Subaru used air assist injection on some (but not all) Phase II normally aspirated EJ series models starting in MY01 in European / ‘rest of world’ spec models. It feeds the fuel injectors with not only fuel, but also air. The two mix within the injector, and the amount of air flowing to the injectors is increased at idle by a solenoid valve which is not used on any of the other engine models. The throttle body, inlet manifold, fuel pipe assemblies, and injectors are all different on engines with this type of air assist injection, and the most obvious visible clue that an engine has this system is the extra solenoid valve. It has a purple connector, and is mounted very visibly between the power steering pump pulley and coil pack. These models also have an atmospheric air pressure sensor mounted on the RH front suspension strut, and do not have exhaust gas recirculation. They use a wideband main / upstream lambda sensor.
A similar version of this air assist injection system was used on some US spec models from the same era (Subaru refer to these as the ‘AAI UJ’ system), and a different version without the extra solenoid valve was also used in some non European / ‘rest of world’ spec models.
Note: air assist injection is not the same thing as secondary air injection.

Active Valve Control System (variable valve timing). Hydraulic vane type continuously variable valve timing. Some models with AVCS are ‘single AVCS’ – i.e. DOHC engines with AVCS only on the inlet cams. Some other, later models used dual AVCS – variable timing on both the DOHC inlet and exhaust cams

Active Valve Lift System. ECU control of the lift of one of the intake valves on each cylinder, the valve can be operated by one of two cam lobes.

CAN bus was the first networking standard widely used in vehicles. Very much like LAN for computers. Prior to CAN bus, every signal was connected via it’s own dedicated wiring, but as cars got more and more complex, that made the wiring harnesses get bigger and more expensive. The different systems in pre CAN bus cars were largely ‘stand alone’, so relatively easy to isolate from the rest of the car. There usually were connections between various systems, but a minimal number of them, doing relatively little.
CAN bus was developed (like most automotive electronics technology) by Bosch, specifically for use in vehicles in the 1980’s, and became common in the ’90’s typically initially only in the most complex cars (where there was the most to be gained from minimising the size and cost of the harnesses).
With CAN bus or any of the other automotive networking standards which have come since, one pair of wires can handle vast amounts of signals, with the limit being the bandwidth rather than an actual number of distinct signals. Therefore it is most effectively used where many different signals need to be shared between multiple systems. The almost limitless CAN bus bandwidth, high reliability and requirement for centralised diagnostics as car complexity increased meant the systems became less and less ‘stand alone’. Most systems are constantly also checking the presence and functions of other systems. This makes using them in engine conversions considerably more complex and expensive.
Subaru started using CAN bus in European models in MY00, initially only in Legacy models with VDC. The bus communicated between the ABS/VDC, some of its sensors such as yaw rate and accelerometers and the automatic transmission controller, but not initially the engine management. They first fully embraced CAN bus in the Generation 4 Legacy (MY04 –> in Europe), and later in the Impreza and Forester models (mostly MY08 –>). All models from them on use CAN bus. These models all use CAN bus for the engine management, and mist use it for the dash instrumentation and the then new ‘body control module’. The body control module is a new ECU which took over the functionality of the typical numerous individual modules which were previously used to control things like wipers, electric windows, alarm. It also converts various conventional (i.e. non CAN bus) signals to CAN bus, including various signals which the engine management requires such as instrumentation (tacho signal, warning lights, etc) immobiliser, and sometime vehicle speed signal. etc.

Using a CAN bus Subaru engine in a typical engine conversion project like a VW conversion requires a few things:

  • Faking the presence of other systems which the engine management needs to be able to ‘see’ to be able to operate normally, such as the ABS, automatic transmission controller, dash instruments, etc
  • Conversion of various signals which either the engine management needs top operate normally or the user needs to be able to integrate the engine into their VW properly between conventional signals and CAN bus

See Why does a Subaru engine having CAN bus management make it considerably less practical to use in VW conversions for more details

A coolant tank specifically intended to catch coolant expelled from the cooling system during abnormal conditions, to prevent spills. These are required for some race applications, to minimise the chances of lost coolant getting onto the track. In OEM applications such a catch tank will never have maximum or minimum level mark, and should be designed to be easy to empty. Its only purpose is only to catch coolant vented from the closed circuit during abnormal running conditions to prevent spills. These are required for some race applications, to minimise the chances of lost coolant getting onto the track.

Canister Purge Control is the system which sucks fuel vapour stored in the charcoal canister into the engine to burn it when the engine running conditions suit. The engine part of this system is very simple on Subaru engines. It is just an ECU controlled solenoid valve mounted on the inlet manifold which opens when there is sufficient manifold vacuum to draw the vapour in.

A ‘drive by wire’ throttle has no mechanical link between the throttle pedal and the throttle plate. Only pedal position sensors and throttle plate actuators. Because of the safety aspect of an electronic system having total control over the throttle plate opening, OEM’s put a great deal of effort into the ways in which the system can fail. Not unlike aircraft electronic controls, a lot of redundancy is built in. Both the pedal position and throttle plate positions are measures by two sensors, the signals from which are checked against one another. They are wired independently, fed from different power supplies and even monitored by separate microprocessors. This means if any one component fails, the other can still allow some control in a ‘limp mode’. Just like how mechanical cable throttle engines typically have dual throttle plate return springs to ensure that the throttle closes if the cable snapped, the actuator which moved the throttle plate has springs which close the throttle if the motor loss power.

Subaru started to use ‘drive by wire’ throttles in European / Rest of world’ spec models in MY04, in the then new Gen IV Legacy models – the same models in which CAN bus engine management was first used. Some but not all Impreza and Forester models started using ‘drive by wire’ throttles in MY05 – 07, and in these models the ‘drive by wire’ introduction did not happen at the same time as the CAN bus management, but 1-2 years earlier.

‘Drive by wire’ throttles make Subaru engines considerably more complex and expensive to install in VW’s, and not particularly practical to use at all in the VW models which has the majority of their pedal system mounted outside, under the cab floor (i.e. split window and bay window buses). In terms of the normal engine management functions, there are no advantages to using a ‘drive by wire’ Subaru engine in a VW conversion at all – only numerous disadvantages. The only time there can be any benefit to using them is if your donor Subaru also had cruise control, and you want to use it in the VW. Fitting the cruise control becomes much simpler and cheaper than it is with a cable throttle engine, mainly because less additional hardware is needed to add cruise control to a ‘drive by wire’ engine, and much of the wiring work needed to add cruise control have already been installed as part of the ‘drive by wire’ system.

Double overhead cam. i.e. two cam shaft per bank of cylinders, for a total of 4 cam shafts on a horizontally opposed engine. Easily distinguished by the cam belt covers having two round recess per cylinder head, positioned vertically one above the other, corresponding to the two cam pulleys per head.

A dual mass flywheel does most of the torsional vibration damping inside the flywheel assembly. The flywheel is effectively two flywheels back to back, one of which is attached to the end of the crank, and the other is what the clutch bolts to. Between the two flywheels is a torsional spring and friction damping mechanism, rather like the one at the centre of a clutch disc with a damped bub, but bigger. This allows it to be tuned to different vibration ranged relative to the hub damper previously used in almost all (later model) clutch discs. Earlier dual mass flywheels such as the Valeo ones that Subaru started using in some (but not all) normally aspirated Legacy models in MY00 typically used a rigid clutch disc (i.e. one with no damper hub). Some later models, such as the Sachs ones used with the EE20 Subaru diesel engines used both a dual mass flywheel and a disc with a damper hob, obviously carefully tuned to match one another so no strange resonances can occur.

Dual mass flywheels generally have a bad reputation, typically because they are expensive service items. A conventional single mass flywheel, if not abused, with typically outlive the car. However, dual mass flywheels are only designed to last as long as a clutch before the damper mechanism, and or friction lining is at the end of it’s useful life. The former being because of general wear and tear, but the latter often being because the thickness of the friction lining face has been minimised to squeeze the dual mass flywheel in to space originally only intended for a single mass flywheel. In many cars the dual mass flywheel took up more space, and either used spacers between the engine and gearbox, or a longer gearbox bell housing to make room, and their dual mass flywheel still had a reputation for being trouble.

The Valeo DMF that Subaru introduced in some MY00 Legacy Models was unusual, in that it took up no extra room. They squeezed it into the gap between the same engine and gearbox designs, without using spacers. Not long after they were introduced Subaru changed the release bearing design, making it thinner, so effectively making a bit more space, so it seems they didn’t get this right 1st time. The friction lining face on these is incredibly thin, so it’s no surprise that they can’t take much abuse, and don’t like having the clutch replaced without also fitting a new DMF. Their clutches do not interchange with any other Subaru clutch types, and no uprated clutches or even OEM parts with different torque ratings seem available. There is no stock VW clutch disc which us suitable for using in these clutches in VW applications. Aftermarket discs which could be used for this application do exist, but are not recommended – you’re very much better off spending the money on replacing the DMF with a conventional single mass version and the relevant clutch.

Engine Control Unit, or Electronic Control Unit – terminology used interchangeably. Typically if something automotive related just says ‘ECU’, it means Engine Control Unit, but if other information is included such as ‘Immobiliser ECU’, it meand Immobiliser Electronic Control Unit.

An engine management system is a system in which one electronic system manages everything about how the engine runs. With petrol engines, this originally meant the fueling and the ignition, but as engines got more complex, it included more and more other systems (variable valve timing, for example). Engine management is the successor to fuel injection systems. In a fuel injection system, the fueling only is controlled. The ignition may be coil and points, or an independent electronic system.

This basically means not Japanese or US spec. Both of those markets have unique emissions laws, and therefore unique engine management systems relative to all other markets. Europe and the ‘rest of the world’ all have near identical requirements in terms of the emissions laws that manufacturers build their cars to, so tend to have very similar (often near-identical or actually identical) spec engine management systems.

Exhaust gas recirculation (EGR) is a system which enables some of the air flowing onto the cylinders of an engine to be replaced with exhaust. It is very widely misunderstood in the aftermarket and by amateur car modifiers. It’s actually a very clever idea, which has few if any downsides. See our EGR article for a lot more information of what it does and why.

In a vented coolant system as used by Subaru and the rear engine VW  models, the expansion tank (also regularly and correctly called a reservoir or reserve) is the unpressurised tank which holds the coolant which passes in and out of the header tank during cooling and warming of the system under normal operating conditions. They are always vented to the atmosphere, and will be around 1/3 full of coolant when cold. They always have  maximum and minimum marks, and the coolant pipe to the pressure regulating cap always connects to the bottom of the expansion tank / reservoir/ reserve tank . Not to be confused with a catch tank.

as well as being one of the cycles of a 4 stroke engine, components referred to as the induction system, in the aftermarket world at least, are usually the air intake system on an engine – everything upstream of the inlet manifold. This usually consists of pipework, the air filter arrangement. Subaru documentation also included the throttle body in a description of induction components, so we do too

The header tank in a cooling system is the tank which the pressure regulating cap is attached to. A header tank is always pressurised when the cooling system is up to working temperature on all modern engines. In a vented cooling system, the header tank can be (and often is) combined with the radiator itself. In a closed cooling system it is always a separate tank (i.e. never combined with the radiator). The header tank also often gets called an expansion tank. This is only correct for closed cooling systems though (in which the air in the header tank absorbs coolant expansion be getting compressed). In a vented cooling system, the header tank has nothing to do with expansion (the expansion is entirely handled by the header tank cap and expansion bottle / reservoir). The header tank of a vented cooling system getting (incorrectly) referred to as an expansion tank is a major cause of the confusion about how the different coolant systems work, and in identifying them.

see Pressure Regulating Cap We usually refer to these as header tank caps, but it makes more sense to describe their function here under ‘pressure regulating cap’.

A widely misunderstood but essential component required to make Subaru cooling systems compatible with the heater(s) in the T25 / T3 / Vanagon (or any other heater which throttles coolant flow to control passenger compartment temperature). Included in all of our coolant plumbing kits.

Note 1: there are some Subaru engines which do not require a heater bypass, especially from certain model later than MY04, in which Subaru used a very different design of cooling system. However, this later system is complex, does not suit using reversed coolant manifolds, and used other hardware in the Subaru which is not practical to use in VW conversions. In general converting them to the earlier cooling system design and using an earlier reversed coolant manifold where required is preferential for all VW conversions.

Note 2: an absolute lack of understanding of the Subaru cooling system and the requirement for a heater bypass is the reason that many early VW to Subaru engine conversions very frequently involved modifying or even removing the thermostat. This includes many fitted, in the UK at least, by companies promoting themselves as ‘specialist Subaru engine conversion installers’ over many years. Despite whatever total crap they’d tell their customers to hide the fact that removing the modifying the thermostat is the only way they could ensure coolant circulation, the fact that they are or were prepared to hack the cooling system to work around the problem which they do not understand tells you everything you need to know about how little they really know or knew about what they are doing. Nobody with even a moderate understanding of engines would ever suggest removing a thermostat, as there are multiple serious downsides to doing so. The only thing remotely ‘special’ about the ‘specialists’ who did this was their ability to successfully market some of the worst quality engine conversions we’ve ever seen for very high prices. The number of obscure mods they did to the systems on Subaru engines which can only be explained by a near total lack of understanding of those systems is really quite shocking.

Japanese Domestic Market – Models produced for sale in Japan (only). Due to their unique emissions laws and higher octane petrol, all JDM Subaru models use different specification engines to those used in any other markets. Mechanically some are quite similar to a particular model used in another market, others have many different parts. Electrically, all the JDM models from MY95 onwards have engine management systems unique to Japan. All service information for JDM models was only published in Japanese, and is very hard to get outside Japan. Parts unique to JDM models are not available fron Subaru outside Japan. JDM models are best avoided unless you’re in Japan – they can be a lot more trouble than they are worth.

Manual Subarus have an input from a switch which detects when the gearbox is in neutral.

Pre OBD II Subaru models: All, or possibly all but one of the pre OBD II models (i.e. the 4 cylinders from MY90 to MY01-03 depending on model or the EZ30 6 cylinder from MY00 onwards for European / ROW spec Subarus – they all have one or no lambda sensors apart from the SVX, which has one per bank of cylinders) are perfectly happy with the neutral position input always in one state – i.e. not connected to a working switch, with no symptoms or error codes. However, whether that state is open or closed does vary from one ECU part number to another. Due to the vast amount of ECU part numbers, the easiest way to determine whether your pre OBD II model is happy with the input open or closed is by trial and error. All of our harnesses have the NPS wires identified, and open by default, so this is very easy. A working neutral position switch can be fitted to the OBD I models in VW conversions, but there is generally no point doing so.

OBD II Subaru Models: Most OBD II Subaru models (i.e. the 4 cylinders from MY01-03 onwards depending on model and EZ30 6 cylinder from MY00 onwards for European / ROW spec Subarus – they all have a lambda sensor after the catalytic converter as sell as before) are generally not happy with a neutral position signal which never changes state. They give NPS error codes. With many of the engines there are no symptoms of this other then the engine check light being on because of the NPS error code, but those which also give symptoms will be similar to if no vehicle speed sensor is fitted. Therefore if you have symptoms, you should fit a neutral position switch.
Note, if you have no vehicle speed sensor, fitting a neutral position switch will not achieve anything – you’ll need both the speed sensor and a neutral position switch.

On Board Diagnostics. When cars first got diagnostic features built into their engine management (typically starting around the late 1980’s), this was not standardised between manufacturers – each manufacturer used their own system. All of those ‘first generation’, incompatible diagnostic systems later became known as ‘OBD I’. Subaru used their OBD I diagnostic system from MY90 to MY00, MY01 or MY03 on European models, MY96 on US spec models, and somewhere inbetween (around MY98) on Japanese models. Error codes can be read from the Subaru OBD I models without any tools.
Subaru call their OBD I diagnostic system SSM (Subaru Select Monitor), and there are two dealership tools for it, SSM I and SSM II. The SSM I tool is very limited in terms of what you can do with it, and barely worth having. The SSM II tool is considerably more useful. SSM I and SSM II tools both show their age now, being early and mid 1990’s technology, both relied on plug-in ROM cartridges (like games consoles from the same era) containing all the relevant memory locations, scaling factors and offsets for all of the parameters that they can measure. However, they’re still surprisingly useful, if they are accompanied by the relevant cartridges for the job you’re doing. As expected for dealership diagnostic gear, both the tools and the ROM cartridges were insanely expensive when new.

Car manufacturers were forced to accommodate standardised diagnostic tools between the mid 1990’s and early 2000’s – exactly when depends on the market the model was built to be sold into. Most still also used more powerful diagnostic protocols for their dealership tools. The standard is widely known as OBD II. Initially good generic diagnostic tools were expensive, with limited features, but they are readily available for next to nothing now,. If you are going to buy one, get one which can display live data – they are vastly more useful, and cost hardly any more. Don’t buy one for use with 4 cylinder European spec Subaru models from MY00 or MY01 before you have read ‘OBD II Connector’ below. Note many OEM’s, including Subaru removed the ability to read error codes without tools (quite common in OBD I systems) when they introduced OBD II compatibility.
Not all OBD II tools are the same in terms of what cars they can communicate with. Different manufacturers transmit their ODB II data via a few different protocols. For European / ‘rest of world’ spec Subaru models from MY00 to MY03 (Legacy / Liberty) or MY07 (Impreza / Forester) use K-line. Later model then that use CAN bus. All of the better, more modern generic OBD II tools can read all of the common protocols, but some older ones may not.
Don’t bother with any aftermarket ‘SSM’ or ‘SSM II’ diagnostic tools *. While they can communicate with the ECU’s non of them have a good enough library of all the memory location, conversion factors and offsets for large numbers of ECU specs (i.e. the data in the OEM diagnostic cartridges) to work reliably, especially on normally aspirated engines.
* There is one tool from EcuTek which works for abut 2 years of OBD I Subaru models, and all the OBD II models, but it is very expensive.

All manufacturers are supposed to use the same connector for OBD II diagnostics in 12V vehicles. A few somehow get around this (BMW), rather like Apple’s refusal to use the same connectors as every other manufacturer. This 16 pin connector has become very widely known as an ‘OBD II connector’, or ‘OBD II port’, etc. This is incorrect, and causes a lot of confusion. It is an SAE J 1962 type A connector. OBD II is one of a few diagnostic protocols which it was widely used for. The confusion comes from those who call it an ‘ODB II connector’. They see the connector in a car such as some MY98 – MY01 Subaru’s which are not OBD II compliant, assume the engine management is OBD II compliant, go and buy a generic OBD II tool, and wonder why it won’t communicate with their car.

Single overhead cam. i.e. one cam shaft per bank of cylinders. Sometimes causes confusion amongst those not familiar with engines, as because there are two banks of cylinders on a horizontally opposed engine, an SOHC horizontally opposed engine has 2 cam shafts. Those unfamiliar with engine terminology can confuse this with ‘twin cam’, which exclusively means double overhead cam (DOHC). Easily distinguished by the much taller cam belt covers having only one round recess per cylinder head, corresponding to the one cam pulley per head.

The original EJ series Subaru engine design, used from MY90 – MY99 in European and ‘rest of world’ spec Subaru models, most US spec models, but not all Japanese spec models.
4 engine to gearbox bolts (actually 2 bolts and 2 studs)
All 6 cylinder head bolts accessible without taking the rocker covers off on all the Phase I SOHC cylinder heads
spark plugs do not fit through holes in the rocker covers all the Phase I SOHC cylinder heads
the phase I SOHC cylinder heads have horizontal ribs cast into them along with ’16 Vave’. Note, all EJ series engines are 16 valve, but none of the other cylinder head designs say so like this one does

The first significant redesign of the EJ series engines. Many changes were made, but the most obvious external ones are as below. Introduced in MY00 European and ‘rest of world’ spec Subaru models, most US spec models, but not all Japanese spec models (some of them used Phase II engines from MY98 or MY99).
8 engine to gearbox bolts (actually 6 bolts and 2 studs)
new cylinder heads design for the SOHC models with smooth rocker covers (i.e. no ribs or text)
cylinder head bolts not accessible without taking the rocker covers off on any Phase II engines
spark plugs fit through holes in the rocker covers on all the SOHC and DOHC Phase II engines

The cap on the header tank or radiator which controls the coolant system pressure. In the rear engine VW bus world, these are usually referred to as header tank caps, because they fir to the header tank. In Subarus, they may fit to the radiator (normally aspirated) or header tank (turbo), so could be referred to as radiator caps or header tanks caps. Explain, include terminology from book, explain unbranded cap issues inc pressure tests, call out JP group and Heritage

A type of clutch in which the release bearing moves from the engine towards the gearbox, pulling against the clutch pressure plate, as the clutch pedal is pressed. Note this has nothing to do with the motion of the clutch slave cylinder pushrod or cable – only the release bearing, as this is widely misunderstood. As used in all the turbo Subaru model from MY90 – MY04, the manual EZ30 Legacy models, and all the higher powered turbo models (> ~250 bhp) from MY05 onwards. ‘Pull to release’ clutches are not just a Subaru thing. They are used by various other manufacturers  too including Porsche, Mitusbishi, Lamborghini, Nissan, pretty much exclusively in higher powered applications, as their benefits only really justify the extra cost and complexity in such applications.

A type of clutch in which the release bearing moves from the gearbox towards the engine, pushing against the clutch pressure plate, as the clutch pedal is pressed. Note this has nothing to do with the motion of the clutch slave cylinder pushrod or cable – only the release bearing, as this is widely misunderstood. As used in all rear engines VW models and all the normally aspirated Subaru engines except the manual EZ30 Legacy models, and the lower powered turbo models from  (< ~250 bhp) from MY05 onwards. ‘Push to release’ clutches in general are far more common than ‘pull to release’.

Secondary air injection is a system which makes the catalytic converter(s) warm up faster following a cold start. The mixture is rich following a cold start, as cold start enrichment is used to make the engine start easily and run acceptably when it is effectively too cold. A rich mixture burns cold. A big proportion of an engines emissions happen before it is up to temperature, and catalytic converters do not work correctly until they are how. Adding extra air to the exhaust as it leaves the engine can solve all of these issues. The extra oxygen added to the exhaust means the unburned fuel from the rich mixture can be re-ignited, and this happens before or in the catalytic converter, increasing the heat and allowing it to warm up to operating temperature much more quickly.
Extra air is added as the exhaust leaves the cylinder hears by pumping it in using an electric pump and two electric valves which are controlled by the ECU.
Note: secondary air injection is not the same thing as air assist injection .

A conventional flywheel is a single lump of iron or steel (i.e. a single mass) which does no active damping of torsional vibrations created by the engine. It’s inertia does some passive vibration damping, and active damping was typically done by a clutch disc with a damper hub. A clutch with a damper hub is easily recognised as it has a visible as a mechanism with a ring of springs around the inside of the friction lining, inside which there is also a friction mechanism.

The thermostat is a cooling system is a temperature controlled valve which changes the route taken by the coolant depending on the temperature of the coolant which is circulating in the engine. It achieves this by using a ‘wax motor’ or wax capsule to turn the heat energy from the coolant into motion. Some vehicles use a thermostat in which the wax motor control two valves simultaneously, one controlling the flow to or from the radiator, and the other controlling the flow through an alternative bypass circuit. No Subaru models use this system. They all use a thermostat which operated a single valve controlling flow from the radiator to the water pump.

A key which as well as the conventional key blade, has an RFID chip inside. The chip verifies the authenticity of the key to the engine immobiliser system. The transponder chip is read over a very short distance by an antenna coil surrounding the ignition lock barrel. Subaru started using transponder key immobilisers in MY96 in certain European spec Legacy and Impreza models. These used a fixed code immobiliser system from Phillips / Siemens , and were used until MY99, by which time most European spec models used them. European Forester models used a transponder key system from Texas Instruments / Denso from MY97 – MY01. From MY00 onwards, Legacy and Impreza models switched to a cryptographic transponder key system from Texas Instrument s/ Mitsubishi, and Foresters used the same system from MY02 onwards.
The fixed code transponder keys have always been clonable with the right equipment, which was why they were superseded. The cryptographic transponders were much more secure when introduced, but as with many automotive transponder key systems, the cryptography used has since been completely reverse engineered, so they can now also be cloned.
We have all the Subaru dealership diagnostic tools to re-match all of the above mentioned Subaru transponder key systems if the system is incomplete or mismatched (i.e. not all from the same donor Subaru). We also stock the transponder chips (only available as part of a key from Subaru), and have the tools to clone Subaru transponder chips too. We only offer these services for VW engine conversion customers.

Tumbler valves, also known as tumble generator valves look like individual throttle body throttle plates, positioned right next to where the injectors inject fuel. They are electronically controlled, and are not throttles. They fully close at low rpm, forcing all the air through a small passage where the fuel is injected. This increases the air velocity, improving the fuel atomisation for emissions reasons. At higher rpm the tumbler valves are always fully open.
Subaru typically used tumbler valves on some turbo engines from about MY03 onwards, and from about MY06 onwards also on some normally aspirated models.

United States Domestic Market – models built to the spec required for sale in the USA and Canada. The USA has emissions laws which are unique relative to all other markets, and all the Subaru engine management systems from MY94 –> for that market are unique to suit.