The new 6-speed gearbox is 85 pounds heavier than the 5-speed units. It has an internal oil pump for pressure lubrication. The change to pressure lubrication assures that local temperatures in the gear box are kept under control, a valid criticism of the 5-speed design under racing conditions. The 3.9 final drive unit has a separate oil galley, isolating possible gear chips and further managing internal thermal loads. It is also very expensive and in relative short supply.

The Japanese market 6-speed has six gears in the space of the five previous WRX gears. In 6th gear is turns some 350rpm HIGHER at 60 mph than our WRX gearbox does in 5th.  The Japanese 6-speed is the similar to the 5-speed rally gearbox with a 6th gear added. The European STI 6-speed has had 5 and 6th gears opened up slightly and would be the gearbox the possible US WRX would have. It probably makes the best sense for the US driver. Personally, I do not mind buzzing down the freeway at 3500-4000 rpm for the shear pleasure the Japanese ratio set provides in the mountains and at the track.

The Japanese gearbox offers a standard 50/50 torque split center differential or the electronically controlled 38/62 split that can be progressively set toward a 50/50 and locked position. When considering which center diff to use, please realize that the STI cars have larger hubs bearings, drive axles and 20mm larger ring gear in the differential to handle the increased torque loads caused by sending over 200hp to the rear of the car. The weakest part of the WRX drive train is not the transmission, but the small R-160 differential. In constant hard running with a higher power engine it easily develops rattles from the spider gear area and a very irritating whine can develop from bearing noise. A person going to the trouble to install the variable torque split differential in their WRX without the STI hubs, shafts and R180 differential better know how to rebuild R160 differentials. They will doing it fairly often. In my 260hp WRX I rebuild my R160 diff every 40-50,000 miles.

Kits for conversion to the 6-speed can be assembled on request. Lead time for a new 6-speed gearbox can run 8-10 weeks. Cost estimate is $6500 for the parts to install a standard 6-speed 50/50 torque split. Conversion to R180 and torque splitting center differential is considerably higher. Please contact me by email at mshields@spdusa.com for details.