Subaru EJ18 Tuning

"All you need to know about tuning the Subaru EJ18 engine!"

Our aim here is to review EJ18 tuning and highlight the best mods that work. Subaru EJ18 make a good tuning project and with a few sensible tuning mods like ECU maps, turbo kits and camshafts you will certainly improve your driving enjoyment.

History of the Engine

  • Impreza 93-99 GC6 series
  • Legacy (except USA) 90-96 BC2, BC3, BD2, BD3, BG3 series
  • Isuzu Aska (1990–1993)


  • 108 bhp @6000 rpm 110 lbft @ 3200 rpm


  •  113 bhp @ 6000 rpm 114 lbft @4500 rpm


  •  118 bhp @5600 rpm 121 lbft @3600 rpm

Tuning the Subaru EJ18 and best EJ18 performance parts.

Best EJ18 tuning parts

When talking about the best ultimate for your EJ18 engine, we are going to focus on the upgrades that give the biggest return for your cash.

Significant gains on the EJ18 can be made from camshaft upgrades. Altering the camshaft profile alters the intake and exhaust durations on the engine and can dramatically change the bhp and power output.

Fast road camshafts tend to bump the bhp through the rev band, you may sacrifice a little bottom end torque but your top end will be lifted.

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

In a daily driver should ideally to optimize your bhp range to your driving style.

You'll never have ever thought a EJ18 Motorsport camshaft is a pleasure to live with when driving in heavy traffic.

Each engine responds better to mild cam durations than others.

The map and fuelling also will make differences on the torque gains you'll get.

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.

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

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

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

Carefully think through your options and then source your mods and set yourself a power target to avoid wasting your time and money.

Remaps will help to establish the full potential of all the tuning mods you've fitted to your EJ18.

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 often differs on the tuning mods you've fitted and the condition of your engine.

Shoving more fuel and air into your EJ18 is the aim to any car tuning task.

Headers transmit the air during the suck phase from the air filter and allow it to be pulled into the engine cylinders with fuel for the squish phase.

The size of bore and shape and flow characteristics of the Intake can make a noticeable difference to to fuel engine efficiency on the EJ18.

It's not uncommon that plenum chambers are in dire need of a performance upgrade, although some manufacturers provide reasonably well designed plenum chambers.

Adding a EJ18 larger valve kit, doing a bit of port matching and head flowing will also boost torque, and as an added benefit will give you increasing the torque increase on other tuning 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 EJ18 but to be realistic we wouldn't recommend trying this.

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

However you'll find engines will need better parts at higher power limits

See where you'll find these limits and upgrade to better pistons and crank to cope with the power.

It is common that there is a restriction in the air flow sensor (AFM/MAF/MAP) on the EJ18 when considerably more air is being sucked into the engine.

We note 4 bar air sensors coping with quite large power gains, whereas the OEM air sensor sapped bhp and torque at a much lower level.

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


When you boost the performance you will need to pay attention to to the fuel system.

More performance needs more fuel. It is important to over specify your flow rate on the injectors.

The accepted safe increase is to add 20% when fitting an injector, this takes into account injector deterioration and allows 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 turbocharged engines

  • 58 PSI 340cc/min 200hp
  • 58 PSI 511cc/min 300hp

4 Cylinder NASP engines

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


You may need to improve your exhaust if your current exhaust is actually creating a restriction.

On most factory exhausts you should find that your 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 can usually air flow from the engine but avoid an exhaust that is too big or you may end up will reduce the flow rate. Stick to 1.5 to 2.5 inches as a rule of thumb.

Common exhaust restrictions are traced to the emissions filters installed, so adding a freer flowing performance alternative will help avoid this restriction.

Weak spots Issues & problem areas on the EJ18

The EJ18 engines are generally reliable and solid as long as they are regularly serviced and maintained.

Regular oil changes are vital on the EJ18, especially when tuned and will help extend the life and reliability of the engine.

For more information on Tuning your EJ18 engine please join us in our friendly forum where you can discuss tuning options in more detail with our worldwide members with the EJ18 engine, or read our ej18 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 modifications work best for them on each model of car. Comments are used to improve the accuracy of these EJ18 articles which are continually updated.

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