Tuning the Subaru EJ25

"Comprehensive guide to tuning the Subaru EJ25 engine!"

The Subaru EJ25 are awesome to work on and with the optimum sports enhancements like ECU maps, turbo upgrades and camshafts you will dramatically maximise your driving enjoyment.

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

When talking about the best best for your EJ25 engine, we are going to focus on the tuning parts that give the biggest return for your cash.

Significant gains on the EJ25 can be made from cam upgrades. Altering the cam profile alters the intake and exhaust durations on the engine and can dramatically change the engines power and power output.

NB: Fast road camshafts commonly raise the torque across the rev band, you may lose a little low down bhp but higher rpm power will be higher.

Motorsport camshafts, raise the higher rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

A Motorsport cam won't do well if driving in heavy traffic.

You should ideally match your power band to your preferences so for a car used daily stick with a mild fast road EJ25 cam

Each engine responds better to extreme cam durations so set your engine up on a rolling road.

The ECU mapping and injectors and fuel pump also have an effect on the torque gains you'll make.

Altering valve durations can alter the torque 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 TorqueCars Video tutorial on car tuning. And be sure to subscribe and support our new channel.

Stages of Tune

Stage 1 mods: Remaps/piggy back ECU, Drilled & smoothed airbox, Fast road camshaft, Intake headers, Panel air filters, Sports exhaust manifold.

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

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

Remaps helps unlock the full potential of all the modifications you've fitted to your EJ25.

It will usually give around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but your results usually differs on the modifications you've applied and the condition of your engine.

Feeding more air into each cylinder is vital to any engine upgrade task.

The intake plenum take the air during the suck phase from the air cleaner and allow it to be fed into the engine and mixed with fuel.

Shape and flow rate of the Intake headers can make a large improvement to fuel atomisation on the EJ25.

On popular production engines intake headers are in desperate need of a performance upgrade, although a few car makers provide well optimised intake headers.

Increasing the EJ25 valve size, carrying out 3 or 5 angle valve jobs and porting and head flowing will also raise performance, and significantly will raise potential for increasing the performance increase on other parts.

Turbo upgrades

NASP 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.

When the engine is fitted with a turbo tuning parts are going to make more power and you will discover turbo engines use more solid components.

There are tuning limits for every engine, with some being extremely strong and some only just able to handle stock power

Research these restrictions and upgrade to higher quality components to handle the power.

We've seen car owners spending a loads of money on turbo upgrades on the EJ25 only to experience the engine explode when it's completed.

Big capacity turbos commonly experience a bottom end lag, and low capacity turbos spool up quickly but won't have the top end engines power gains.

In the last 10 years the world of turbochargers is always moving on and we commonly find variable vane turbochargers, permitting the vane profile is altered according to speed to lower lag and increase top end torque.

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

It is common that there is a limitation in the air flow sensor AFM/MAF on the EJ25 when loads more air is being fed into the engine.

You'll see that 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor limited power at a much lower level.

Adding a supercharger or additional turbo will make large torque gains, although more difficult to setup. We have this guide to twinchargers if you want to read more.

Fuelling upgrades and mods

You will need to ensure that the engine is not starved of fuel so will have to increase the fuelling when you start extending past 20% of a torque increase.We strongly recommend you to be generous with your injector capacity.

The accepted safe increase is to add 20% capacity when specifying an injector, this accounts for injector deterioration and allows some 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.

Exhaust upgrades and mods

Only look to improve your exhaust if your current exhaust is creating a flow problem.

On most factory exhausts you'll see the flow rate is still ok 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 will certainly help air flow out of the engine but do not go too big or you could will reduce the flow rate. Stick to 1.5 to 2.5 inches for best results.

Usual exhaust restrictions can be located the emissions filters installed, so adding a higher flowing sports alternative will help avoid this restriction.

Weak spots, Issues & common problem areas on the EJ25

The EJ25 engines, if regularly serviced and maintained, are generally very reliable and have few issues.

Regular oil changes are vital on the EJ25, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

History of the Engine

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 190Nm(140 lbft) of torque at 2800 rpm
• DOHC (JDM)(1996–1998) - SAE - 172.6 hp (129 kW; 175 PS) at 6000 rpm and 230.5Nm(170 lbft) of torque at 3800 rpm
• DOHC (USA)(1997–1999) - SAE - 165 hp (123 kW; 167 PS) at 5600 rpm and 220Nm(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

• 162 hp at 5600 rpm, 226Nm(167 lbft) torque at 4400 rpm
• 175 hp at 5600 rpm, 229Nm(169 lbft) torque at 4400 rpm
•  173 hp at 5600 rpm, 225Nm(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

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

EJ255

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

• US  Impreza WRX STi MY 2004~2007
  300 bhp (304 PS; 224 kW) at 6000 rpm (New SAE standard) and 407Nm(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 393Nm(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)

For more information 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.

We need your help improving this article, so please send us your feedback in the comments box below and pass on any tips, points or facts we have wrong or have not covered.

We really like hearing from our readers, and hearing about which tuning parts were the most effective for them, it helps us improve our recommendations and articles to reflect current trends in modifications and ensures that our EJ25 guides and tips are kept up to date.

Please Check out my YouTube channel, we're regularly adding new content...

Click to accept YouTube Cookies & Play.

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.

If you liked this page please share it with your friends, drop a link to it in your favourite forum or use the bookmarking options to save it to your social media profile.