HOME VERSION 1.1 BLUEPRINT

TRANSMITTING TORQUE: WRX STi 6-SPEED

MAKING THE STi DRIVING EXPERIENCE TOTALLY EXCLUSIVE AND THRILLING!

The Subaru WRX STi is the first Subaru in the United States equipped with a 6-speed manual transmission, and it features a short-throw shift mechanism and a pressurized lubrication system. The Subaru 6-speed was designed to handle the massive 300 lb-ft of torque developed by the STi’s turbocharged 2.5-liter boxer engine. The design also enhances shifting comfort and action.

SIX SPEEDS, NO WAITING

The 6-speed manual is the WRX STi’s only transmission. Specs tell you that the two top gears are overdrive, which helps with top speed and fuel economy (should that be an issue!). More importantly, the 6-speed was developed to help extract maximum power from the WRX STi’s engine. Having six gears from which to choose also gives the driver potential for greater control ... and makes the car a lot more exciting to drive!

There are a number of benefits to having six forward gears rather than five. The ratios between the gears are numerically closer to each other in the six-speed than the gears in the WRX 5-speed. Closer ratios allow the driver to keep the engine in the rpm range that maximizes performance.

THE SHIFTING LINKS

The 6-speed comes equipped with a factory short-throw shifter, which allows quicker shifting while reducing the driver’s arm movement. With the front-to-back movement of the shift lever defined as “shifting” and the side-to-side movement as “selecting,” short-throw shifters typically reduce the travel distances of both movements from average shifters. (See the chart for a comparison between the shifting and selecting distances in the WRX STi and WRX 5-speed manual.)

“The driver can shift by mere wrist movement,” explains Hitoshi Harashima, who oversees operating systems at the Powerunit Mounting & Operating System Design Section in the Subaru Engineering Division. “For the WRX STi, we shortened stroke length to the optimal point for minimizing shifting mistakes. We claim that our shift lever is one of the shortest-stroke levers of any sedan in the world market.”

A JOINT LIKE THIS

Another part of developing the 6-speed transmission was to give the driver the feeling of the gears clicking into place. Subaru Engineering designed the shift linkage for a positive feel, with shift smoothness of prime importance.

The shift-linkage joints ride on tubes that guide them within the transmission. Bushings between the guide tubes and the bolted linkage sleeves are coated with Teflon^® and reinforced by thin metal bushings, improving both smoothness and durability.

To help prevent an accidental shift into reverse, the transmission has a lockout. The driver must pull up a ring on the shift lever to engage reverse.

UPWARD SPIRAL

Adding to shifting smoothness in the WRX STi is the design of the transmission’s gearing. Helical gears with spiral-shaped teeth mesh easily with smoother action during gear changes. Synchronizers for all gears contribute to firm, smooth shifts using reduced force, with doublecone synchronizers on first, third and reverse gears and triple-cone synchronizers on second gear to further assist in smooth, fast shifts.

The gear set is mounted to a steel adaptor plate, which is bolted to the front of the differential casing. The transmission casing supports the gear set at the rear. This straightforward design adds to strength and durability.

To withstand the engine’s 300 lb-ft of torque, transmission gears require added hardening of their surfaces. Hardening the interior metal would make the gears fragile and susceptible to breakage, so the hardening process is applied only to the gear surfaces.

Subaru uses two processes to harden the gear teeth a carburizing heat treatment and quenching process, and a shot-peening process. Carburizing the steel gears involves exposing them to heat and carbon to form surfaces that are then hardened by quenching in a liquid. Shot peening bombards the surfaces of the gears with small metal pieces at high velocity, which is similar in concept to a blacksmith hammering steel into shape.

But remember, unlike a traditional two-wheel-drive vehicle where wheelspin is a common occurrence, Subaru All-Wheel Drive provides rock-solid traction. This same resistance to wheelspin can put a tremendous strain on a transmission gear set. The choice is yours. Drag racing any vehicle exposes a drivetrain to forces well beyond those of normal day-to-day driving. These types of failures are not a consideration of any manufacturer’s warranty. Sorry.

OILED AND PUMPED

The 6-speed transmission is lubricated by a trochoidal oil pump mounted at the rear of the unit. The pump has a 4-lobe inner rotor and 5-lobe outer rotor. Gear-driven by the center differential, the rotors force oil to the transmission’s main shaft, pinion shaft and transfer gears at a rate that depends on the center differential’s speed. A regulator and pressure-relief valve maintain oil pressure.

A pressurized system ensures a constant oil supply to the gearing while minimizing the amount of oil required within the transmission case. The oil level lies underneath most of the gearing, which reduces both friction and oil foaming. Reduced friction improves efficiency and performance, and keeping the oil from foaming helps to maintain oil quality and effectiveness.

While lubricating the transmission, the oil system also helps to cool it by carrying away heat from the gearing, which enhances durability and reliability.

WAY TO GO

For the WRX STi to be the impressive performer that it is, all components and systems must contribute to the driving experience. Allowing the driver to direct 300 horsepower through a performance 6-speed transmission is an excellent way to start!