Subaru EJ15 Tuning

"All you need to know about performance tuning the Subaru EJ15 engine!"

The Subaru EJ15 are good project engines and with the ultimate uprated modifications like remapping, turbo kits and camshafts you will substantially increase your driving experience.

Let us examine the options for your EJ15 tuning and summarise the best modifications.

We look into EJ15 tuning and show the premier upgrades.

History, Power & Specs of the Engine


  • 96 bhp at 6000 rpm 95 lbft at 3600 rpm


  • 101 bhp at 5600 rpm 101 lbft at 4000 rpm


  • 94 bhp at 5200 rpm 103 lbft at 3600 rpm


  • 99 bhp at 5200 rpm 105 lbft at 4000 rpm

Tuning the Subaru EJ15 and best EJ15 performance parts.

Best EJ15 tuning mods

Just because particular tuning parts are are common on EJ15 it doesn't mean you should fit it, we shall focus tuning parts that will give your EJ15 the biggest power gain return for your cash.

Significant gains on the EJ15 can be made from camshaft upgrades. Altering the camshaft profile alters the intake and exhaust durations on the engine and can dramatically change the power band and power output.

Fast road camshafts commonly increase the bhp through the rev band, you could sacrifice a little low down power but the top end will be better.

Motorsport and race camshafts, increase the top end band but as a result the car will not idle smoothly and low end power nearly always suffers.

For a car used daily, really you should, ideally aim to optimize your engines power to your usage of the car.

I'd be shocked if you find a EJ15 Motorsport 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.

Some EJ15 engines respond better to extreme camshaft durations check your engine on a rolling road.

The engine timing and injectors and fuel pump also will say much 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.

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Best Engine Mods for your car

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

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

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

The EJ15 engines are great to work on and thankfully there is an increase of mods and performance parts out there.

Remaps will help unlock the full potential of all the tuning parts you've fitted to your EJ15.

(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 may rely on the tuning parts you've carried out and the condition of your engine.

It is the whole point to any engine performance tuning project to push fuel and air into your EJ15

Your intake manifold will direct the air from the air cleaner and allow it to be fed into the engine cylinders.

Design and rate of flow of the Intake manifolds can make a big change to fuel mixing and power on the EJ15.

Commonly we find the intake manifolds are improved through aftermarket parts, although some manufacturers provide fairly well optimized intake manifolds.

Larger EJ15 valves, doing some EJ15 port enlargement and head flowing will also improve bhp and torque, and importantly will give you an improved bhp and torque increase on other parts.

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 EJ15

Frankly the work required to turbocharge and EJ15 could be much better spent elsewhere.

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

It is important to find these limits and fit better pistons and crank to cope with the power.

We see many car owners spending a lots of money on turbo charger upgrades on the EJ15 only to experience the motor go up in smoke soon after it's finished.

Large capacity turbo chargers tend to suffer a bottom end lag, and small turbo chargers spool up much more quickly but won't have the high rpm torque gains.

Over the last 20 years the choice of turbo chargers is always increasing and we are seeing variable vane turbo chargers, where the vane profile is altered according to speed to lower lag and increase top end performance.

You'll commonly see there is a restriction in the air flow sensor (AFM/MAF/MAP) on the EJ15 when considerably more air is being sucked 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 performance at a much lower level.

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


Don't forget to look at the fuelling when you are increasing the power - it makes the car more thirsty. It makes sense to over specify your injector capacity.

The accepted safe increase is to add 20% to the flow rate when specifying an injector, this takes into account injector deterioration and gives you 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.

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 NA (naturally aspirated) engines

  • 58 PSI 285cc/min 200hp
  • 58 PSI 426cc/min 300hp


You should look to uprate your exhaust if your current exhaust is actually causing a restriction in flow.

On most factory exhausts you'll find the exhaust 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.

Please dont run with the biggest exhaust you can find this will 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 are in the emissions filters 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 EJ15

The EJ15 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 EJ15, 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 EJ15 engine please join us in our friendly forum where you can discuss tuning options in more detail with our worldwide members with the EJ15 engine, or read our ej15 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 modifications 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 EJ15 tuning guides which get regular updates and revisions.

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