Technical Documents

Evolution Of The STi

Subaru Tecnica International (STI) was created with the foremost task of establishing a full scale Subaru entry into the World Rally Championship. STI managed the contract to build the first factory Group A Liberty as well as developing much of the technology behind it.

Whilst preparing to enter the WRC, STI campaigned the Liberty in the local Japanese domestic rally championship and from the beginning supported and worked with Possum Bourne when he ran the Group A Liberty and later the Impreza. Interestingly Possum had a motorsport connection to Subaru several years before STI was formed. STI supplied engines for the Group A Liberty and later the Impreza WRX and Impreza World Rally Car.

STI was still a small group in 1992 when the Impreza Group N car was first tested. It was at this time that Australian engineers started working on tests and events with STI engineers.

Overwhelmed by the success of what was initially a small group, Subaru has invested significantly in STI, given the people and the technology to support a diverse range of Subaru motorsport activities. Along with WRC, STI develops the Subaru technology and components behind the Production World Rally Championship, significant regional Rally Championships such as the ARC, and SCCA Pro Rally Series in the US. In the Japanese circuit STI supports teams in the All Japan GT Championship and Super Endurance Race Series.

STi GD Chassis Drivetrain

Focusing on Group N Rally, Australia’s outright rally class, STI controls the development, technology and homologation of the Subaru Group N vehicle competing in the PWRC, ARC and all Group N Rally Championships around the world. STI works closely with all the factory and leading Group N teams, intensively helping the teams to collectively develop the technology of their cars and each year co-ordinating the feedback from these teams into a more competitive set of parts or more advanced complete Impreza WRX STi vehicle for the next homologation. A lot of this technology also filters down from STI’s involvement in the development of the WRC car. In fact this is the process that drives the development of the Subaru and especially Impreza WRX STi that you drive today.

So, from the beginning of the Group A and Group N Impreza, STI began developing a number of suspension and engine competition parts for race and rally. This has grown into a comprehensive STI catalog, covering Impreza and Liberty for dress up, performance and special motor sport parts.

Subaru and STI have won many times in the history of WRC and ARC competition, but the core technology and motorsport philosophy has remained the same.

Premium technology represents both excellent engineering and the intensive effort of all the people who work for STI and Subaru competition teams around the world with passion and craftsmanship.

Historically, the STi (like all other Subaru models) has taken to referring to the car in terms of model years: MY00, MY02, etc. As with many car makers, Subaru starts selling its next year’s model late in the preceding year. For instance, in September 2007 the MY08 went on sale in Australia.

This history will focus on Australian spec WRXs which ofter differ from JDM (Japanese Domestice Market) and other foreign market WRXs (such as the UK and the US). The exception to this are the MY94-97 STi models that were released as JDM models only, but are included to give a starting point of the Impreza STi’s origins.

This history will not include limited special edition STis such as the S20x series, Type R, Type RA or Spec C models unless they were offered and sold by Subaru Australia.


STi Version I

The first Impreza STi was launched in January of 1994 and was a response to Mitsubishi’s Lancer GSR Evolution.  The Lancer GSR was quicker than the standard Impreza so WRX’s were taken to Subaru Technica International to be fitted with a tuned and blueprinted engine. This STi came with forged pistons, lightweight valve adjuster system, polished intake ports, a revised TD05H turbo and a remapped ECU. The spec was the same for both the Saloon and Estates with the engine producing 185kw at 6500rpm and 310Nm of torque at 3500rpm.

Extras to come with the STi included Intercooler watersprayer, uprated intercooler ducting, a larger bore Fujitsubo exhaust system, loads of pink STI badges, an STi chassis plate on the offside front suspension tower, front strut brace, uprated brake pads, stiffer STi springs and dampers and the now familiar high level rear boot spoiler.

Inside was red stitched Nardi steering wheel, STi gearknob, STi dials and STi semi bucket seats. The STi was also available with a large range of options including a limited slip diff for the rear axle. The Impreza STi was available in Red, Silver and Black with the Version 1 STi using the same gearbox as the WRX.

The Type RA (Race Altered) STi was launched in November of ’94, it was similar to the STi but was lighter, more powerful, having 202kw at 6500rpm  and 320Nm at 4000rpm. The Type RA also had a diver controlled centre differential (DCCD) with a rear limited slip differential which allowed between 50 and 65 percent of power to the rear wheels via a 5 step adjustment. The Type RA also came with closer gear ratios which made acceleration quicker, WRX style front foglamp covers, 16inch gold alloy wheels, silver and black strut brace, rally style roof vent, a faster steering rack and Type RA badging on the boot lid.

Both the Version I STi and STi RA models were built to order only.


In August 1995 the C Spec STi Version 2 series was released. This Impreza was similar to the original STi but power was hiked to the same levels of the old RA model:  202kw at 6500rpm and 320Nm at 4000rpm. Interestingly, the standard STi came with the red induction manifold but the Version II RA which followed shortly did without. The RA also had less power with 195kw and 312Nm. Also released was the 555 WRX STi saloons and Estates which came which a deluge of stickers, a roof vent.

In early 1996, STi released 1000 units of the the Version 2 V-Limited edition and was based upon the earlier Type RA but came without stickers. A white only V-Limited Type RA was released a month later, this had a polished exhaust system and minor internal changes.

Like the Version I, the all variants of the Version II STi were released as JDM models only.


STi Version III

Subaru brought out a heavily updated STi (D Series) Version III STi at the end of 1996 which reached the Japanese market ceiling of 206kw at 6500rpm. Torque increased to 343Nm at 4000rpm. The intercooler on this car was mounted flat as opposed to the slightly tilted position on earlier versions. The Type RA now gained the red inlet manifold and a carbon fibre strut brace. The open decked engine boasted lighter forged pistons, molybdenum coated valves, a metal head gasket, a better radiator an improved single bore exhaust system, stronger clutch and an STi quick shifter. The car also featured From mechanical as opposed to hydraulic lifters.

A new face appeared in January 1997 in the form of a 2 door STi Type R. It had a stiffer 2 door body shell, a front lip spoiler, side skirts, a deeper rear valance plus Type R badging and tinted rear windows. It also had an even closer Group N gear ratios, manual control of the intercooler water sprayer, air conditioning, folding mirrors and blue faced gauges. This model was available in Feather White, Sonic Blue Mica or Chase Yellow. Today the STi Type R is one of the most sought after Imprezas today.


The Ultimate STi - STi 22B

The E Series STi version 4 was released in September 1997 with the main changes being in the interior. This consisted of a new dashboard, revised instrument clusters and a central speedometer. Power remained at the maximum allowed, 206kw but torque was raised to 355Nm and 4000rpm.

The E series also had one of the most desirable Imprezas ever. In September March 1998 the 22B (WRX STi Type R 22B) was released with a run of only 425 vehicles of which 5 were brought to Australia. Somewhat of an Automotive legend, this very special 2 door STi was created to celbrate Colin McRae’s winning form in the WRC. They were very expensive at AUS$125,000 each, and that did’t include compliance to enable it to be road registered here in Australia. Only later did Subaru Australia assist owners with registering the car for road use in Australia.

Hand built by STi in Japan, the 2.0L engine was enlarged to 2.2L by boring the cylinders (hence the 22B nameplate) fitting them out with forged pistons and specially matched cylinder heads and cam profiles and punched out  206kw and almost all of its 363Nm at a much lower 2800rpm.

STi 22B Rear

A VF23 turbo was fitted as standard, but the car could also be specified with the larger VF22 as a no cost option. A unique twin-plate clutch fed power to a revised transmission with DCCD. Subaru tests in Japan resulted in a boasted 0-100kmh time of 4.2 seconds which was never repeated elsewhere with a 4.8 sprint more realistic, but nevertheless still frighteningly fast for its day.

Most striking of all was the 22B’s coupe body shape, complete with pumped-out flared front and rear guards over the wider track (20mm front, 40mm rear), aggresive front spoiler and rear wing. Styling cues were taken directly from the championship winning World Rally Car version, giving the 22B a much more muscular appearance than previous 2 door STi models. Wheels were specially made BBS 17×8.5″ wheels with Z rated tyres.

Other notable features of the 22B included a unique interior, better brakes, suspension, and steering rack.


STi Version 5 - first Australian delivered STi

Whilst released in September 1998, early 1999 saw the arrival of the two-door STi Version V coupe in Australia. Only 400 were imported making it highly sort after with its popularity remaining undiminshed even to this day. Available in white or blue, these had the full Japanese domesitc spec 206kw engine, readily identified by its red intake manifold, complete with roller bearing turbo, revised cams and comparatively high boost pressures.

Other Version V STi goodies included close ratio transmission, aluminium bonnet, rear screen wiper and the biggest rear wing this side of a 22B. Oddly enough, Australian delivered models had the old Version IV suspension and not the inverted strut type fitted on the JDM version.

STi Version 5 Rear Wing

The Australian spec car also cam with semi-automatic climate control, and a special in-car security package offering anti-hijack protection.

This car proved to be an instant hit with both customers and the motoring press, who praised the car for its impressive performance and handling, as did customers who voted with their cheque books, ensuring that the entire shipment of 400 cars sold out in double quick time. It was not all good news for Subaru as a certain number of cars fell foul to mysterious piston failures, possibly due to a mismatch between the Japanese domestic engine calibration and Australian 95 RON fuel. Subaru’s response was to reduce peak boost from 17.5 to 16 psi when owners brought their cars in for normal servicing, which effectively reduced power slightly from 206kw.


STi Version 6

Whilst Subaru Australia promised that the import of 400 Version V cars would’t be quickly repeated, the strong demand resulted in Subaru Australia immediately importing an additional 400 STis with the release of the Version VI in late 1999.

Mechanically the Version VI was nearly identical to the previous version with the primary difference being that the Version VI was a four door body shape. Other differences included the adoption of inverted struts, a fully automatic climate control system, a slightly re-styled front spolier, and minor engine management tweaks in response to the piston failures of some Version V cars.

MRT Power Kits

As with the MY99-00 WRX, the MY99-00 STi models come with ability to upgrade the factory ECU using EcuTeK software, negating the requirement to replace it with an aftermarket unit when chasing significant power increases. The ability to tune the factory ECU also allows direct access to factory calibration data, and the ability to use most of the default factory settings whilst just changing those required.

MRT Power Kits

There are no Power Kits for the MY99-00 STi, however MRT have done heaps of work on this model and are able to put together a package to suit your needs.


The Version 7  STi was freleased in 2001, but only in Japan. Essentially the Version 7 is the same as the Version 8 (MY02) model releases in Australia in and is reviewed below. From the MY01/2, the STI was less commonly known by its version number and like other Subaru models known generally by its manufactured year.


MY02 Bug-Eye STi

The MY02 was the first STi in Australia not sold in limited numbers, and was the first STi since inception to have a new chassis (the GD chassis as opposed to the GC chassis for all previous STis)

Based on the “bug-eyed” Impreza, the STi too featured dominant round headlights, however the STi ones were WRC inspired with eyelids that look more purposeful and less like a startled dear! Like the WRX, the body was completely new with improved torsional stiffness, in addition to improved crash safety due to the addition of a large “U” shaped reinforcing brace bolted to the underside of the front chassis rails. Whilst smoothness, feel and NVH all improved directly as a result of the greatly increased torsional rigidity of the new body shell. Like, the WRX, the downside of the new chassis was the weight with the STi now tipping the scales at 1475kg – 75kg more than the WRX and nearly 200kg more than the Version 6 STi.

The biggest change to the STi was the engine which was the first Subaru engine in Australia to feature variable valve timing. Known as AVCS (Advanced Valve Control System), the timing was on the inlet cam only. Other new engine features were forged pistons as standard, the deletion of the up-pipe catalytic converter found on the WRX, intake manifold tumble valves, bigger intercooler, manual intercooler water spray and a new VF35 turbocharger. However, unlike previous Australian delivered STis, the MY02 arrived detuned for our 95RON fuel and produced less power than the JDM versions (and previous Australian versions), with a peak power output of 195kw and 343Nm of torque. Whilst Subaru claimed a mid 5s time for the 0-100kmh sprint, the reality was more like a high 5, flat 6 second time.


STi 6 speed gearbox MY02 STi Detuned 195kw Engine


The weight gain, reduced power, detuned engine map and continued use of a single scroll turbo as opposed to the use of twin scroll turbos on JDM STis produced an engine that delivered above 4000rpm, but suffered from a huge torque hole up until that point. Reviews of the engine were harsh from many automotive magazines, with Autospeed stating that: “there’s no excuse for the fact its tuned-for-Australia turbo engine has the worst torque range of any vehicle on the market.”

MY02 STi Interior

It was’t all bad news though. The highlight of the MY02 STi was the new 6 speed gearbox that was much stronger than the WRX 5 speed, and had a close spread of ratios that helped keep the Australian spec STi’s narrow power band engine on the boil. The 6MT was built completely from scratch, with a new transmission design to fix the strength and shift quality issues of the old 5MT transmission.

Other drivetrain enhancements included a larger R180 rear diff (as opposed to the R160 on the WRX) and stronger driveshafts. The STi also scored larger disc rotos and Brembo brakes on both the front and rear. Externally, the STi looked much like the WRX except for a slightly larger bonnet scoop, foglight covers and headlights.

Like the MY01/02 WRX, the interior was much improved over previous STi models with a new dashboard, instrumentation with a large centre tacho, centre console, 6-stack double DIN CD player, front and rear blue-trimmed seats and interior trim.

Despite the criticism of many automotive magazines, the MY02 won Motor’s BFYB (Bang For Your Bucks), showing that it was still a hard-core machine when driven hard.


MY04 STi

Mechanically, the MY03 STi delivered to Australia was identical to the MY02 with the same detuned 195kw engine and drivetrain.

The obvious change was the exterior styling, which the ditching of the “bug-eyes” headlights for more shapely candles more in keeping with the Subaru brand, a new front grille, re-touched bumper bar and lightly re-styled rear lights in line with the MY03 WRX. The most striking changes were the huge bonnet scoop, the biggest ever seen on any Subaru and large rear wing(the MY02 STi had the same flat-lid boot spoiler as the WRX).

Like the WRX, the MY04 saw very minimal changes – indeed the addition of a pollen filter to the A/C system was the only change of the previous model.

The MY05 STi featured the first mechanical changes since the MY02, and the most significant was the inclusion of DCCD (Driver Control Centre Differential) – the first time it had been included for an Australian spec STi despite the fact it had been available on JDM models since the late 1990s.


In short, DCCD is a sophisticated mix of electronic and mechanical elements combining together with the aim of maximising the amount of engine power transferred into the road surface, in addition to the basic task of providing differential action between the front and rear axles. DCCD is controlled by a stand-alone ECU that takes inputs from a wide range of sensors and parameters, including lateral “G”, brake light switch, handbrake switch, ABS activation status, throttle pedal position , rear differential oil temp, individual wheel speeds and finally DCCD auto/manual status. The DCCD on the MY05 STi allowed for a torque split from between 50/50% to 35/65% from front to rear.

Other significant changes were bigger wheel hubs that housed much larger bearings, necessitating a change to a bigger wheel PCD of 114.3mm, up from the previous size of 100mm, a slightly increased rear track covered by larger flared rear guards and new HVAC controls and centre console.

The MY05 STi represented perhaps the end of the dominance of the STi in its market, winning Motors BFYB (Bang For Your Bucks) – the last time a WRX/STi has won the award.

MRT Power Kits

MRT Power Kits

If there was ever a car that qualified for most requiring a better engine map, the MY02-05 STi would be it. There is so much potential in the engine that is wasted by the ECU tune, and a retune using EcuTeK software transforms this car with significant power and driveability increases. The ability to tune the factory ECU also allows direct access to factory calibration data, and the ability to use most of the default factory settings whilst just changing those required. As a result, in addition to all other custom modification work offer, MRT offers three Power Kits based using EcuTeK.

  • XA kit: Consists of Type 1 EcuTek upgrade, 3″ muffler and air intake modifications for a guaranteed 20kw power increase and up to 15% torque increase over standard.
  • XB kit: Consists of Type 1.5 EcuTek upgrade, full 3″ exhaust system from the turbo, cold air intake and new intank fuel pump for a guaranteed 40kw power increase and up to 28% torque increase over standard.


MY06 STi

Like the MY06 WRX model, the STi saw the Subaru style and design statement make another big statement with the contraversial new front grille based upon the new corporate grill based on FHI’s (Fuji Heavy Industry) aviation beginnings. Whilst not as poorly received as the bug-eye, it nevertheless received much press attention and was known by Subaru fans as the “pig nose”.

The exciting news with the MY06 model was replacement of the old EJ207 2L engine with the new EJ257 2.5L motor that had been introduced in the US back in late 2003. Complete with a revised AVCS system, electronic throttle control and larger exhaust system with a single tip muffler, the engine gave the STi back its old power figure of 206kw and torque increased to 392Nm, giving the STi the genuine ability post mid 5s for the 0-100kmh sprint.

The increase in engine capacity in addition to a retune of the ECU removed much of the torque hole that had plagued the MY02-05 STi, giving the car much improved tractability and driveability. Another significant change was the addition of a steering wheel input to the DCCD system and a changed max torque split of 41/59 fron to rear, giving the car even better handling on the road and track.

EJ257 2.5L Engine

Internally, the MY06 and MY07 models were pretty much identical to the MY05.


The MY07 is virtually identical to the MY06, with only small changes such as the addition of a roof wind spoiler.

Limited Editions

  • Whilst special editions of the STi such as the Type R and RA were available for most versions of the STi in Japan, it was not until the release of the MY07 did Subaru Australia offer a special edition STi with the sale of the track only STi Spec C Group N FIA homologated car from their Motor Sports division. Sporting the full treatment from stripped body shell, Japanese specification EJ207 2L engine and upgraded suspension, these cars represented the ultimate weekend car fro use in competition and targas.

MRT Power Kits

The introduction of the relatively low stressed 2.5L engine and electronic throttle control gives a fantastic platform for enhancing the MY06-07 STi. Using EcuTeK software to upgrade the factory ECU and using parts developed, MRT offers three Power Kits.

  • XA kit: Consists of Type 1 EcuTek upgrade, 3″ muffler and air intake modifications for a guaranteed 20kw power increase and up to 15% torque increase over standard.
  • MRT Power KitsXB kit: Consists of Type 1.5 EcuTek upgrade, full 3″ exhaust system from the turbo and air intake modifications for a guaranteed 35kw power increase and up to 40% torque increase over standard.



MY08 STi

The release of the MY08 STi saw the first all new car since the 1994, and a demonstrated a significant shift in Subaru’s focus. Like the WRX it was based upon, the STi was more refined and “user-friendly” than previous models – gone was the harsh ride for a more agreeable ride and manners.

The MY08 STi in terms of body panels this is the most differentiated STI ever. Only the front doors, bonnet and roof panels are carried over from the WRX. Underpinning the MY08 Impreza WRX STI is a strengthened version of the same body that sees the standard Impreza achieve impressive safety ratings and much improved NVH (noise, vibration and harshness).  The STI’s wheelbase is a scant 5mm longer than the WRX (put that down to suspension geometry) though the track increases are a substantial 35mm at the front (to 1530) and 40mm at the rear (now 1540).

MY08 STi Rear

Overall the cars are the same length, but thanks to those flared guards, the STI is 55mm wider than the WRX.

Guards have been flared WRC-style front and rear — the former scoring vents that actually help cool the engine bay. At the rear there’s a four-outlet exhaust system and an aggressively styled under bumper diffuser. Combined with the surprisingly subtle hatchtop spoiler, the diffuser generates enough downforce to negate any aero lift at high speed. The STI also gets unique headlamps (with xenon low beam) and it’s the only Impreza to feature mirror-mounted front indicators.

At the heart of the new STI is a revised version of the 2.5-litre boxer engine that graced the last generation STI. The new engine complies with Euro IV emission requirements and pumps out an impressive 221kw at 6000rpm. Peak torque is 407Nm at 4000rpm. The engine also features a new full CAN-Bus technology ECU that controls the dual Active Valve Control System (variable valve timing and lift on both inlet and exhaust cams) for better driveability.

STi DCCD Diagram

This MY08 STi is also the first Impreza to feature SI-Drive (Subaru Intelligent Drive), offering the choice between three throttle/engine control maps. Intelligent (I) mode improves economy, while the Sport (S) and Sport Sharp (S#) modes offer better throttle response and unfettered access to the engine’s full output.  Other changes to the STI’s powerplant include a larger capacity intercooler and revised exhaust layout with two twin exhaust outlets.

The MY08 STi retains DCCD as implemented on the MY06-07 STi, with torque split through a range from 50:50 (fr:rr) to a rear-biased 41:59 depending on conditions. Thanks to a blend of mechanical actuation and electronic control it responds in milliseconds.

Another first for the STi is the three-mode Vehicle Dynamics Control (VDC) system. The default setting is a conventional, if generously calibrated, stability system. The driver then has the choice to disengage the system completely (one touch of the lower dash-mounted button) or choose a Traction mode which allows more leeway for sideways antics.

Lastly, for the first time the STi was available in two variants, the base STi and the Type R. Both cars are identical except for the 18″ lightweight BBS alloy wheels and Recaro sears

Whilst the spec sheet of the MY08 STi is impressive, it ca’t disguise the fact that the MY08 is not the hard core STi of old. Softer and less explosive than previous models and obvious not targeted to the boy-racers, it is nevertheless a serious car that is much more focused and cohesive than the MY08 WRX that it is based upon.

MRT Power Kits

Whilst the MY08 STi is undoubtably more advanced and softer than previous models, it still has much potential lurking under its skin. With a new, more powerful ECU that can be tuned using using EcuTeK software, the MY08 STi presents many opportunities to regain much of the old STi character. In addition to all other custom modification work offered, MRT offers three Power Kitsbased using EcuTeK.

MRT Power Kits

  • XA kit: Consists of Type 1 EcuTek upgrade, 3″ muffler and air intake modifications for a guaranteed 20kw power increase and up to 15% torque increase over standard.
  • XB kit: Consists of Type 1.5 EcuTek upgrade, full 3″ exhaust system from the turbo and air intake modifications for a guaranteed 35kw power increase and up to 40% torque increase over standard.



Special thanks to: Subaru Australia, AutoSpeed, Carsales, Strike Engine, and Graham Berry for the majority of content and images contained in this article.

Leave a Reply

Your email address will not be published.

No products in the cart.