Subaru Impreza history and technical infos

Subaru, part of the Japanese Fuji Heavy Industries,  is a relative newcomer to the World Rally Championship. Nevertheless its grip is firmly set on the FIA. The company managed to push the WRC class car regulations on Max Mosley (president of the FIA) and got hold of all major decision makers in the WRC. Subaru’s marketing strategy is enormously focused on rallying. That fact constitutes an excellent basis to put in place the world’s best rallying team using some of the best drivers, advanced technologies and cars today.

The Subaru Imprezas are prepared for competition at Prodrive‘s plant in England. Prodrive is an extremely professional and well managed team (lead by David Richards). Their drivers for the 1999/2000 seasons are:

  • Juha Kankkunen (gravel and snow specialist, does not like tarmac very much)
  • Richard Burns (gravel specialist, sufficiently fast on tarmac)

Most WRC events are run on slippery surfaces so Prodrive’s choice of drivers that perform best on this type of surface seems logical. But enough politics lets see what the car is like. There is no special history behind the Impreza. The car was chosen as a potential WRC champion mainly because it was smaller (lower inertia) than the Legacy, its predecessor. Subaru clearly wanted to be world champion so they put a lot of hard work into the Impreza, Prodrive took on from there to lead the car to the world title.

Launched in 1994 the Impreza was immediately available in a 4 wheel drive turbo charged version delivering 211Bhp (217Bhp for the 1999 European model). The car’s main peculiarity is its engine. It’s a 4 cylinder boxer, all aluminum construction mounted longitudinally at the front. Boxer engines have the advantage of lowering the center of gravity of the whole structure. In the Impreza’s case things got quite complicated due to the fact that the engine is turbocharged. The intake tubes are on the upper part of the engine while the exhaust tubes are, partly, on the lower part. The turbo charger is mounted on the right side of the engine. Mounting the turbo this way is quite new and certainly one of the many achievements of Subaru in the Impreza. Since the exhaust tubes use significantly different lengths to reach the turbo charger it is relatively difficult to assure a constant charge of the compressor wheel. The biggest drawback of the Impreza boxer engine is the proximity of the inlet manifold to most heat sources in the engine bay. Additionally the intercooler is located above the engine, which is definitely not the best place to put it, hence the big central air vent on the bonnet. A similar choice, for the intercooler placement, in the Nissan Sunny GTi-R of old resulted in engineers calling it inter-warmer…Locating the intercooler this way results in relatively high intake air temperatures of around 50 °C and thus reduces the engine’s efficiency. The racing version of the Impreza uses an intercooler which is relocated in the front bumper.

The engine being mounted longitudinally, the gearbox and transmission are much easier to fit and service than they would have been in a transversally mounted engine configuration (like in the Lancia Integrale or the Toyota Celica GT4 and Corolla WRC for instance). Speaking of transmission nothing but classic things here in the street Impreza version. Free differential in the front axle (like most other 4WD turbocharged cars with the exception of the Mitsubishi Lancer Evolution V and VI), viscous coupler in the central epicyclical differential while a second viscous coupler is fitted in the rear differential. The street version of the Impreza has three major drawbacks: low performance tires, an imprecise, slow gearbox (dating 30 years back to the original Subaru design) and too much steering wheel travel from lock to lock. While the first is easy to correct (you’ll have to fit 16″ or 17″ wheels and performance tires, note that since 1998 the Impreza turbo uses 16″ wheels but is still fitted with low performance tires) the second and third ones are a pain in the back. Kits are available to shorten the gear-lever travel while mounting the whole lever structure on Teflon cushions (quick-shift). There are no kits to improve on the too low ratio steering however. These modifications done (plus a suspension and injection chip upgrade resulting in a total of 240Bhp or more, at the expense of overall reliability) the street Impreza is a very fast car. I’ll state the English magazine CAR which tested the (modified) Impreza on track some years ago and declared: “If you can’t drive this car fast you can’t drive fast.” I think that says it all.

Nothing really revolutionary, except for its engine architecture, is to be found in the commercial Impreza GT turbo. Classic McPherson struts front and rear, vented disks (and recently 4 pot fixed brake calipers) at the front while plain disks and floating calipers are used at the rear. The central differential locking device (a viscous coupler) has been “detuned” in order not to interfere with the standard fitted ABS and this slightly affects the car’s handling. A stiffer viscous coupler would have been a better choice from that perspective. The turbocharger used in the European model comes from the Japanese manufacturer IHI and is relatively small compared to those used in similar cars (the Toyota GT4 and Lancia Integrale for instance not to speak of the truck sized turbo fitted in the Ford Escort RS Cosworth). This choice favors low end torque and reduces turbo lag but can, sometimes, be cruelly felt at high engine speeds where the “small” compressor wheel’s output drops significantly. The torque split set by the central epicyclical differential is 50-50 front-rear and serves well, in all conditions, the judiciously balanced and stiff body of the car.

As you realize the street version of the Impreza, below its cheapo outfit (which get less cheap looking as the years go by), is a car for drivers that want a car providing the thrills with no frills. Japanese manufacturers are the only ones able to still produce that much of a car for such a low price.

The commercial success of the Subaru Impreza turbo is already impressive. No other four wheel drive turbo charged car has been produced and sold in so important numbers. Rumor has it that Subaru is selling more Impreza turbos in Europe than “normal” Impreza versions. The main reasons for this success are the Impreza’s price to performance ratio, its racing successes and its reliability. Add the fact that most car related magazines largely overrate its abilities and you get the huge commercial success the car enjoys. The Impreza turbo is, essentially, a hassle free car to be used everyday the whole year round. Under the family sedan outfit however lurks a road hungry speed monster.

Things (and performance) are, of course, quite different in the competition Imprezas. The GroupA Impreza GT Turbo has scored a total of 11 WRC wins while its WRC class brother has scored 19 WRC wins already. Only the Audi Quattro has scored more WRC wins with a total of 21 so it is no wonder the Impreza has captured the attention of so many fans and is almost a cult car by now.

The car’s commercial success has lead Subaru to produce many awesome versions of the Impreza turbo most of which are, unfortunately, not available outside Japan. The most impressive Impreza to date is certainly the WRX STI 22-B version. The car was designed by Peter Stevens the designer of the McLaren F1 road going car himself and is arguably one of the most attractive ones on the market today. The 22B, a WRC Impreza look-alike, contains some very elaborate parts in both its suspension, transmission and engine internals (a one piece steel crankshaft for instance). The car’s output is only limited by Japanese regulations which set the maximum engine output of any Japanese car to 280 Bhp (which is exactly the WRX’s engine theoretical output but rumor has it the real output is closer to 350 Bhp). The chassis, brakes, engine management and cabin equipment are all top-spec and clearly express the car’s main purpose: the fastest A to B times possible. The WRX version of the Impreza turbo was engineered by STi (standing for Subaru Tecnica International). This version sports a GroupN spec gear box, stronger front and rear differentials and heavy duty drive shafts. Additionally die-cast suspension parts and custom suspension springs and dampers allow the driver to fully exploit the car’s potential (if he/she dares). In the engine department the WRX Impreza adds a steel crankshaft, new pistons, new cylinder heads and a bigger turbo than the standard version. The exhaust is also specific and made, in part, from stainless steel. These engine modifications allow the red line to be pushed to 8,000 rpm! Subaru finally used adequate wheel sizes on the WRX Impreza eliminating one of the major drawbacks of the standard version. The rims are now 17″ in diameter.

The result is a four wheel drive, two door saloon that’s got the potential to leave the line as fast as a Porsche 911 RS, cracking the 0-100Km/h dash in just 4.9 sec on its way to a 235Km/h top speed. On top of all this version of the Impreza can corner much faster than the standard spec and is at least 1.5 sec faster per Km on open roads.

Unfortunately if you wish to buy one of these beasts and you live outside of Japan you are on your own. The only way to get your hands on one is through a personal import channel, which is still feasible but complex, and the total cost of such a purchase adds up to approximately US$ 50,000.- It’s still worth the money though.

Note that the Impreza GT Turbo and the Mitsubishi Lancer Evolutions, as most other “homologation specials“, are not street legal in North America mainly because they fail to meet Emission Control Regulations in these areas.

Apparently Japanese manufacturers are the only ones, today, capable of producing such cars. Why is it that Europeans seem not interested (capable) in designing and producing thoroughbreds such as the Subaru Impreza WRX or the Mitsubishi Lancer RS Evolution V is a mystery to me. Having such arguments in any manufacturer’s catalog is a powerful marketing tool which can boost the sales of the whole line of cars produced.

Ultimately the Impreza GT turbo has naturally evolved to become a multiple World Rally Champion with three world championship titles already on its account. The car continues to progress as the years go by but changes occur now mostly under the hull. The latest (and greatest) WRC Impreza yet is the P2000 version that appeared for the first time in the 2000 Portuguese rally only to win it, driven by Richard Burns. The main evolutions that took place, as compared to the previous version, are a better under the hood air circulation, refinements in the suspension layout and geometry as well as more elaborate differential control units. The Impreza P2000 WRC also innovates in the exhaust domain with the back-box now holding an integrated catalyst. The exhaust tubing is now made out of the extremely expensive INCONEL alloy like in many other WRC cars. These latest cars are fitted with fly-by-wire throttle control (no cable is used between the gas pedal and the inlet manifold) as well as sequential gearboxes with switch gear knobs fitted on the steering wheel.

The current competition version of the Impreza is a WRC class car. It has been engineered by Prodrive‘s chief engineer and technical director David Lapworth. Lapworth has worked with Peugeot in the rallying world where he collaborated in the development of the famous Talbot Sunbeam Lotus Group B car. This car was world champion in 1981 driven by the legendary Henri Toivonen.

Lapworth met with Richards, who was already managing Prodrive at the time, in 1981 and has worked with him ever since. Prodrive was preparing Porsche 911s for Toivonen and other drivers, they have been involved in rallying for more than 17 years now.

The competition Impreza is a car clearly built to win. The car is relatively compact and its mass distribution is almost optimal (56% at the front wheels and 44% at the rear). Its overall weight (unlike most modern cars) is relatively low, the car weights only 1235Kg in its street version. Prodrive had no trouble meeting the lowest weight limit for both Group A cars and WRC cars (1230Kg).

Subaru and Prodrive have been very active in developing and testing active (hydro-electronic) differentials. These were first introduced on the Toyota Celica (in 1986 I think) but were not used on racing cars at that time since Toyota Team Europe were unable to build them reliably enough (they went back to viscous couplers at the time). For the record I’ll mention that Toyota Team Europe were also the first to introduce the ALS (a.k.a. bang-bang) engine management system in their cars in 1993. Subaru are the current masters in electronically controlled differentials. The current WRC version of the Impreza carries 3 computer controlled, high pressure hydraulic pumps that manage the front, rear and center differentials.

This system works as follows:
Each differential holds a kind of clutch which is controlled by the hydraulic pressure applied by a pump (these are high pressure pumps that can deliver upwards of 50 bar pressures). If the sensors, connected to the in-coming and out-going axles, detect slip the hydraulic pump applies more pressure to the clutch inside the differential in order to lock it. Depending on the hydraulic pressure applied the lock force varies constantly as related to the information sent by the sensors to the computer that controls each differential. Using the same system the driver can distribute torque between each axle from inside the cabin while the car is running. By doing this, the driver overrides the computer controlled circuitry and decides whether the front or rear axle should get more torque from the engine. The driver can, thus, apply more torque on the rear axle when racing on tarmac, equal torque between axles on gravel and even convert the car to a simple (well almost) front wheel drive. Additionally there’s a switch on the car’s dash that allows the driver to lock the center differential in order to allow spin-free starts on wet tarmac or snow stages.

Note that the system’s principle is not new. The Ford RS200 and Lancia S4 Group B cars had a similar mechanism (although not as sophisticated and mechanically controlled) that could change torque distribution between axles from inside the cabin through a lever. The RS200 had this device installed even in its street version although it was disconnected on most road cars.

The year 2000 version of the Impreza (the P2000 WRC car) adds even more sophistication to the technology used in previous cars. The center and rear differentials, although still active, are almost fully locked all the time. This results in a car that handles better at the limit, both on gravel and tarmac, but is less versatile in tight corners. A redesigned roll-cage further rigidifies the car’s body. The 1999 version of the car introduced an electronically actuated accelerator (there’s no cable linking the pedal to the inlet manifold anymore, this system is known as drive-by-wire). This device, in conjunction with the ECU has a non-linear fuel injection characteristic whereas classic butterfly systems always use linear airflow to position functions. Subaru now use a semi-automatic, sequential gearbox which is operated by switches mounted on the steering wheels (just like F1 cars). It took more than four years to develop and test the new gearbox which now seems a big advantage over those used in other cars. The driver doesn’t have to leave the steering wheel to shift gears.


The Impreza in its WRC version

The year 2000 Impreza (the WRC versions) gear boxes were custom built at Prodrive. The company tried the XTrac boxes some years ago but had a big enough budget to develop its own gear box. Sequential gear boxes work much like the ones fitted in motorbikes.

You push the gear lever to switch to the lower gear and pull it to switch to the higher gear. Most rally teams use sequential boxes and Subaru were the last ones to join the club. Although Prodrive did test their sequential box years earlier they never homologated it with the FIA and it was only fitted in 1999 in the competition cars. Sequential gear boxes are more troublesome in cars than they are in motorbikes.

The high torque output of car engines is much more difficult to transmit and that’s the reason why you see so many competitors having trouble changing gears after some special stages. By the way do not think that cars such as the BMW M3, Ferrari 355 F1 or the Alfa Romeo 156 have solved the problem. The “sequential” option on these cars is, in fact, a normal 5 or 6 speed gear box controlled by an hydraulic circuitry. The gear switching is still H-pattern based. A classic (real) sequential box (i.e. non H-pattern based) goes for US 30,000 and up from specialized companies such as XTrac. Quaife and Hewland.
A last word on sequential gearboxes: how do they compare to the classic dog-boxes used in top rally cars until recently? Well drivers say that after testing the sequential versions they will not go back to dog-boxes. I find that although the convenience of a sequential box is very important, not being able to “jump” gears (i.e. 6th to 2nd in one movement) may be an inconvenience. The time gained when normal shifting occurs is lost whenever gear “jumps” must be performed. The major advantage of sequential boxes, apart from the quicker gear shift, is that they free up the driver’s left foot (more than classic dog-boxes do) since no use of the clutch is required except for engaging first gear. This fact enables those drivers that brake with the left foot (a driving technique introduced on early Saab 96S cars back in the late sixties and currently used by Kankkunen, Mäkinen, Sainz and many others) to be more efficient.

One can only compliment Subaru for offering the general public a car such as the Impreza turbo. In these days of speed limitations, emission controls, excessive passive and active “security” features Subaru were able to offer a package really destined to the car-loving driver. The Impreza was, up to recently, the only 4WD turbo charged pure breed to be commercially available almost everywhere.

A word on the 2001 Subaru Impreza Turbo WRX

Commercially available Impreza models from 1996 through to 2000 suffered common shortcomings among which one can list:

  • A body that is far from being rigid and can cause handling imprecision when over-stressed
  • An engine sump that lacks baffles and can lead to oil starvation and catastrophic engine damage in hard-cornering
  • Weak brakes with spongy feel
  • The worst intercooler position one can use (on top of the engine)

Subaru tried to address some of these issues in the 2001 Impreza whose technical specification can be found here. In some cases it succeeded but failed for the most. The 2001 Impreza mainly addressed the body softness issue and displays a torsional stiffness increased by 185% when compared to previous models while longitudinal stiffness is up 250%. These figures are impressive but underline the previous model’s main deficiency. The 2001 car handles much better and is far more precise than previous models.

The new edition’s brakes are improved over those of previous versions, both in size and feel, they are not, however, more efficient as the car’s overall weight was increased by more than 120kg and now reaches 1430kg, far too much.

The engine has undergone a vast modifications program which, unfortunately, only addressed emission control issues. Consequently the power output is similar to that of the previous Impreza and can hardly compensate the increased weight. The car’s performance figures are way off those of the previous version with an additional second in 0-100km/h times. This fact alone is a big disappointment to the sporty driver. Note that although the intercooler capacity was increased by 11% its position has not varied as it is still located on top of the engine. Engine modifications include a new, Mitsubishi sourced, turbocharger which is bigger than the one fitted to previous versions and now blows a 0.93 bar maximum boost. The intake manifold is also new and is designed to create inlet gas turbulence at certain engine speeds and throttle positions in order to increase the engine’s responsiveness. An additional catalytic converter has been fitted ahead of the turbocharger and greatly limits the gases flowing to the turbine. As you see most of the modification regarding the engine are targeting its compliance with the most stringent emission control regulations. As a result the new car complies with the latest US Emission Controls and is therefore imported to this country whereas previous versions weren’t. Although we do not want to disillusion  our American friends they have to know that the Subaru Impreza WRX Turbo imported in the United States is a long way from previous vintages of the same model.

Chassis modifications were also extensive and the car now sports 17″ wheels, one of the greatest shortcomings of previous models were its undersized wheels and the inability to fit bigger aftermarket ones as the wheel arches would not allow it.
Another problem carried over the multiple Impreza models from 1995 on has been addressed on this latest model. We’re talking about the huge lock to lock travel which used to be 3.9 turns and is now reduced to only 3 turns still a bit high but far better than before.

The car is 65mm longer than the previous version but its wheelbase only gains 5mm. This of course means additional inertia and an emphasized tendency to understeer due to greater overhangs both front and rear.

A downside is the absence of bonnet vents which, although fitted in previous models, were blanked by plastic caches. The 2001 Impreza lacks the vents altogether. Bonnet vents were useless on standard, showroom cars but could be made functional and effective on customer upgraded, “chipped” cars by removing the caches. The total absence of vents on 2001 models can prove tricky to overcome on modified cars as under bonnet temperatures rise sharply when the power output is increased.

Overall Subaru could no longer constitute an exception in the motor world with its top of the line 2001 Impreza. The car was designed to appeal more to Motor Vehicle Office officials than to thrill loving drivers. This is certainly a shame but was there any other choice? We feel the answer to this question is, No. The era where cars could be designed to charm the sports driver over the environmentalists is certainly over and the latest Impreza WRX is sufficient proof of this claim.

At least the 2001 Impreza doesn’t have ESP…yet.

Note that the presentation above deals only with the street version of the car. The competition, WRC class car, can only have improved over the previous version!



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