Subaru Impreza RB320oc Owners Club RB320

To advertise your company click here  

Unveiling the hidden beauty of Subaru's mechanical parts

Mechanical Parts

Torque converter



Anyone interested in motor vehicles must have heard mention of the torque converter at least once.

Even if you do not use the term torque converter, you will have heard of the "automatic transmission clutch." Anyway, I think that many people are aware of the principal mechanisms of this device, and so in this article we will examine the torque converter in greater detail.

The torque converter connects the engine and the transmission. Internally, the torque converter is filled with a fluid called automatic transmission fluid (ATF) and also contains three bladed wheels -- one directly connected to the engine, another directly connected to the transmission, and a third fixed bladed wheel. When the engine bladed wheel turns, it causes a flow of ATF. This causes the transmission bladed wheel to turn in accordance with the flow of ATF. The fixed bladed wheel works to magnify delivered torque. Engine torque is delivered to the transmission as a result of this process.

People who are more knowledgeable should know that in the torque converter a lock-up clutch is installed. For example, when cruising at a constant speed, the lock-up clutch operates automatically and directly engages the engine and the transmission to reduce power loss. The lock-up clutch is an electronically controlled clutch designed to handle this role.

The principle behind the torque converter was first outlined about 100 years ago by a German engineer, and the torque converter has now become an indispensable mechanism for vehicles with automatic transmission. The concept behind the lock-up clutch was first put into practice 20 years ago.

I asked Keiichi Maruyama of the Subaru Engineering Division if this was correct. Mr. Maruyama is a specialist who has been continuously involved in the research and development of the clutch and transmission for the 18 years since he entered Fuji Heavy Industries.

"The explanation seems okay. The torque converter is a kind of continuously variable transmission." He then described it in more detail.

In engineering terms the engine side bladed wheel is called the impeller, the transmission side bladed wheel is called the turbine, and the fixed bladed wheel is called the reactor. These bladed wheels achieve a rotational balance and rotate with precision. This is important because, if there is uneven rotation, the vibration of the bladed wheel own and the uneven flow of ATF would lead to uneven torque delivery and noise.

The three-dimensional shape of the blades on the bladed wheels are curved and have the appearance of a living thing.

"The shape and strength of the blades on the bladed wheels are fundamentally determined by calculation. The final judgment, however, is determined though experience." This indicates the level of technique demanded of technology by an skilled engineer. The design on the blades determines how well the torque converter functions.

For example, when taking off from a standing start, we step on the accelerator pedal and the engine begins to revolve, and the pump directly connected to the engine revolves at the same rate as the engine. In addition, this produces a rotational flow of ATF. The design of the blades on the bladed wheel makes it possible to make the necessary changes to deliver any level or condition of torque. In the case of a turbocharged engine, the design of the blades plays such a role that makes us not feel turbo lag. When we step on the accelerator pedal, torque is delivered instantly, producing sudden acceleration. This is the key feature derived from the bladed wheel's design.

On the other hand, when putting the vehicle away in its garage, by using creep (a feature of automatic transmission vehicles whereby the vehicle moves slowly without using the accelerator), the vehicle can be moved slowly and smoothly. This is another feature determined by the shape of the blades on the bladed wheel.

The number of blades on the bladed wheel is also important. If there aren't enough blades, the flow of ATF is not smooth, and if there are too many blades, it will cause resistance for the flow of ATF.

"Accordingly, the final assessment of the torque converter lies in sensory evaluation. In the creation of a Subaru car we place particular importance on driving performance. Therefore, these measures of evaluation take on greater importance. After an evaluation of strength and durability, and the efficiency of fuel consumption, a sensory evaluation is carried out. This evaluation cannot be represented in numerical figures. As a result, the engineers in charge of the development of the torque converter evaluate the vehicle, driving together with members of the vehicle research and experimentation department. At Subaru, engineers are required to have the ability to express their feel of a car's driving performance."

Bearing in mind Subaru's particular emphasis on the driving performance of the car, there is one more secret to the development of the torque converter.

"Since many vehicle manufacturers have transmissions of similar shape, the torque converters they use are also similar. However, Subaru is different. Subaru has fitted a torque converter that is especially narrow. When the torque converter is wide, the front overhang is large, and this means that we cannot make use of the special feature of the Horizontally-Opposed engine -- namely, its advantage of short length. In addition, because an all-wheel drive (AWD) is major and important feature of Subaru vehicles, one of our important objectives is to make the transmission, including the AWD system, lightweight and compact. Since we are fastidious about the driving performance of the vehicle, we place the same amount of importance on the size of the torque converter as on its performance. With size as a strict constraint, it is a test of our professional skill to produce a torque converter with excellent performance."

To enhance fuel efficiency, the lock-up clutch minimizes the number of engine revolutions while the engine and transmission are connected directly, and deliberately slips the lock-up clutch at a low rpm range. It was once widely thought that this technology would be difficult to realize in the four-cylinder engine.

When you actually look at the bladed wheels for the torque converter and the lock-up clutch, it is not as big as you might have thought. However, this seemingly small thing can multiply engine torque two-fold when taking off from a standing start. Moreover, when driving off, the engine easily delivers an output of more than 200 horsepower, an amazing feeling. Mr. Maruyama puts into words his spirit of challenge, "I want to create a smaller, more efficient torque converter that delivers better fuel efficiency."

Appearing much like compact jet-engine turbines, the three bladed wheels bring to mind an image of rotating ATF and delivering terrific power and torque.

I see it not so much as a mechanism but a natural phenomenon. When the car is running, I see in my mind's eye the swirling ATF that powers a vehicle.

Quick Links

Richard Burns

Subaru rally legend, Richard Burns Britain's only FIA World Rally Champion died in November 2005 of brain cancer at the age of 34.

Click here for more information on the 2001 World Rally Champion.

RB320 Features

  • 6,000 of extra equipment
  • 320PS/450Nm Prodrive tuned
  • build limited to 320 units
  • finished in Obsidian Black
  • bespoke 18" alloy wheels
  • unique numbered tax disc holder
  • Prodrive tuned suspension system with Bilstein dampers, Eibach springs & rear ARB
  • lowered ride height
  • polished s/steel mesh front grille
  • STI front lip spoiler
  • STI quickshift gearchange
  • STI side sill plates
  • RB branded gear knob
  • RB branded car mats
  • RB exterior badging
Click here for full list