We've been wondering aloud for many years; why doesn't Subaru release a turbo version of their relatively large 2.5-litre boxer four? Well, it's finally happened. On January 6th 2003, it was announced that the North American market would be treated - exclusively - to a 2.5-litre single turbo engine for their newly introduced Impreza STi. It will be the most powerful four-cylinder production car available in that part of the world.
Under that scoop'd aluminium bonnet resides an EJ-series flat-four with a 99.5mm bore and 79.0mm stroke - its total swept capacity of 2457cc is around 25 percent larger than the usual 2.0-litre Subie turbomotor (which uses a smaller bore and stroke). The block is a strong 'semi closed' design, containing forged aluminium molybdenum coated pistons, forged high-carbon steel con-rods and sodium filled exhaust valves. Static compression ratio is apparently 8.2:1.
The DOHC, 16-valve heads feature variable inlet camshaft timing (which Subaru dubs AVCS, or Active Valve Control System) and serves to maximise efficiency across the rev range. Note that AVCS also improves emissions - something that, up until now, has kept the STi out of North America.
A single IHI VF-series turbocharger blows into the 2.5-litre's belly through a top-mount air-to-air intercooler, which is larger than that found on the conventional WRX. An airflow 'splitter' is integrated into the high-rise bonnet scoop, and a water spray system incorporates updated nozzles. The water tank lives in the boot.
The North American STi thrusts out a substantial 224kW (300hp) at 6000 rpm and 408Nm (300lb-ft) of torque at 4000 rpm. This compares favourably to the Japanese-spec STi, which - with a 2.0-litre capacity but running higher-octane fuel - produces 206kW (276hp) at 6000 rpm and 394Nm (291lb-ft) at 4400 rpm.
Given the 25 percent increase in swept capacity, it's safe to assume that the 2.5 STi will offer much improved low rpm torque than conventional 2.0-litre versions. Note that, for the first time on an Impreza, electronic throttle control is also employed. The 2.5 turbo's quoted redline is 7000 rpm - a few hundred rpm lower than the 2.0-litre STi.
Driving through a 6-speed manual gearbox (with a short throw shifter) the US-market STi should accelerate from zero to 62 mph (99.8 km/h) in around 5.0-seconds, depending largely on the launch.
So what's the reason Subaru has only now produced a 2.5-litre turbo? We can only surmise it can be tied to strong sales of the WRX in North America and the existing 2.0-litre STi was unable to meet US emissions standards. It was probably also compromised by the relatively poor US-grade pump fuel.
The driveline of the new STi is a quantum leap ahead of the previous model; like the Mitsubishi Lancer Evo 7/8 it now uses a driver variable centre differential. A Driver Control Centre Differential (DCCD) is used to alter the front-to-rear torque split. In auto mode, DCCD varies torque split depending on conditions and driving style. For manual adjustment, a small thumb wheel can be used to send up to 65 percent of drive to the rear wheels. This reduces understeer and provides handling more like a rear-wheel-drive vehicle.
In addition to the centre differential technology, a Suretrak viscous limited slip front diff improves handling agility and stability. The Suretrak unit "responds to a torque differential between left and right wheels to increase traction and reduce understeer". A mechanical rear differential is also fitted, ensuring maximum traction.
Chassis rigidity is also a strong point (uh-oh, nearly a pun). A "super stiff ring-shaped reinforcement frame body structure with a hydroformed front sub-frame" provides a very rigid platform. In addition, strengthened mounts for the front transverse suspension linkages aids handling and steering.
Similar to the current twin-turbo Liberty (Legacy), the STi runs inverted front and rear struts in order to minimise unsprung mass. Spring and damper rates are firmer than those in the normal US WRX, and the STi rides 0.4-inch closer to the bitumen providing a lower centre of gravity.
Road holding is a task for beefy 225/45 Bridgestone Potenza RE070 'summer radials' mounted on forged aluminium 17 x 7.5-inch BBS rims.
Braking is by virtue of mammoth 12.7 x 1.2-inch (323 x 30mm) ventilated front discs and 12.3 x 0.8-inch (312 x 20mm) ventilated rear discs. Biting onto these are four and two pot calipers respectively. EBD (electronic brake force distribution) and recalibrated 'Super Sport' ABS are also fitted.
A quick-rack steering arrangement delivers a sharp 15.0:1 ratio compared to 16.5:1 in the standard US WRX.
In order to keep kerb mass to a minimum, the standard US-spec STi comes with thin gauge rear glass, aluminium front control arms, forged aluminium wheels and void of a sound system - an optional sound system can be purchased at extra cost. On the other hand, standard equipment includes power windows and mirrors, climate control, central locking, airbags, Momo wheel and wrap-around seats - it's hardly a 'gutted' racer. Kerb mass will be similar to the Australian-spec 2.0-litre STi, at 1470kg (3234lb).
Aerodynamics are improved with the 2003 Impreza facelift (MY04 in America) and the STi sports front bumper corner spoilers, HID (high intensity discharge) headlights, 'ground effects' and a dual element rear wing to reduce rear-end lift. This wing is very similar to that fitted on the current WRC STi.
For further information on the US-spec 2.5-litre STi check out www.Impreza.Subaru.com
Other Developments Within Subaru
Crankcase and Pistons
One of the inherent advantages of the Subaru flat-four engine layout is its relatively short length; this improves packaging and weight distribution. To combat the weight penalty associated with a boxer (or 'V') layout, however, the crankcase and heads are made from aluminium. Furthermore, the flat-four crankshaft doesn't require the counterweights that are necessary on conventional in-line engines.
Through experimentation Subaru determined that "a piston that moves up and down in a smooth rolling-like motion is ideal." In practice, though, a conventional piston skims the side of the bore at various points - hardly the optimal smooth rolling-type motion. To combat this, a new barrel-shaped piston 'rolls up and down the bore' while improving friction, vibration and noise. Piston skirt design also strays from the norm with short, thin, soft skirts - this minimises weight. Combustion efficiency is improved by reducing the size of the cavity between the top of the compression ring, the side of the piston and cylinder bore. Ring groove wear is minimised through use of an alumite treatment (which is applied to the whole piston head), while the skirts are coated with molybdenum in order to reduce friction. With these technologies combined, the new Subaru pistons are internally referred to as LF (low friction) pistons. Fuel consumption is reputedly improved by 0.5 to 2.3 percent.
Subaru claims the boxer engine layout is ideal for a turbocharger application. The large space available toward the rear of the engine allows engineers to freely choose the appropriate turbo size. With the turbocharger in a rearward location, the path into the intake manifold is also very straight and short, which aids throttle response. It is also said that the smoothness of the flat-four layout reduces the chance of turbocharger break down.
The turbocharger fitted to an Impreza WRX spins at 130,000 rpm with an engine speed of 3000 rpm. This is a fairly typical crank to turbine rotational speed difference. Of course, the upshot of the Impreza turbo system is engine performance similar to a 3.0-litre engine, but with better fuel efficiency and emissions standards.
A considerable 25 percent of all Subaru vehicles sold are fitted with a manual gearbox. Subaru says, "the basic design for the 5-speed transmission currently fitted to the Legacy and Impreza has been in use before these models appeared and dates back to the Leone era". While the design of the gearbox is along old lines, the quality of the gears has been much improved; this is required to cope with the significant torque output of the turbo engines. All gears are put through a heat treatment called carburizing and quenching and the surface is further hardened through shot peening. Note, however, an all-new 6-speed gearbox is used in the current STi models.
For further information on current Subaru technology, visit the company's worldwide site www.subaru-global.com/about/parts/index.html