Subaru EJ255 Tuning

"All you need to know about tuning the Subaru EJ255 engine!"

A site visitor asked for more information on the EJ255 and how to tune it up and modify it, so this page has been assembled to go over what we feel are the primary areas we rate and are the greatest enhancements you can do to this engine.

In this article we review EJ255 tuning and report on the optimum mods that work. Subaru EJ255 really good project engines and with carefully chosen sports tuning mods like remapping, turbo kits and camshafts you will really enhance your driving experience.

History of the EJ255 Engine

Featuring sodium filled valves to prevent hot spots and distributed around the world the EJ255 has quite a loyal fan base.

There were 2 versions of this engine, one used the heads and short block from the STi  EJ257 then from 2006 AB630 short block and AB820 heads were used. If you swap the pistons out, you will pretty much have an EJ257.

  • 210–265 hp (157–198 kW; 213–269 PS)


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.

Tuning the Subaru EJ255 and best EJ255 performance parts.

Best EJ255 tuning parts

When talking about the optimum modifications for your EJ255 engine, we are going to focus on the upgrades that give the biggest return for your cash.

Performance camshafts work better at higher RPM power which makes it a best mod for NASP and turbo petrol engines, but not so much on diesels. This will usually provide good top end power benefits through performance camshafts on most engines

So altering your EJ255 camshaft will make a difference to the engine power band. Choosing a higher performance camshaft profile raises the power band|torque curve|power curve|power range accordingly.

Fast road cams tend to bump the torque over the rev range, you might lose a little low down torque but higher rpm power will improve.

Competition cams, bump the higher rpm power band but as a result the car will not idle smoothly and low end power nearly always suffers.

In a car driven daily you should ideally to optimize your torque band to your cars usage.

I'd be gobsmacked if you have found a EJ255 Motorsport camshaft is a pleasure to live with when in heavy traffic.Because your low end power will be very lumpy. Competition cams are designed for maximum power at the top end of the RPM range, a place that most daily commutes will not permit!

Each engine responds better to mild camshaft durations check your engine on a rolling road.

The ecu map and fuel pump and injectors also have a large bearing on the torque gains you'll hit.

Extending exhaust or intake 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.

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

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

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

The EJ255 power trains make great tuning projects and thanks to their popularity there are quite a few choices of parts and tuning parts about.

ECU flashing should help to release the full potential of all the parts you've done to your EJ255.

(Sometimes flashing the ECU 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 around 30% more power on turbocharged vehicles and you can expect to see around 15% on NASP engines, but the outcome usually differs on the parts you've fitted and the condition of your engine.

It is the main goal to any car tuning project to push air into your EJ255

Air Intake manifolds carry the air from the filter and allow it to be drawn into the engine cylinders with fuel for the squish phase.

The shape and flow rate of the Air Intake manifolds can make a big difference to to fuel atomisation and engine efficiency on the EJ255.

Most intake manifold are begging for performance upgrades, although a few makers provide well optimised intake manifold.

Big valve conversions on the EJ255, doing some port matching and head flowing will also lift torque, & more importantly will raise potential for a better torque increase on other tuning mods.

EJ255 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 EJ255

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

When the engine is fitted with a turbocharger tuning mods are going to make more power and you will discover turbocharged engines are built with more solid components.

There are tuning limits for every engine, with some being incredibly solid and some only just able to handle stock powerIt is important to find these restrictions and fit forged components to cope with the power.

We see many mechanics spending a loads of money on turbo charger upgrades on the EJ255 only to suffer the humiliation of seeing the whole thing throw a rod on it's first outing after it's completed.

Large capacity turbos tend to suffer low end lag, and small turbos spool up much more quickly but don't have the peak rpm torque gains.

In recent times the world of turbochargers is always improving and we are seeing variable vane turbochargers, allowing the vane profile is altered according to speed to lower lag and increase top end power.

Twin scroll turbochargers divert the exhaust flow into 2 channels and flow these at differently angled vanes in the turbo charger. They also help the scavenging effect of the engine.

It is not unusual that there's a restriction in the air flow sensor AFM/MAP on the EJ255 when loads more air is being pulled 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 at a much lower level.

Adding a supercharger or additional turbo will make large performance gains, although more challenging to configure. We have this feature on twinchargers if you want to read more.


When you increase the performance you will need to uprate to the fuelling.

More performance needs more fuel. Most tuners we speak with say to over specify your injector capacity.

The rule of thumb is to add 20% capacity when fitting an injector, this allows for injector deterioration and affords a little spare capacity should the engine require 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

4 Cylinder supercharged engines

  • 58 PSI 312cc/min 200hp
  • 58 PSI 468cc/min 300hp
  • 58 PSI 625cc/min 400hp
  • 58 PSI 937cc/min 600hp

EJ255 Performance Exhausts

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

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

Do not go with the widest exhaust you can source you'll slow up the exhaust flow rate - the best exhausts for power gains are usually between 1.5 to 2.5 inches. It is the shape and material more than the bore size.

Typically exhaust restrictions come around the filters installed, so adding a faster flowing high performance aftermarket unit will improve air flow, and rather than doing an illegal decat, will keep the car road legal.

Weak spots, Issues & problem areas on the EJ255

The EJ255 engines are generally reliable and solid as long as they are regularly serviced and maintained.

Regular oil changes are vital on the EJ255, especially when tuned and will help extend the life and reliability of the engine.

If you want to know more, or just get some friendly advice on Tuning your EJ255 engine please join us in our friendly forum where you can discuss EJ255 tuning options in more detail with our EJ255 owners. It would also be worth reading our unbiased Subaru tuning articles to get insights into each modification and how effective they will be.

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 parts work best for them on each model of car. Comments are used to improve the accuracy of these EJ255 articles which are continually updated.

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