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.

The EJ20 series engines were launched in 1989 to replace the EA engine series by Subaru and remained in production until 2021. The long production run makes it evident that these were great engines that were loved by car aficionados.

They were highly reliable four-stroke engines and were available with both single and double overhead camshaft arrangements.

Subaru presented these engines as both naturally aspirated as well as turbocharged variants and power ranged from 96hp to 320hp.

In this article, we will mainly discuss the EJ203, EJ205E & EJ20K engines and the mods that work best for these 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.

Watch our video that accompanies this article for troubleshooting and diagnosing check engine light problems.

History, Power & Specs of the Engine

Different regions had different configurations primarily to meet emissions regulations, but availability of parts and target market costs were also a factor.

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
  Europe
  '94-'99 115ps(85kw 113 hp)
  GC, GF series
  Isuzu Aska ('90–'93)'90-'93 125ps (92kw 123hp )

EJ201

• 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

Difference Between An Open Deck & Closed Deck Engine Block Design

An open deck engine is a type of engine block design in which the area surrounding the cylinders is open or exposed.

In an open deck design, the upper part of the cylinder walls receives little support from the surrounding block structure. There are open spaces or gaps between the cylinders that are not filled with material.

A closed deck design is the opposite of an open deck design, with the cylinder walls surrounded by a solid, continuous block structure.

Closed deck designs give the cylinder walls more rigidity and support, increasing their strength and resistance to movement or distortion under high pressure and temperature conditions.

EJ20K

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

1. Shim Under Bucket Design: In a shim under bucket design, the valve lifters (tappets) are located below the camshaft. These lifters are usually hydraulic and automatically adjust to maintain proper valve clearance. However, in some engines, shims are used to adjust the valve clearance. A shim is a thin piece of metal or composite material that is placed between the lifter and the valve stem to set the correct clearance. By using different thickness shims, the valve clearance can be adjusted as needed.
2. Shim Over Bucket Design: In a shim over bucket design, the valve lifters (tappets) are located above the camshaft. In this configuration, the lifters do not directly contact the valve stems. Instead, there are small buckets or cups that sit on top of the lifters, and the valve stem contacts these buckets. Shims are used in this design as well, but they are placed between the bucket and the valve stem to adjust the valve clearance.

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

EJ205

• 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.

Camshaft Upgrades

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.

Are Fast Road Cams Worth It For The EJ20?

If you want fast road cams for your EJ20, you must first have the ECU remapped. Only after the remapping will you be able to reap the full benefits of these fast road cams.

It is worth noting, however, that fast road cams are an expensive modification, and some experts believe that the minor headline figure gains obtained with such a large investment are not worth it.

It is a matter of personal preference, and for those who have completed all other modifications to their vehicles, getting fast road cams may be the way to go. It should also be noted that when we say "all other mods," we mean "building a stronger engine with ported heads."

It is also worth noting that the standard cams on these engines are capable of handling power figures of up to 400hp. That is why few people install fast road cams on these engines, believing that the money would be better spent elsewhere.

It should be noted, however, that some internet users claim that once these engines reach 450hp, other issues such as internal cylinder head weaknesses begin to emerge.

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.

Competition cams are designed for maximum power at the top end of the RPM range, a place that most daily commutes will not permit!

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.

Please watch our video which covers the 5 principles of tuning your car. Be sure to keep up with our latest YouTube content and subscribe.

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

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

Typical stage 3 mods often include: 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.

Engine Tunes/Remapping/ECU Flashing

Remapping the Subaru Forester vehicles with the EJ203 engines shows a power increment of around 33hp whereas the torque rises by around 50Nm.

Similarly, Subaru Legacy models with the EJ203 see a power increase of around 17hp whereas torque increases by approximately 19Nm.

The EJ205E was fitted to the Subaru Impreza WRX & STI variants. Typical power gains with remaps on this engine are around 30hp whereas you can expect a torque increment of around 60Nm.

A power increase of around 20 to 40hp can be obtained by remapping the vehicles with the EJ20K engines.

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

(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.)

Aftermarket ECU's we've had recommended to us on the EJ20 are...

• Utec
• Accessport
• Ecutek.

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 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

EJ20 Intake Mods & Upgrades

Let's now talk about one of the most popular performance upgrades for all types of vehicles: air intake systems.

Your intake manifold will direct 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 manifolds 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.

However, before we go over the options available for EJ20 series engines, we need to think about whether or not these modifications are useful.

Stock cars typically have very little air intake system restriction, so there isn't much need to upgrade these components.

However, when you make significant upgrades, your car might begin to feel restricted, and at this point the advantages of a performance air intake system become evident.

You can choose an air intake upgrade from one of the following manufacturers:

• AEM Intakes;
• K&N;
• Perrin Performance;
• Takeda.

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

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 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.

A turbocharger is an excellent way to quickly increase your vehicle's power.

Although some of the cars with the EJ20 series engines came with turbochargers fitted from the factory. Still, here are a few options if you want to install an aftermarket turbocharger on your vehicle:

• BorgWarner EFR 76/70;
• BorgWarner EFR 83/74;
• BorgWarner EFR 91/80.

Besides this, you can also use the universal part from BorgWarner AirWerks.

Some other companies that offer turbochargers that fit the vehicles with the EJ20 series engines include:

• GrimmSpeed;
• Dorman;

Turbochargers are among the most expensive performance upgrades for any vehicle thus complying with the manufacturer’s instructions is vital.

small turbo upgrades, +49lb/min

• FPGreen
• Frank50
• SZ49

Then for more power there are ...

• TD05H-18G's
• TD05H-20G spools as fast but offers around 25hp more Good on stock internals,

20psi of boost can cause problems on the stock block, although some have run around 21psi we would recommend strengthening the essential components to handle this power. You'll also see fuel issues at these boost levels, and the wastegate really should be ported slightly as this becomes a restriciton. ECU's such as Utec, accessport, ecutek can help mitigate these issues and help you get the engine running properly.

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. Wastegates also sometimes need a small amount of porting on larger turbos to maximise flow.

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.

Intercooler

An intercooler is an effective upgrade that is vital for EJ20-powered Subaru vehicles. It is basically a radiator that provides a constant source of cold air to the engine.

Because cold air is more oxygen-rich than warm air and oxygen is a key component for the combustion process, by providing cold air to the engine, the intercooler aids the engine in delivering higher power figures.

It is worth noting that an intercooler does not add power to a car rather it just gives it the ability to deliver higher power for longer.

The stock intercooler can heat up too quickly when running higher levels of boost so can actually become a restriction.

Choosing the right intercooler is important if you don’t want to face the issue of heat soak. Heat soak happens when the intercooler itself heats up and thus it is no longer capable of delivering cold air to the engine.

Furthermore, while installing an intercooler, you must also stay conscious that you don’t end up adding additional drag to the vehicle.

Some of the manufacturers that offer intercoolers for these vehicles include but are not limited to:

• COBB;
• GrimmSpeed;
• Perrin;
• Mishimoto.

EJ20 Fuelling Upgrades & Mods

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.

The stock fuel components are good enough for regular cars. However, once you tune the vehicles, you need more fuel and thus fuelling mods become vital.

Some of the common fuel mods that are performed on the EJ203, EJ205E & EJ20K engines involve both injectors, fuel rail and fuel pumps, so let's break these down and discuss the options open to you:

Upgraded Fuel Injectors

The aim of using these upgraded fuel injectors is to improve the fuel quantity that is delivered to the engine.

When more fuel is delivered to the engine and it is mixed with a higher quantity of air, the rate of combustion increases. As a result of this, the power output of the engine increases.

Companies that offer fuel injectors for the EJ20 series engines include DeatschWerks, Injector Dynamics & Bosch. Here are a few examples of fuel injectors for vehicles with these engines:

• DeatschWerks® Fuel Injectors;
• Injector Dynamics®ID2600-XDS Fuel Injectors.

High-Flow Fuel Rails

If you want to ensure a constant flow of fuel to the injectors, particularly at high power levels, high-flow fuel rails can be of great help. Here are some of the options that you have:

• PERRIN Performance® Top Feed Style Fuel Rail Kit;
• Aeromotive® Black Top Feed Fuel Rails.

Upgraded Fuel Pump

An upgraded fuel pump is important if you want to ensure constant fuel delivery to the fuel injectors.

You can use upgraded fuel pumps from Walbro, AEM, DeatschWerks & Aeromotive for this purpose.

Here are a few examples of fuel pumps you can use:

• DeatschWerks® DW400™ Electric In-Tank Fuel Pump with 9-1041 Install Kit;
• AEM Performance Electronics® High Flow Fuel Pump.

Fuel Pressure Regulator

Another important fuel upgrade that most car enthusiasts go for is a fuel pressure regulator.

This part comes in handy when the intention is to precisely control the pressure of the fuel and optimize the fuel delivery.

• Standard® Intermotor™ Fuel Injection Pressure Regulator;
• Delphi® Fuel Injection Pressure Regulator.

Issue Related To Fuel Starvation

Multiple Subaru EJ20 engine owners have reported that when they corner their vehicle with the EJ20 engine at a sharp angle, the engine appears to experience a fuel cut and the vehicle seems to lose power to move forward.

Several people have reported experiencing this problem when accelerating their vehicle after a sharp corner.

Most people report that this problem occurs when they turn their car to the right. However, when cornering on the left, the fuel starvation problem does not appear.

Some owners of these vehicles believe the problem is caused by the performance tune, which is causing a misfire, but a few owners have reported similar issues even with the stock tune.

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

Exhaust

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 x 3 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 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.

Aftermarket Radiators

Another performance upgrade that you might need to install on your car with the EJ20 series engine is an aftermarket performance radiator.

Once a vehicle is delivering more power than the stock version, there is always a risk of overheating which can cause serious damage to the car.

Aftermarket performance radiators help keep tuned vehicles cool even when you are getting a lot more power output from them.

Some of the companies that offer radiators for vehicles with the EJ20 series engines include:

• Mishimoto;
• Koyo;
• Skunk2.

By performing the above-mentioned modifications on cars with the Subaru EJ20 engines, you can considerably increase the power output of your car.

Weak spots Issues & problem areas on the EJ20

The EJ20 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.

High oil consumption has been a noted issue on the turbocharged engines. We would recommend a comprehensive overhaul on WRX and STi engines around the 70,000 mile mark, and that owners perform a compression test and verify valve clearances and tolerances.

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.

Cylinder 4 is a potential weak spot, it is not cooled as effectively as other cylinders and can run much hotter. It will cause problems such as a knocking noise intially on cold starts and then will worsen to be there all the time.

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.

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.

If you would like to know more, or just get some friendly advice 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.

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