Recently we swapped a 6 speed STI gearbox into the 1999 2 Door WRX STI. Check out the video here: [embedded content]
The original joke Nissan drivers told about go-fast Subarus is they came with glass gearboxes. Early WRXs ('93-'97) and Libertys definitely spent plenty of time on the back of tow trucks with 1st or 2nd gear sitting in the transmission pan thanks to bulk grip, plenty of power, and a weak design.
This led to the afternarket taking a leaf out of the rally book, with straight-cut gearsets replacing the factory helical 1st-4th gears. There was also dog-engagement boxes, which could allow near-clutchless gear-changes but were also difficult to drive compared to normal synchromesh transmissions.
While later 5-speed manuals got tougher they don't hold a candle to the "unbreakable" six-speed manual which debuted in the GD-series STi Impreza. With design input from Porsche Engineering the 6-cog 'box is physically wider and much, much heavier than the 5-speed.
Among many upgrades the 6-speed boasts an oil pump (for improved lubrication), fatter gears and improved synchromesh for faster shifting performance, while the clutch and flywheel have a larger diametre, providing more surface area to clamp on. Some STi models also featured front limited-slip diffs (for a maaaad handling upgrade), and an adjustable centre differential called Driver Controlled Centre Differential (DCCD).
Thanks to Subaru's inherent Lego nature the 6-speed provided a bolt-in upgrade for Libertys, Foresters, Imprezas, and Outbacks, connecting to both 4- and 6-cylinder engines, and connecting to factory tailshafts. This meant you didn't need a noisy aftermarket gearset fitted to your original gearbox, you had a factory solution with an extra gear for sweet freeway cruising.
One of the downsides, apart from the weight of the 6-speed, was the cost. A decade ago a 6-speed swap would run nearly $10,000, though this has halfed today. If you then also fitted the full R180 rear-end from an STi that could add several thousand dollars more, though it truly provided a nuclear-tough drivetrain you could abuse even with 300kW at the wheels.
If you're wondering why someone would spend so much changing their gearbox and diff for another factory unit, one 7000rpm launch should cure any doubts you have!