performance mods.

So, let’s take a look at some of the options that you have when it comes to fitting a turbocharger to the EJ25 engine.

Upgrade Turbocharger up to 420 hp for Subaru Impreza WRX/WRX STI EJ20 & EJ25;

• IHI PERFORMANCE OEM+ VF48 SUBARU EJ25 TURBO UPGRADE; 300hp region
• MHI Subaru EJ20 / EJ25 Stage 2 Upgrade 49S36-A0000;
• TD05-18G is a good mid power spec turbo for 300-400hp projects
• Gottuned Hybrid Turbocharger (upgrade) TD05-20G Subaru Impreza WRX STI, WRX Turbo, WRXR8.
• Garrett GTX 3076R has seen power figures approaching 500hp

All these turbocharger upgrade options offer considerable power gains. However, it is to be kept in mind that a turbocharger upgrade is one of the, if not the, most expensive mod that you will do on your car.

In most cases you'll also need to upgrade the fuel, and strengthening the block with forged components is strongly recommended.

So, be extra careful while getting turbochargers and ensure that whichever option you go for conforms to your engine’s requirements and is well suited for it.

Big upgraded turbos often suffer a bottom end lag and slower spool up, and low capacity turbos spool up quickly and deliver power at lower RPM ranges, but don't have the peak end torque gains.

In the last 10 years the selection of turbo units is always improving and we are seeing variable vane turbo units, allowing the vane angle is altered according to speed to lower lag and increase top end torque.

Twin scroll turbo units divert the exhaust flow into a couple of channels and flow these at differently designed vanes in the turbo. They also improve the scavenging effect of the engine.

It is not unusual that there's a restriction in the air flow sensor MAP/MAF/AFM on the EJ25 when a lot more air is being drawn into the engine.

Going up you'll find 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor was restricting bhp and torque at a much lower level.

Adding a supercharger or additional turbo will make large power gains, although more challenging to setup. We have this article on twincharging if you want to read more.

EJ25 Fuelling Upgrades

Don't overlook the need to improve the fuel system when you are increasing the performance - it makes the car more thirsty. We would recommend you to be generous with your injectors flow rate.

When you want to get more performance from your EJ25, you need to supply more fuel to the car. The process of combustion requires air and fuel. So, the greater is the quantity of this mixture, the higher is the rate of combustion.

Some of the fuel-related performance mods that are performed by owners of vehicles having the EJ25 engine include:

• High-Pressure Fuel Pump;
• Fuel Pressure Regulator;
• Injector Driver;
• Fuel Injectors.

The rule of thumb is to add 20% to the flow rate when buying an injector, which takes into account injector deterioration and provides a bit of spare capacity should the engine need more fuel.

We think this one is common sense, but you'll need to match your fuel injector & fuel pump to the type of fuel your car uses as well.

High-Pressure Fuel Pump

If you are looking to upgrade the high-pressure fuel pump of your vehicle having the Subaru EJ25 engine, you can get the one manufactured by one of the following:

• Walbro;
• DeatschWerks;
• COBB.

Fuel Pressure Regulator

A fuel pressure regulator as the name gives away regulates the fuel pressure and thus getting this part from a reliable brand is always a good choice.

So, here are a few companies that offer high-quality fuel pressure regulators for vehicles having Subaru EJ25 engines:

• Hybrid Racing;
• DeatschWerks;
• BLOX;
• AEM.
• Bull Boost Performance also offers fuel pressure regulators for the EJ25.

Injector Driver

Hondata offers a high-quality injector driver for this engine but it is an expensive part mainly because of the superior performance that it delivers. Let me know in the comments if you recommend other parts?

Fuel Injectors

The purpose of fuel injectors, as the name implies is to supply enough fuel to the engine so it can produce power.

So, when the fuel requirement increases, you need to get better and higher capacity fuel injectors for your vehicle having the EJ25 engine.

TLF Performance Parts is a well-known brand in this field, and the company offers a wide range of fuel injectors with varying flow rates.

The flow rates are measured in cc and fuel injectors are usually available with the following flow rates so in most cases, choose the next size up when you've found your cars requirements:

• 565cc;
• 600cc;
• 700cc;
• 720cc;
• 900cc;
• 950cc;
• 1000cc.

Typically, depending on other components used, an 800cc injector will support around 350hp and 1000cc will cope with around 500hp.

Radiators

Once you have installed a turbocharger and have performed other associated mods on your vehicle with the EJ25 engine, you will be making much more power than the stock vehicle.

Heat is the primary cause of problems and loss of compression/piston slap and oil leaks so it is sensible in my book to improve the EJ25's ability to cool itself down.

How Good Are Stock Cooling Components?

At this point, it is vital to keep in mind that the factory-fitted cooling components of the vehicle have been fitted according to the requirements of the stock vehicle.

So, when higher power figures are produced, more heat is also generated as a by product.

It is also pertinent to mention that overheating is one of the most common issues associated with modified vehicles and overheating leads to multiple other issues as well.

Upgrading To A Performance Radiator

This necessitates the upgrading of the stock radiator to a high-performance one so that the engine does not overheat.

A few of the manufacturer options related to high-performance radiators include:

• M2 Motorsport;
• Mishimoto;
• TYC;
• Koyorad.

Exhaust

You only need to uprate your exhaust if your exhaust is creating a restriction. This is typically around the headers and catalyst so catback exhausts do very little in the grand scheme of things.

On most factory exhausts you'll see the exhaust flow rate is still fine even on modest power gains, but when you start pushing up the power levels you will need to get a better flowing exhaust.

Sports exhausts generally help improve air flow from the engine but do not go too large or you could very well end up with a reduced flow rate and damage the engine scavenging. So generally speaking, keep to around 3 inches as a rule of thumb.

Typically exhaust restrictions are traced to the catalyst installed, so adding a better flowing performance catalyst removes the restriction. We note that performance cats perform similarly to decats and have the added benefit of keeping your car street legal, as decats or catalyst removal is illegal in most territories for road going cars.

Intercooler

Now that you have performed several other mods on your car and are getting much higher power, you would like to have this power for an extended period.

However, a major issue is that the engine needs cold dense oxygen-laden air for this purpose which is not generally available and for this reason, an intercooler is added to the vehicle.

It is to be kept in mind that an intercooler does not add power on its own, rather it just allows the engine to keep operating optimally for an elongated period.

Know Your Car’s Requirements

Another factor that you must consider at this point is whether or not the large intercooler you are fitting is ideal for your car’s dynamics.

At times, a large intercooler adds drag to the car and thus instead of making the situation better, starts deteriorating it.

What Is Heat Soak & Why Does It Happen?

Besides this, choosing the ideal intercooler size is also vital to prevent heat soak from happening.

So, heat soak is a situation in which the car’s intercooler itself becomes so hot that it fails to cool the air. When this happens, the car is no longer able to deliver higher power for long durations.

Some of the intercooler options that are often chosen by owners of vehicles having the EJ25 engines include but are not limited to:

• Grimmspeed;
• Mishimoto.

Weak spots Issues & problem areas on the EJ25

The EJ25 engines are generally reliable and solid units, as long as you follow the manufacturers service schedules, and use a good quality oil to ensure longevity. Few problems should happen as long as they are regularly serviced and maintained.

Carbon build up in the head, particularly around the valves which will sap power or create flat spots, this is a larger issue on direct injection engines but should be looked out for on all engines. We have tips on removing carbon build up.

Turbo Issues

The oil filters, which clog over time, are the primary cause of the turbo issues. When this occurs, the oil begins to bypass the filter and, because it has not been filtered, it contains several impurities that accumulate in the banjo bolt.

This limits the oil flow to the turbocharger, which is when it begins to suffer.

How To Avoid This Problem?

Simply installing a larger oil filter that is suitable for the EJ25 engine and replacing the banjo bolt, if necessary, will help to prevent a turbocharger problem.

What Are The Symptoms?

A louder turbo whine and smoke coming from the exhaust are signs that your EJ25 turbo may be malfunctioning.

Valve Cover Gasket Oil Leaks

As the mileage on these engines increases, the valve cover gaskets degrade which can lead to oil leakage.

What Are The Symptoms?

Symptoms that indicate that the gasket is causing an issue include the smell of burning oil, visible oil leaks and smoke emitting from the engine bay.

Ringland & Piston Problems

Piston problems arise when cracks develop on the ringlands (areas of the piston which are adjacent to the rings and ring grooves). So, when these cracks expand, it starts affecting the compression on the cylinders.

The cooling on the EJ25 is not uniform and typically the end (4th) cylinder takes most of the heat. This expansion and heat fatigue really ruins the engine.

What Are The Symptoms?

Symptoms of this issue include compression and power loss, ticking noise from the engine as well as smoke emission from the car exhaust.

Some of our members have had issues with flat spots or glitches after applying mods and upgrades or tuning, this is not usually related to this engines design, so instead see our article on diagnosing flat spots and problems after tuning which should help you get the bottom of this issue.

Regular oil changes are vital on the EJ25, especially when tuned and will help extend the life and reliability of the engine.

If you would like to know more, or just get some friendly advice on Tuning your EJ25 engine please join us in our friendly forum where you can discuss tuning options in more detail with our worldwide members with the EJ25 engine, or read our ej25 tuning articles to get a full grasp of the benefits and drawbacks of each modification.

Please help us improve these tips by sending us your feedback in the comments box below.

We love to hear what our visitors have got up to and which tuning parts work best for you on your car. Which helps us keep our guides and tips up to date helping others with their modified car projects. Your feedback and comments are used to keep this page up to date, and help improve the accuracy of these EJ25 tuning guides which get regular updates and revisions.

This article is flagged for an update, so please pass on specific tips below that I can include. All articles, and content was written by me, Waynne Smith TorqueCars founder. Some outlines are just too general so I appreciate your feedback and suggestions. This entry was filed under Subaru. You can leave a response below or join our forum to discuss this article and car modification in detail with our members.

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