September 13, 2006

Subaru's SI-Drive

2007 Subaru Legacy spec B SI DRIVE control. Click image to enlarge

By Jim Kerr

Subaru has this knack for letting drivers have fun with their vehicles. Take their stability control systems for example: while some other manufacturers decrease engine power as soon as the vehicle slips a little off the intended line, Subaru programmers let the car slide a little, almost rally style, before the stability control intervenes to help keep occupants safe. Now Subaru is incorporating another driver-modifiable system into their vehicles to enhance driving performance and pleasure. They call it Subaru Intelligent Drive (SI-Drive).

SI-Drive is being introduced first on the new 2007 Legacy 2.5 GT, Legacy 2.5GT spec.B, and Outback 2.5XT models. SI-Drive uses the drive-by-wire throttle system and multiple computer programs to tailor the car's performance to driver's desires.

The use of electronic throttle operation instead of cable-operated systems is key to SI-Drive design. When the throttle pedal is pressed, voltage signals from the pedal sensor go to the engine computer. The computer then operates a motor on the throttle shaft to open the throttle plates. The rate of opening the throttle plates in relationship to the amount of movement of the throttle pedal is a much-debated item among various vehicle engineers. Some want the vehicle to feel spirited.

2007 Subaru Legacy specB. Click image to enlarge

To accomplish this, the computer is programmed so the throttle opens faster in relationship to the pedal, but this can also make the vehicle more difficult to drive smoothly in stop and go traffic. Others want a smooth application of power, so they delay the throttle opening in relation to the amount of pedal movement.

General Motors has used both modes of operation on their 4x4 trucks for several years. In two-wheel drive or 4WD high range, the throttle plates open faster for that feel of acceleration, but in 4WD low mode the rate of throttle opening is delayed so the driver can apply smooth power over rough and bumpy terrain. Subaru's SI-Drive uses similar principles but lets the driver control the settings.

2007 Legacy spec B SI DRIVE dash. Click image to enlarge 2007 Legacy spec B SI DRIVE Intelligent Mode . Click image to enlarge

The Subaru system allows the driver to select three different modes of operation with a rotary dial located on the centre console. The driver has the choice of "Intelligent, Sport or Sport Sharp" modes. Not only does the switch allow the driver to select a different programmed rate of throttle opening, it also modifies automatic transmission shift performance as well.

When Intelligent mode is selected, the system reduces engine torque and maximum power and switches to a more relaxed throttle response curve. By controlling throttle angle and response of the Electronic Throttle Control system, peak output is restrained by about 20 per cent, and peak torque is limited to 228 lb.-ft. Why limit power? The answer is to maximize fuel economy. Subaru estimates that real world fuel economy figures can improve by as much as 10% in this driving mode. This mode also works well in slippery conditions so that traction can be maintained better by Subaru's all-wheel drive system.

Sport mode provides quick throttle response and linear acceleration. The characteristics of this mode make it suitable for freeway or highway driving and mountain roads or hilly terrain. The driver feels more closely connected by the vehicle response to gas pedal movement.

Sport Sharp mode selects a computer program mode where throttle response is lightning quick and the rate of opening is faster, with more maximum power. The transmission shift points are also changed on automatic models so that gears are held longer for faster acceleration. This mode is designed for twisty roads, passing vehicles or merging into traffic - anywhere the driver wants maximum performance and response from the vehicle.

Few other vehicles allow the driver to select the type of response they desire from their vehicle. Some vehicles such as Mercedes and BMW do it automatically, but letting the driver select adds to the fun of driving a sporty vehicle. The Corvette is one of the few that allows a driver to select a Competitive Driving mode, where the stability control and traction control programs are modified to allow the car to "hang out" on corners for maximum driver control yet still have the benefit of stability control if the driver gets really silly.

Using different programming in the computers for different driving styles and situations isn't really new, but Subaru's SI-Drive is one of the few systems on the market that lets the driver have control of what they want. Thank you Subaru. For a complete list of Jim Kerr's Auto Tech columns, click here.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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