Subaru EJ25 Tuning

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

The Subaru EJ25 are awesome to work on and with carefully chosen performance upgrades like ECU maps, turbo kits and camshafts you will substantially maximise your driving pleasure.

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

History, Power & Specs of the Engine


  • 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)


  • Legacy/Outback 00 - 02 (5MT only)


  • 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


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


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.


  • 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.

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.

Altering your EJ25 camshaft will make a dramatic difference to the engine engines power. Choosing a higher performance camshaft profile raises the engines power accordingly.

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.

Race cams, increase the high end rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

On a daily driver must carefully try to match your engines power to your driving style.

I would be surprised if you have ever thought or claimed that a EJ25 Competition cam is a pleasure to live with when driving around busy urban areas. This is because a competition cam causes a very lumpy idle, and makes the car more prone to stall or jerk along in stop start traffic, sadly though many ignore this and end up ruining a perfectly good car and having to revert back to a fast road, or OEM cam profile.

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.

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Best Engine Mods for your car

  1. Mapping - 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. Head work - The goals of porting and flowing the head are to get air flowing into the engine while removing flow restrictions and turbulence.

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.

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 and you can expect to see around 15% on NA (naturally aspirated) engines, but the end result often rely on the tuning mods you've done and the condition of your engine.

It is vital to any tuning job to feed air into the EJ25 engine

The intake plenum carry the air from the air filter and allow it to be sucked into the engine and mixed with fuel.

Design and flow characteristics of the Intake manifolds can make a big change to fuel atomisation on the EJ25.

We often see intake manifolds are ripe for aftermarket tuning parts, although a few car makers provide decently flowing intake manifolds.

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.

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.

There are weak spots for every engine, with some being incredibly solid and some just sufficiently able to handle stock power

Discover these limits and fit more solid crank and pistons to cope with the power.

We've seen guys spending a lots of money on turbocharger upgrades on the EJ25 only to experience the motor explode soon after it's completed.

Big upgraded turbos often suffer a bottom end lag, and low capacity turbos spool up quickly 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.


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.

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 to the type of fuel your car uses as well.

All the following flywheel power targets will assume an injector duty cycle of 80% and a base of 58psi of fuel pressure at idle.

4 Cylinder turbocharged engines

  • 58 PSI 340cc/min 200hp
  • 58 PSI 511cc/min 300hp
  • 58 PSI 682cc/min 400hp
  • 58 PSI 1022cc/min 600hp

4 Cylinder NA (naturally aspirated) engines

  • 58 PSI 285cc/min 200hp
  • 58 PSI 426cc/min 300hp
  • 58 PSI 568cc/min 400hp
  • 58 PSI 853cc/min 600hp


You only need to uprate your exhaust if your exhaust is creating a restriction.

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. So generally speaking, keep to 1.5 to 2.5 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.

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.

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.

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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.

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