The best high-performance cars are the result of a lengthy design and engineering process that comes to defines the brand. Building cars according to strict engineering principles over a long period of time establishes authenticity, especially with regard to performance cars. In addition, successful motorsports competition offers the opportunity to prove the validity of those engineering principles.

The 2004 Subaru Impreza WRX STi represents the ultimate development of Subaru core technology, the Symmetrical All-Wheel Driving System that combines a lightweight, high-power boxer engine, full-time All-Wheel Drive and a long-travel, low-friction 4-wheel fully independent suspension. Subaru has proven its core technology for three decades, as the world leader in All-Wheel Drive automobiles and as one of the most successful brands in international rallying, including three consecutive World Rally Manufacturers' Championships with the Impreza.

The 2004 Impreza WRX STi , built for the North American market, combines the largest-displacement engine ever offered in this model with an advanced Symmetrical All-Wheel Driving System previously available only in other markets. As a result, North American driving enthusiasts enjoy the best STI high-performance elements in a package designed specifically for them.

It is important to note that the 2004 WRX STi is not a modified version of the WRX. Nearly every major mechanical system in the WRX STi is unique to this model - engine, transmission, All-Wheel Drive system, suspension, brakes - even the instrument panel.

The "Boxer" Engine Advantage
Subaru introduced its first horizontally-opposed engine more than 30 years ago and remains a staunch proponent of this engine layout. The Subaru boxer engine is ideal for the All-Wheel Drive application because it is inherently more compact and has a lower profile than "V" or in-line engines. The layout concentrates the engine's mass in a small area and helps provide a lower centre of gravity, contributing to quick handling and steering response. Aluminum-alloy construction of the engine and transmission results in a lightweight drivetrain.

Mounting the boxer engine longitudinally (front to back) allows the transmission to be mounted directly behind it. Power travels in a straight, near-horizontal line to the rear differential, minimizing frictional loss. This symmetrical, uniform layout also provides excellent left-right balance. In contrast, an all-wheel drive system in a vehicle with a transverse (sideways) mounted engine requires additional components to reroute the power from transverse to longitudinal orientation. Such a system adds friction and weight, resulting in power loss.

The horizontally opposed layout also results in an inherently smooth-running engine because the motions of the pistons from one cylinder bank and the vibration the motion creates cancel the vibrations of the opposing bank. Lower vibration contributes to durability, a Subaru hallmark. Some inline and V-type engines require a balancer shaft to help reduce reciprocating vibration, and this feature adds both weight and complexity.

300 Horsepower and 300 lb.-ft. of Torque
Impreza WRX STi is powered by a turbocharged and intercooled 2.5-litre DOHC boxer engine that produces 300 horsepower at 6,000 rpm and 300 lb.-ft. of peak torque at 4,000 rpm. A curb weight of 1,484 kg (3,263 lb) gives the WRX STi an excellent power to weight ratio.

A stiffer engine block was necessary to provide durability under higher boost and power conditions. The 2.5-litre horizontally opposed 4-cylinder engine is based on a specially reinforced semi-closed deck engine block with rigid cast aluminum pistons and forged high-carbon steel connecting rods. A semi-closed deck design provides the same cooling efficiency as an open-deck cylinder block but with the greater strength of a full closed-deck. Special reinforcing ribs in the block provide additional strength.

Active Valve Control System
The STi's dual over-head camshaft (DOHC) cylinder heads use hollow camshafts to reduce mass. Shimless cam followers reduce valvetrain mass to enhance response. The WRX STi powerplant incorporates Subaru's Active Valve Control System variable valve timing technology to optimize the engine's volumetric efficiency throughout the rev band.

Active Valve Control adjusts intake camshaft timing to employ the ideal intake and exhaust valve overlap under all engine operating conditions. The system helps eliminate the traditional compromise between low-end torque and high-rpm horsepower and also helps to enhance fuel economy and reduce emissions.

The Active Valve Control System can rotate the intake camshafts through a range of 35 degrees relative to the crankshaft. The Engine Control Module (ECM) regulates this movement, based on input from various sensors - airflow, engine coolant temperature, throttle position and camshaft position. The ECM generates an electrical output signal to an oil control valve positioned at each intake camshaft sprocket. Oil pressure is then varied within "advance" and "retard" chambers inside the Active Valve Control System actuator. The system calls upon computer maps to respond to various driving conditions, providing ideal valve timing for stable engine idling and optimum torque across the engine speed range.

High-Boost Turbo
The high-boost turbocharger in the WRX STi 2.5-litre engine produces a maximum of 14.5 PSI of boost, compared to 13.5 PSI in the standard WRX 2.0-litre engine. Because the Active Valve Control System allowed Subaru engineers to tune the WRX STi engine to produce greater low-end torque, they could use a turbo optimized for high-end power. This combination effectively addresses an old compromise: smaller-type turbos require less exhaust energy to achieve boost in response to throttle opening, but as a result of their small size they produce less pressure and therefore lower boost and power. Larger turbos can produce more power but their larger turbines take longer to build boost, resulting in greater lag.

Intercooler with Manual Intercooler Water Spray
A turbocharger compresses intake air, increasing its density and therefore allowing the engine to burn more fuel for more power. Compressing the air also heats it, which in turn reduces power output. An intercooler is used to reduce the intake charge temperature, which creates additional power. This also allows the engine to run more advanced ignition timing, for more power, without causing detonation, or "knock."

The intercooler in the WRX STi is significantly larger than the unit in the WRX, offering greater heat transfer capacity.

The large-capacity intercooler features a manually operated water spray system to provide additional cooling. Under high-performance driving conditions, the driver can operate the spray to reduce intercooler temperature and keep power and performance more consistent. Pressing the dash-mounted button operates the spray through a single nozzle in two-second bursts. A reservoir is located in the trunk to enhance weight distribution and ensure a cool water supply.

Electronic Throttle Control
Further enhancing quick throttle response in the WRX STi is a drive-by-wire Electronic Throttle Control. This system eliminates the physical linkage between the throttle pedal and the engine. The throttle pedal is connected to an electronic sensor that sends a signal to the Engine Control Module (ECM), which controls an electronic motor that automatically adjusts the engine's throttle plate. Electronic Throttle Control provides more precise control and more flexible response than a conventional throttle and also helps to reduce fuel consumption and emissions.

Handling The Heat
Subaru expects WRX STi owners to take advantage of the engine's high-performance capabilities and so has fine-tuned every detail to provide durability, especially in regard to temperature control. Piston crown thickness was increased to handle high thermal loading.

Using technology adapted from racing engines, the exhaust valve stems are sodium-filled to aid valve cooling. At temperatures over 138°C (280° F), the sodium melts and splashes up and down in the stem, absorbing heat from the valve head and transferring it to the cooler stem, where it is dissipated into the valve guide and cylinder head.

Iridium-tipped sparkplugs provide greater protection from heat than platinum-tipped sparkplugs. Iridium is one of the densest materials found on Earth and an Iridium-tipped sparkplug requires less voltage to spark, burns fuel more efficiently, sparks at leaner air/fuel mixtures, and delivers higher horsepower and better fuel economy.

The exhaust system features a low backpressure muffler and a single-outlet chrome sport tailpipe.
Low Emissions Vehicle (LEV)

All '04 Subaru models comply with Low Emission Vehicle (LEV) standards. The turbocharged WRX models apply several technologies to meet this stringent standard.

The fuel injectors use a top-flow multi-spray design. The 12-hole pattern improves fuel atomization into the cylinders and allows a leaner mixture during warm-up. Tumble Generator Valves above the fuel injectors remain closed during engine warm-up to create a tumble effect, which improves atomization of intake air entering the cylinders. This action creates a leaner air/fuel mixture to reduce emissions.

Exclusive 6-Speed Manual Transmission
Impreza WRX STi is the first Subaru sold in North America equipped with a 6-speed manual transmission. This new unit has been designed to handle high-torque capacity while providing short, smooth shifts. All gears are helical-type with diameters and thickness chosen for high-strength. The use of double-cone synchronizers on 1st, 3rd and reverse gears and a triple-cone synchronizer on 2nd gear provides for firm, smooth shifts. A new parallel link gear selector shortens the shift throws, and sliding bearings on the shift fork rods reduce friction.

As a safety measure, the transmission uses a reverse lockout feature. The driver must pull up a ring on the shift lever before reverse can be engaged.

A trochoidal oil pump near the centre of the transmission case pressurizes and supplies lubricating oil to the mainshaft, pinion shaft and transfer gears, while a regulator and pressure relief valve maintain oil pressure. While the car is being driven, oil is collected in a chamber mounted on the front of the drive pinion shaft. This reduces oil level in the pan and therefore foaming caused by gear rotation.

The STi's clutch is larger than the one in the WRX with a higher clamping load. A temperature-compensating hydraulic damper helps to prevent too-abrupt clutch engagement for smoother take-up.

Driver Controlled Centre Differential (DCCD) All-Wheel Drive
Impreza WRX STi is equipped with the brand's most performance-directed All-Wheel Drive system, called Driver Controlled Centre Differential (DCCD).

In the latest DCCD system, an intelligent automatic mode actively adjusts power distribution through the centre differential to suit road and driving conditions. Manual mode in the current system allows the driver to vary the power distribution to suit specific performance demands.

A planetary centre differential provides a performance-oriented 35 percent front / 65 percent rear power split. In Automatic mode, the electronically-managed, continuously variable transfer clutch can vary the split ratio as needed to suit driving and road-surface conditions. The transfer clutch will vary the centre differential power distribution ratio according to throttle opening, cornering force (lateral g), acceleration and deceleration (longitudinal g) and wheel slip.

The transfer clutch uses a pilot clutch, allowing the system to use a small, light electromagnetic main clutch. There's no need to establish hydraulic pressure, eliminating a need for pumps, pipes, valves and an actuator.

Automatic mode provides the best all-around performance for most drivers and driving conditions, varying the front/rear torque split automatically depending on driving conditions and driver input. The system responds to acceleration, deceleration, cornering force and wheel slippage to determine the ideal power distribution.

Taking A Curve With DCCD
As the driver releases the throttle and applies the brakes approaching a turn, the centre differential locking force is minimal. As the driver turns the car into the apex of the curve, the lateral g force increases. As a result, DCCD begins to lock the differential, transferring power to the front wheels. As the car exits the corner and the driver applies the throttle, DCCD continues to progressively lock the differential to help prevent oversteer and maintain stability.

Manual Mode for Special Driving Conditions
In Manual mode, DCCD allows the driver to vary the front-to-rear torque distribution to custom-tailor All-Wheel Drive performance to specific driving conditions. The driver first selects Manual mode with a console-mounted button and then turns a thumbwheel on the centre console to select from among six levels of centre differential locking. Increasing the lock factor keeps more power at the front wheels to suit certain driving conditions, specific road surfaces, or a personal driving style. Locking the centre differential essentially creates a 50/50 power split with no differential action, making this setting suitable mainly for slippery road surfaces that will allow some wheel slippage.

Limited-Slip Differentials
Limited-slip front and rear differentials compensate for side-to-side traction differences and enhance cornering by helping to prevent the inside wheels from losing traction and spinning. Impreza WRX STi employs a cam-type limited-slip front differential and a mechanical clutch-type limited-slip rear differential. The cam-type differential is a gearless unit that responds to a torque difference between left and right front wheels to increase traction and reduce understeer.
Compared to the viscous limited-slip rear differential used in other Subaru models, these two types of differentials react more quickly to help Impreza WRX STi maintain optimal traction under high-speed cornering maneuvers.
The cam-type limited slip differential does not use conventional pinion and side gears. Instead, it uses a series of cams and cam followers to provide differential action and to transfer torque in the event of a traction loss. It offers instantaneous response because transfer is based on a difference in torque, not rotational speed. As a result, locking action is smooth and progressive to enhance steering response.


Inside the cam-type differential unit, there are two opposing face cams splined to the front axles and cam followers splined to the differential case. The face cams are formed with a series of inclined ramps that resemble teeth. The cam followers have asymmetrically shaped profiles to match the face cams. Drive through the differential is via the cam followers to the face cams, and then to the axles.

When the car is moving in a straight line, there is no relative speed difference between the face cams. When cornering with no load, each of the face cams is free to rotate independently from the other. The cam followers move back and forth freely from left to right. When torque is applied, the cam followers are forced against the face cams, transmitting driving force to the wheels. If one wheel begins to slip, the relative movement between the face cams, combined with the angular force of the cam followers, sends torque to the wheel with better traction.

Subaru chose a conventional mechanical limited-slip rear differential (pinion gear, side gears, clutch mechanism) because it provides tighter control (less traction difference) between the wheels.

High-Integrity Platform With Rally-Derived Suspension
A super-stiff Ring-Shaped Reinforcement Frame body structure with a hydroformed front subframe is the basis of the high-integrity Impreza platform. The Impreza was designed for high-performance power from its inception so the body structure required no modifications to accommodate the performance potential of the 300-horsepower WRX STi.

The 4-wheel independent suspension follows Impreza architecture - struts with lower L-arms and a stabilizer bar in front and a dual-link rear strut layout with stabilizer bar, but the components are unique to the WRX STi. The main differences from the WRX are that the WRX STi has been lowered slightly, yielding a lower centre of gravity, and that the WRX STi employs forged aluminum front lower L-arms and rally-derived inverted struts at all four wheels.

Inverted struts increase bending resistance for quicker and more consistent performance under hard cornering. This variation on the MacPherson-type strut essentially functions like a conventional shock absorber set upside-down and covered by another tube. The strength of the outer steel tube, rather than the internal piston rod, bears the brunt of bending forces acting upon the strut under cornering. The piston rod rides in the lower portion of the strut tube (rather than the upper section), and the secondary upper tube mounts to the inner fender where the piston would mount in a conventional strut. This second tube adds bending strength to the strut. The larger strut tube allows a larger piston area, and there is a bump stop located at the bottom of the main tube.

With an inverted strut, the piston's stroke remains consistent during hard cornering, helping to maintain the ideal tire-to-road surface contact pattern for increased traction and cornering power.

Since the Impreza WRX STi is capable of extremely aggressive cornering, Subaru added a cross bar to reinforce the front crossmember, and brackets to reinforce the rear crossmember to help prevent flex. Higher-rate springs are used than those on the standard WRX models. Ride quality is a bit firmer, as the WRX STi suspension has been tuned for maximum cornering performance.

Brembo® Brakes with Super Sport ABS
Subaru has equipped the Impreza WRX STi with one of the most advanced and performance-optimized braking systems on the market today. The powerful Brembo Performance Brake System employs ventilated 4-wheel disc brakes to help ensure stopping power and consistency commensurate with the vehicle's extreme performance capability. The front discs are 323 mm (12.7 inches) in diameter and 30 mm (1.2 inches) thick with four-piston, fixed-position calipers. The rear discs are 313 mm (12.3 inches) in diameter and 20 mm (0.8 inches) thick and use two-piston, fixed-position calipers.

A new Super Sport Anti-lock Braking System (ABS) helps to reduce understeer and enhance stability under hard braking. This advanced system also helps reduce stopping distance when cornering compared to conventional ABS. Super Sport ABS uses a lateral g-sensor, an additional input that gives the system the ability to individually control rear brake pressures under ABS operation. Standard ABS systems control the rear wheel brake pressures together reducing the braking force to both wheels when a loss of traction is detected at either wheel. If an inside wheel begins to lose traction - as it could under simultaneous hard cornering and braking - Super Sport ABS fine-tunes control at each rear wheel to help maintain stability through the curve. By increasing brake pressure on the outside rear wheel, overall braking force to the front wheels is reduced, providing more available traction for cornering as well as reduced understeer and shorter stopping distances.

Electronic Brake-force Distribution (EBD), meanwhile, provides more precise control of braking pressure between the front and rear wheels based on the vehicle load to improve stopping distances and stability.

Quick-Ratio Steering
The power rack-and-pinion steering system in Impreza WRX STi employs a quicker steering ratio than the standard WRX model - 15.2-to-1 compared to 16.5-to-1 for the WRX. The system yields 2.7 turns lock-to-lock and a 35.4-ft. turning circle to give the car excellent maneuverability. Steering assist boost depends on engine speed: the system provides increased boost at low speeds for ease of maneuvering and parking, and reduced boost as speed increases for better feel. In addition, WRX STi steering uses a load pressure-sensitive valve to reduce pump action at highway speeds, enhancing fuel efficiency.

Wheels /Tires
Subaru Impreza WRX STi is fitted with 17 x 7.5-inch BBS® aluminum-alloy wheels and 225/45 ZR17 Bridgestone Potenza RE070 summer-tread directional performance tires. The customer can choose between silver or gold wheel colour.