Subaru Impreza RB320oc Owners Club RB320

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Unveiling the hidden beauty of Subaru's mechanical parts

Mechanical Parts

Manual transmission gears

Subaru cars are driven all over the world, however, they are most numerous in Japan and North America.

A common feature general users are fond of in these two regions, is automatic transmission.

One point of interest is that even in regions where automatic transmission is popular, Subaru vehicles with manual transmission are very popular compared to vehicles made by other manufacturers. Twenty five percent of the production and sales of Subaru vehicles are fitted with manual transmission. This is probably because many Subaru owners have a deep desire for driving enjoyment.

Now let's look at the beauty of the mechanism for the gears and the shift fork of a manual transmission. I asked Atsushi Udagawa, who is in charge of developing the 6-speed manual transmission for the Impreza WRX at the Subaru Engineering Division.

What is a manual transmission?

"This is the mechanism that transfers engine torque to the drive train. With one gear we cannot deliver adequate torque to the drive train over the range of driving speeds from start to high speed. Therefore, we have a mechanism that allows us to select from a number of gears via a manually operated shift lever."

I understand this, however, what is torque? How is torque different to horsepower? "Torque is the twisting force. The Impreza Turbo produces maximum torque of about 30 kgfm. Since a steel shaft with a diameter of 10 mm is snapped by a torque of 6 kgfm that a force of 30 kgfm is massive."

When transferring a torque of 30 kgfm to the gears, a force of 1 ton is exerted on each cog, a tremendous amount of force and powerful torque.

The quality of the material needed for gears is steel hardened by heat treatment called carburizing and quenching. These processes aim to harden only the surface of the gears. Hardening the metal inside the gears has the opposite effect and makes the gears fragile. The strongest gears consist of steel that is soft internally and hard externally.

Another process used is called shot peening. This method hardens the surface of a gear by pelting it with tiny metal shot at high velocity.

Although my perspective might be unusual, listening to these explanations conjure up the image of a swordsmith tempering a Japanese sword with single-minded devotion. The shift fork is the part that changes the combination of gears. When we manually operate the gear-shift lever, the shift fork moves accordingly and makes the sleeve and gear chamfer mesh together.

When they come together, the difference in the speed of revolution between the sleeve and gear chamfer means a "gear clash" would occur under unmanaged circumstances. To avoid this, a synchronizer mechanism is built in to match the revolution and ensure smooth gear changes.

This synchronizer mechanism was first adopted for mass production in 1928 for the Cadillac and is now fitted to all mass-produced manual transmissions. In September 2001 the Impreza WRX STi version with a 6-speed manual transmission made its overseas debut at the Frankfurt Motor Show. The gear ratio for first gear is 3.636. At a standing start with an engine revolution of 1000 rpm, a torque of about 10 kgfm is multiplied 3.636 times in first gear and by 3.9 in the final gear. The torque transferred to the wheels is 140 kgfm.

The above is a sketchy explanation of the manual transmission mechanism, however, I was surprised at another fact.

"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. Originally the transmission was developed for torque estimated at around 20 kgfm. However, the excellence of the fundamental design and successful efforts to improve the strength of the gears, this transmission has grown into a 5-speed manual transmission with the durability and power for the current generation of Legacy and Impreza."

The Leone era was about 20 years ago. Designed with careful thought and developed with attention to detail, the manual transmission has been mass-produced over a long time and has matured well. The manual transmission is a symbol of the excellence of Subaru technology.

However, in the developmental stages of the New Impreza WRX STi, the transmission was required to change to a 6-speed transmission designed to draw out the engine power to its maximum, strengthen gears to cope with greater torque, as well as provide an even greater performance for gear shift operation with short responsive stroke and fresh alive feeling for sports driving.

Accordingly, the 6-speed manual transmission of the Impreza STi version was a new design. "The Impreza WRX STi engine produces 35 kgfm of torque and the 6-speed manual transmission can be adapted to greater torque in the future."

The Subaru Engineering Division conducted research and development aimed at creating the world's best gear-shift operations. This 6-speed manual transmission provides a bracing short stroke shift and provides excellent information to the driver. This is unmistakably the leading 6-speed manual transmission for a mass produced vehicle.

Subaru also increased the performance of the synchronizer and on all five shift rail there are two linear ball bearings. Similar examples of such an effort to minimize the friction of shift operations are rare.

Now I take a look at the gears and the shift fork of the 6-speed manual transmission.

To reduce noise the mass-produced manual transmission uses helical gears with helix angle. The teeth appears to be like a gentle curve. When the gears mesh, the curves fit together and appear as a single part.

Associating this with muscles in animals evokes a strangely physical feel. From long ago people have noted that when we are inspired by the beauty of a mechanism, we see it as flesh and blood.

*1 kgfm = 9.80665 Nm / 7.233 ft-lb

*The specifications and availability of the Impreza WRX STi vary by region.

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