Subaru EJ20 Tuning

"All you need to know about tuning the Subaru EJ20 engine!"

The Subaru EJ20 is a boxer configuration engine and sounds fantastic, it still rates as one of my all time favorite engines.

EJ20's offer very good returns when tuned and with carefully chosen sports mods like a remap, turbo upgrades and camshafts you will certainly enhance your driving enjoyment.

Our aim here is to look into EJ20 tuning and show the best modifications.

History of the Engine

EJ20E SOHC naturally aspirated

  • Legacy JDM '89-'94
    125ps (92kw 123hp )
    BC - BF series
    '93-'99 135ps (99kw 133hp )
    BD - BG series( ECU code EURO, D3 Asia 4H)
    '98- '04 155ps (114kw 153hp )
    BE - BH series( ECU code EURO, D3 Asia 4H)
    '03- '09 140ps (103kw 138hp )
    BL - BP series Europe
    '91-'99 115ps (85kw 113hp )
    BC, BD, BF series
  • Impreza JDM'93-'99
    135ps (99kw 133hp )
    GC - GF series
    '08-current 140ps (103kw 138hp )
    GH - GE series
    '94-'99 115ps(85kw 113 hp)
    GC, GF series
    Isuzu Aska ('90–'93)'90-'93 125ps (92kw 123hp )


  • Subaru BE/BH Liberty
    92kW @5600rpm184Nm at 3600rpm '98-03
  • Subaru GD/GG Impreza
    92kW @5600rpm 184Nm at 3600rpm '00-03

EJ20D DOHC naturally aspirated

  • Legacy JDM '89-'99 150ps (110kw 148hp ) BC - BF and BD - BG series

EJ202 SOHC naturally aspirated

  • Forester JDM SF series, 138ps('97 - '02)

EJ203 SOHC naturally aspirated

  • Forester JDM SG series, 140ps( '03 - '08)
  • Legacy JDM 2.0i BP/BL series, 140ps( '03 - '09)
  • EJ204 DOHC naturally aspirated AVCS
  • Legacy B4 TSR JDM BE - BH series'99- '01 155ps (114kw 153hp )
  • Legacy JDM BL - BP series '03- '09 190ps (140kw 187hp )
  • Legacy (Europe) BL - BP series '03- '07 165ps (121kw 163hp )
  • Impreza JDM GC - GF series'93-'99 155ps (114kw 153hp )
  • Impreza GE - GH series '07-2011 150ps (110kw 148hp )
  • Forester (Europe) SG Series '05- '07 158ps (116kw 156hp )
  • Forester JDM SH Series '08-2011 150ps (110kw 148hp )
  • Exiga JDM YA Series '08 - 2012 150ps (110kw 148hp )

Rocker-style HLA EJ20G

  • Legacy RS 89-93
  • Legacy RS-RA 89-93

Bucket-style HLA EJ20G

  • Impreza WRX 92-96
  • Impreza WRX Wagon 92-96Impreza WRX Wagon AT 96-98
  • Subaru Impreza WRX RA 93-96
  • EUDM Subaru Impreza Turbo 94-96

Closed deck

  • EJ20GDW1HD '92-May to '93-Aug WRX.EJ20G
  • EJ20GDW1HE '92-May to '93-Aug WRX.EJ20G
  • EJ20GDW1HJ '92-May to '93-Aug WRXRA.EJ20G
  • EJ20GDW1HR '92-May to '93-Aug WRXRA.EJ20G
  • EJ20GDW2HD '93-May to '94-Sep S.WRX.MT.EJ20G
  • EJ20GDW2HE '93-May to '94-Sep S.WRX.MT.EJ20G
  • EJ20GDW2HJ '93-May to '94-Sep WRXRA.EJ20G
  • EJ20GDW2HR '93-May to '94-Sep WRXRA.EJ20G
  • EJ20GDW4HJ '93-Jun to '95-Aug WRXRA.EJ20G
  • EJ20GDW5HJ '95-Sep to '96-Aug WRXRA.EJ20G (STI)

Open deck

  • EJ20GDX1ND '93-May to '94-Sep AT.EJ20G
  • EJ20GDX1NE '93-May to '94-Sep AT.EJ20G
  • EJ20GDW1ND '93-May to '94-Sep W.MT.EJ20G
  • EJ20GDW1NE '93-May to '94-Sep W.MT.EJ20G
  • EJ20GDW4HD '94-Jun to '95-Aug S.WRX.EJ20G
  • EJ20GDW4HE '94-Jun to '95-Aug S.WRX.EJ20G
  • EJ20GDW4ND '94-Jun to '95-Aug W.WRX.MT.EJ20G
  • EJ20GDW4NE '94-Jun to '95-Aug W.WRX.MT.EJ20G
  • EJ20GDX4NE '94-Jun to '95-Aug WRX.AT.EJ20G
  • EJ20GDW5HE '95-Sep to '96-Aug S.WRX.EJ20G
  • EJ20GDW5NE '95-Sep to '96-Aug W.WRX.MT.EJ20G
  • EJ20GDX5NE '95-Sep to '96-Aug AT.EJ20G
  • EJ20GDX5HD '95-Sep to '96-Aug S.WRX.EJ20G
  • EJ20GDW5PE '95-Oct to '96-Aug W.WRX.MT.EJ20G (STI)
  • EJ20GDW5PE '95-Oct to '96-Aug WRXSTI.EJ20G
  • EJ20GDW5PJ '95-Oct to '96-Aug WRXRASTI.EJ20G


EJ20K engines fall into either shim under or shim over bucket design

Shim-over-bucket style EJ20K usage

  • JDM WRX 96-97
  • Impreza WRX wagon MT '97-'98
  • Impreza WRX type RA or R MT '97-'98

Shim-under-bucket style EJ20K usage

  • Impreza WRX STI MT 97-98
  • Impreza WRX STI wagon MT 97-98
  • Impreza WRX STI type RA or R MT 97-98


  • Impreza WRX
  • 99-01 (JDM Wagon Body only)
  • 01-06 (all JDM)
  • 02-05 (USDM)
  • 05 SAAB 9-2X AERO
  • 99-06 (all other markets)
  • Forester Cross Sports, S/tb, STI

EJ20H EJ20R EJ206 and EJ208 were fitted to the Legacy and each had differences, with the EJ208 regarded as a new engine design thanks to revised Camshaft, intake manifold injectors and fuel/timing.

NOTE: EJ20T and EJ20TT were not official engine codes, they were used by customers to describe the Tubo and Twin Turbo engines in high performance models.

Tuning the Subaru EJ20 and best EJ20 performance parts.

Best EJ20 tuning parts

The best EJ20 upgrades on an engine are usually the ones that give the biggest return for your cash.

We won't be swayed by popular EJ20 upgrades, they need to be cost effective.

Altering your EJ20 cam will make a dramatic difference to the engine bhp. Choosing a higher performance cam profile raises the bhp accordingly.

Fast road cams commonly boost the performance across the rev range, you may lose a little low end bhp but the higher rpm power will be better.

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

For a daily driver must carefully try to match your engines power to your driving style.

I'd be shocked if you have found a EJ20 Competition camshaft is a pleasure to live with when in heavy traffic.

Different EJ20 engines respond better to less aggressive cam durations so view each engine as unique.

The engine timing and injectors and fuel pump also have an effect on the power gains you'll achieve.

A longer valve duration can alter the power 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, Drilled & smoothed airbox, Panel air filters, Intake headers.

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

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

Review your options and then find your parts and set yourself a power target to avoid costly mistakes.

Mapping should help to to establish the full potential of all the mods you've fitted to your EJ20.

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 rely on the mods you've applied and the condition of your engine.

It is the aim to any performance tuning task to pull more air into your EJ20

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

The bore size, shape and rate of flow of the Intake headers can make a big change to fuel atomisation on the EJ20.

Most air intake manifolds are begging for aftermarket tuning parts, although a few makers provide reasonably well designed air intake manifolds.

Big valve conversions on the EJ20, getting port work and head flowing will also boost bhp and torque, & importantly will raise potential for a greater bhp and torque increase on other upgrades.

We found the following spec list for the EJ20 engine which has been compiled from the most popular mods.

  • Garrett T04E/T3 Ball-Bearing Turbocharger
  • 3" Downpipe (Cat can be removed for off road use) (For T3/T04E Turbo)
  • Cold Air Intake K&N Filter (For T3/T04E Turbo)
  • HKS Racing External Wastegate
  • Up-Pipe (Cat-Less) (For T3/T04E Turbo and HKS Wastegate)
  • 785cc Fuel Injectors with the resistor pack

Other options include the PE1820 or VF30 and a FMIC is a good idea, the top mount position is not the most efficient design out there.

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 EJ20

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.

When an engine is fitted with a turbo parts are simpler to install and you will discover turbocharged engines are built with more solid components.

There are weak spots for every engine, with some being very over engineered and some only able to handle stock power

We recommend you find these restrictions and install forged components to survive the power.

We've seen tuners spending a a stack of money on turbo upgrades on the EJ20 only to see the whole thing catastrophically fail soon after it's been enthusiastically driven.

Big turbos will usually experience no power at low rpm, and low capacity turbos spool up quickly but won't have the peak rpm torque gains.

Thanks to new tech the choice of turbo units is always moving on and we are seeing variable vane turbo units, permitting the vane profile is altered according to speed to lower lag and increase top end torque.

Twin scroll turbo units divert the exhaust gases into 2 channels and push these at differently angled vanes in the turbo charger. They also improve the scavenging effect of the engine.

You'll commonly see there is a limit in the air flow sensor MAP/MAF/AFM on the EJ20 when considerably more air is being drawn into the engine.

You'll see that 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 additional turbo will make large bhp and torque gains, although more complex to configure. We have this article on twincharging if you want to read more.


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

More performance needs more fuel. We would recommend you to be generous with your injector capacity.

The accepted safe increase is to add another 20% when buying an injector, this takes into account injector deterioration and gives you some 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

The Walbro 255 pump is a good solid option for most projects.


Only look to increase your exhaust if your current exhaust is creating a restriction in flow.

On most factory exhausts you'll find the exhaust flow rate is ok 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 equal out the flow of air through the engine.

But if your exhaust pipe is too large, ie: it's over 2.5 inches bore, you will lose much of the flow rate and end up sapping power and torque.

Typically exhaust restrictions can be traced to the emissions filters installed, so adding a faster flowing performance alternative will help avoid this restriction.

Weak spots Issues & problem areas on the EJ20

The EJ20 engines are generally reliable and solid as long as they are regularly serviced and maintained.

The Turbocharged engines such as the EJ20G, EJ205 and the EJ20K are quite highly tuned, and need to be treated with care and regularly maintained.

As piston rings wear they can seep and this results in high oil consumption and burning of oil. Check the exhaust for blue smoke.

Camshaft cover seals regularly fail, so should be examined and replaced to avoid leaks.

The water cooling supply to cylinder 4 is not as good as to the others, so it will typically fail first. The first sign of a problem is an engine tapping noise, sadly a strip down and repair is the only option here.

Regular oil changes are vital on the EJ20, especially when tuned and will help extend the life and reliability of the engine.

For more information on Tuning your EJ20 engine please join us in our friendly forum where you can discuss tuning options in more detail with our worldwide members with the EJ20 engine, or read our ej20 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 parts work best for them on each model of car. Comments are used to improve the accuracy of these EJ20 articles which are continually updated.

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