Fall 2002 Forward to a Friend

A key component of the Subaru drivetrain, the torque converter acts as the bridge between the engine and the automatic transmission. It’s not only a vital link in your vehicle’s drivetrain, it also helps define what many refer to as the “feel” you experience as you drive.

The torque converter contains three bladed, fanlike wheels. The first wheel, sometimes called the impeller, is connected directly to the engine. The second wheel, the turbine, is connected to the transmission.

In between these wheels is a fixed wheel, the stator. The three wheels are held inside a housing that’s filled with automatic transmission fluid (ATF).

When you press on the accelerator, the engine turns the impeller inside the torque converter. The impeller’s blades make the transmission fluid start to flow and rotate inside the housing. The rotating fluid then turns the turbine, and the engine’s power is transmitted to the transmission and on to the wheels.

The stator works between the impeller and turbine to magnify the torque as the vehicle accelerates. At low speeds, the stator remains stationary against the flow of transmission fluid. It works to redirect the fluid flow from the turbine to boost impeller action and multiply the engine torque. As the speed increases, the stator starts to rotate in the same direction as the impeller and turbine. As it does, the torque multiplication stops.

The torque converter features a lock-up clutch. It’s an automatic, electronically controlled mechanism that locks the impeller and turbine together when you drive at a constant speed. This not only reduces power loss, but also increases the drivetrain’s efficiency and the vehicle’s fuel economy.

“The torque converter is a kind of continuously variable transmission,” says Keiichi Maruyama of the Subaru Engineering Division. It links the engine and automatic transmission smoothly as you pull away from a dead stop, and disconnects them when the vehicle comes to rest at stoplights. The design of the blades makes it possible for the vehicle to deliver whatever level of performance the driver demands.

Torque Converter Components

On one hand, the converter’s components help deliver high levels of power instantly when the driver needs to accelerate quickly. Alternately, the converter’s components also will work together to let the vehicle move smoothly at slow speeds, such as when it’s being pulled into the garage at the end of a drive.

The torque converter not only influences how a Subaru performs but also how it “feels.” Its design is influenced through sensory evaluation as well as technical calculation.

“The shape of the torque converter components is fundamentally determined by calculation,” says Maruyama. “But after an evaluation of strength, durability and the efficiency of fuel consumption, a sensory evaluation is carried out. It’s an evaluation that can’t be represented in numerical figures. The engineers in charge of the torque converter’s design work with members of the vehicle research and experimentation department to evaluate the vehicle, and express their feel of a vehicle’s driving performance.”

Subaru engineers also work to make the torque converter lightweight and compact, to work best with the Subaru drivetrain’s horizontally opposed engine and All-Wheel Drive (AWD) system.

“Since many manufacturers have transmissions of similar shape, the torque converters they use are also similar,” says Maruyama. “Subaru is different. We’ve fitted a torque converter that is especially compact. Because All-Wheel Drive is a major and important feature of Subaru vehicles, one of our important objectives is to make the transmission, including the AWD system and its components, lightweight and compact.”

“Since we are fastidious about the driving performance of the vehicle,” he continues, “we place the same amount of importance on the size of the torque converter as on its performance. With size as a strict restraint, it is a test of our professional skill to produce a torque converter with excellent performance.”

The seemingly small component can easily handle hundreds of horsepower from a standing start, or gently move the vehicle forward inches at a time. Subaru engineers like Maruyama have worked for decades to develop a torque converter that will work seamlessly as a component of the renowned Subaru drivetrain to provide an unparalleled driving experience.

“When the vehicle is running, I see in my mind’s eye the swirling fluid that powers the vehicle,” says Maruyama. “I see it not so much as a mechanism but a natural phenomenon.”