Tuning the Subaru EJ15

"Comprehensive guide to performance parts and tuning the Subaru EJ15 engine!"

We shall look into EJ15 tuning and outline the greatest modifications. Subaru EJ15 provide a fun base for your project and with the optimum uprated parts like remaps, turbo improvements and camshafts you will certainly enhance your driving opportunities.

Just because particular parts are are common on EJ15 it doesn't mean its worth having, instead we'll greatest parts that will give your EJ15 the biggest power gain return for your cash.

The cam profile plays a big part in the engines power output so cam upgrades make quite a large difference. The intake & exhaust durations will alter depending on the chosen cam profile, so large bhp gains are on offer for cam upgrades.

NB: Fast road cams normally increase the bhp and torque across the rpm range, you could sacrifice a little low end torque but the top end will be higher.

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

A Motorsport and race cam makes it harder when driving around busy urban areas.

You should ideally match your engines power to your usage of the car so for a typical daily driver stick with a fast road EJ15 cam

Different EJ15 engines respond better to extreme camshaft durations so set your engine up on a rolling road.

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

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.

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Guide to the Best Tuning Mods & Upgrades

  1. Mods that Remove a Restriction

    If the intake or exhaust are restricted in any way this will have an impact on your cars performance, so use an induction kit/filter upgrade and better flowing exhaust whenever your tuning creates a flow restriction.

  2. Mods that Improve Handling

    Braking, Suspension and general alignment of the suspension components is vital for any tuning project even before you start increasing power.

  3. Fit Stronger Parts

    Tuned cars will show up weaknesses, typically in the turbochargers, clutch and internal engine components. Whilst most engines can cope with mild tuning mods, TorqueCars recommend that you upgrade the internal components before these weak spots manifest themselves.

  4. ECU Tunes & Remapping

    While some cars can be easily remapped, others may require piggyback ECU's or aftermarket ECU's but this is the most vital step of your tuning project as it fully releases the power from all of your mods and upgrades. Expect 10-20% on NASP engines and 30-40% on turbocharged units.

  5. More Power Needs More Fuel & Air

    Every tuning project will aim to increase the air supply, but fuel supply is just as vital and will need to match the air the engine can utilise. Fuel to Air ratio is vital so upgrade the fuel pump & injectors. Also you can look to perform head mods (flowing and porting), bigger valves, fast road cams and forced induction upgrades to improve fuel.

Stages of Tune

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

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

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

Carefully think through your options and then acquire your upgrades and set yourself a power target to avoid costly mistakes.

ECU flashing should help to release the full potential of all the upgrades you've fitted to your EJ15.

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 may depend much on the upgrades you've done and the condition of your engine.

It is the aim to any engine upgrade task to get air into your EJ15

The intake plenum carry the air during the suck phase from the filter and allow it to be drawn into the engine cylinders.

The size of bore and shape and flow rate of the Intake can make a big improvement to fuel atomisation on the EJ15.

Commonly we find the intake manifold are improved through aftermarket tuning parts, although some manufacturers provide reasonably good intake manifold.

Increasing the EJ15 valve size, carrying out EJ15 port enlargement and head flowing will also increase torque, and significantly will raise potential for a greater torque 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 EJ15

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

When your car is turbo charged tuning mods are giving better power gains and we find turbo engines are built using stronger components.

There are weak spots for every engine, with some being extremely strong and some just sufficiently able to handle stock power

Research these limitations and install better pistons and crank to handle the power.

There are many drivers spending a lot of money on turbo charger upgrades on the EJ15 only to suffer the humiliation of seeing the engine block go up in smoke when it's used in anger.

Bigger turbo chargers will usually suffer no power at low rpm, and little turbo chargers spool up much more quickly but do not have the peak end torque gains.

Over the last 20 years the range of turbos is always developing and we now see variable vane turbos, where the vane angle is altered according to speed to lower lag and increase top end bhp.

Twin scroll turbos divert the exhaust flow into two channels and flow these at differently designed vanes in the turbo charger. They also boost the scavenging effect of the engine.

You'll commonly see there's a limitation in the air flow sensor MAF/MAP on the EJ15 when considerably more air is being drawn into the engine.

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

Adding a supercharger or additional turbo will make large power gains, although harder to get working. We have this article on twincharging if you want to read more.

Fuelling upgrades and mods

Don't dismiss the need pay attention to the fuel system when you are increasing the bhp - it makes the car more thirsty. Don't forget to over specify your injector capacity.

The rule of thumb is to add another 20% when specifying an injector, helps cope with 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.

Exhaust upgrades and mods

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

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

Don't go with the largest exhaust you can buy this will reduce 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.

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

Weak spots, Issues & common problem areas on the EJ15

The EJ15 engines, if regularly serviced and maintained, are generally very reliable and have few issues.

Regular oil changes are vital on the EJ15, particularly when the engine has been modified and is putting down more power than the manufacturer intended.

History of the Engine

EJ151

  • 96 bhp at 6000 rpm 95 lbft at 3600 rpm

EJ152

  • 101 bhp at 5600 rpm 101 lbft at 4000 rpm

EJ153

  • 94 bhp at 5200 rpm 103 lbft at 3600 rpm

EJ154

  • 99 bhp at 5200 rpm 105 lbft at 4000 rpm

Tuning the Subaru EJ15 and best EJ15 performance parts.

For more information 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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