Tuning the Subaru EJ25

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

Carref prides itself on providing the latest tips and guides to your modification project.

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.

We rely on our visitors to pass on tips and tell us about their projects and what worked on their car, and this article is the culmination of the feedback we have received. First let us look at the history and specs of this engine and then consider which tuning modifications work best on it.

History of the Engine

EJ25D

• DOHC (JDM)(1994-1996) - SAE - 157.8 hp (118 kW; 160 PS) at 6000 rpm and 210.8 N⋅m (155 lb⋅ft) of torque at 2800 rpm
• DOHC (USA)(1996) - SAE - 155 hp (116 kW; 157 PS) at 5600 rpm and 190 N⋅m (140 lb⋅ft) of torque at 2800 rpm
• DOHC (JDM)(1996–1998) - SAE - 172.6 hp (129 kW; 175 PS) at 6000 rpm and 230.5 N⋅m (170 lb⋅ft) of torque at 3800 rpm
• DOHC (USA)(1997–1999) - SAE - 165 hp (123 kW; 167 PS) at 5600 rpm and 220 N⋅m (162 lb⋅ft) 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, 226 N⋅m (167 lb⋅ft) torque at 4400 rpm
• 175 hp at 5600 rpm, 229 N⋅m (169 lb⋅ft) torque at 4400 rpm
•  173 hp at 5600 rpm, 225 N⋅m (166 lb⋅ft) 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 407 N⋅m (300 lb⋅ft) 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 N⋅m (290 lb⋅ft) 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 lb⋅ft (393 N⋅m) at 4000-5200 rpm of torque
• US STI S209 MY 2019
  341 hp (346 PS; 254 kW) at 6,400 rpm and 330 lb⋅ft (450 N⋅m) 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 N⋅m; 289 lbf⋅ft)

Tuning the Subaru EJ25 and best EJ25 performance parts.

What are the most effective 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.

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 a EJ25 Competition cam is a pleasure to live with when driving around busy urban areas.

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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Modifying to Stage 1:

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

Modifying to Stage 2:

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

Modifying to Stage 3:

 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.

ECU mapping will help fully realize the full potential of all the tuning mods you've done 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 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 headers can make a big change to fuel atomisation on the EJ25.

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

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.

Which turbo upgrades are best?

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.

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.

Fuelling

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 NASP engines

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

Choosing the right performance exhaust

One of the most common mistakes and problems we see in tuning projects is usually down to the exhaust, or rather a poorly chosen exhaust for your engine.

You only need to 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 will reduce the flow rate. Stick 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 alternative will help avoid this restriction.

Weak spots Issues & problem areas on the EJ25

The EJ25 engines are generally reliable and solid as long as they are regularly serviced and maintained.

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

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.

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 them on each model of car. Comments are used to improve the accuracy of these EJ25 articles which are continually updated.

This article was written by Waynne Smith on TorqueCars, and I appreciate your feedback and suggestions. This entry was filed under Subaru. You can leave a response below or join TorqueCars forum to discuss this article and car modification in detail with our members.

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