Six speed Manual Gearbox conversion requirements:

There are a few different methods and trains of thought  about fitting a six speed gear box to your wrx, liberty or forester.

Generally they require the same parts with a few exceptions. To simplify it and remove a few misconceptions we have created this blog to help educate everyone and clarify a few things.

The main reason of fitting a six speed sti gearbox is to replace the forever blowing five speed boxes, Even though they are considerably more expensive up front in the long run could end up being the cheaper option after having to replace five speed after five speed in the quest for more power and reliability.

The Subaru Wrx STi 6 speed gearbox was first fitted to the 01 bug eye STi as a standard fitment. The box continues right in to the current model  STi’s however  now they can be found in the liberty and forester ranges. The six speed gear box is basically the same weight and dimensions as each other but they have continuously revised the gear ratios, front diffs, driver controlled centre diff feature.  The transmission code can be found on the transmission, this will explain the internal components and define which model it came out of. Best way to work out which six speed you will need is to call us and discuss the vehicle and purpose for the conversion.

Must when fitting a six speed:

• The Gearbox
• The Clutch release fork (different to five speeds)
•  Six speed Shifter Assembly with the reverse lock out cable 
• The six speed tail shaft has different splines to the 5 speed wrx gearbox also is shorter in length , However an auto tail shaft shares the same splines and is the same length required.
• Rear diff ratio match to what is required for the six speed you are fitting
• Lastly a qualified or experienced mechanic to carry out the conversion.

Cross members:

It’s important to fit the correct cross member to the correct car as the wrong one will cause vibrations in the drive lines as the drive line angles will be to extreme for the tail shaft to rotate in harmony. The Rear rubber mount on the six speed is a full rubber insert which helps strengthen the driveline rigidity and stop the unit from bouncing around. This mount can be fitted to various cross members as they share the same bolt pattern. Generally it’s the foresters that require the standard cross member for the STi box as they have spacers fixed to it.  

Tail shafts:

As previously mentioned the six speed and auto tail shafts are shorter in the front section (first half before the centre bearing) than the 5 speed box. The main difference in the shafts is that the six speed shaft has a universal joint just after the centre bearing where as the auto shaft has a constant velocity joint (slip joint). On the later model six speed STi’s (06  and newer) the  rear diff pinion flange on the tail shaft  is larger and will not mate up to the standard r160 diff flange as previous models.

Rear Diff:

The rear diff will have to suit the six speed you purchase, the trans code will help determine which ratio you will have to match. Up to 08 STi’s the only diff ratios in rear are a 3.9 or a 3.54, The 3.9 ratio was until mid 05 where than changed to 3.54 rear diff ratio, this is also when the larger pinion flange was fitted however this can be replaced with a standard sized flange.

There are two rear diffs in the Subaru range, R160 and the STi’s R180 , The R160 is a standard fitment across the board with multiple variations in ratios, The R180 is an Sti Fitment and takes larger sized rear drive shafts on both the hub and diff sides,  and comes in 4.44, 3.9  and 3.54 ratios. Read the R180 Conversion blog to fully understand what is required to successfully fit the upgrade.

Clutch , Flywheel and starter motor and slave cylinder:

The standard five speed clutch , flywheel and starter motor can remain and be used, however the STi six speed clutch has a larger surface area on the clutch pressure and friction plate and the fly wheel is lighter in weight than the five speed. If you use the six speed set up you will require more clutch bolts. The five speed slave cylinder  can remain.

In General:

The six speed set up will depend on the budget and application,  Any six speed sti gear box will bolt up to a ej series Subaru engine,  The different models of six speed should be considered when purchasing to maximize the compatibility to your application, below are some points for a guide.

• Front diff, male or female ends on gearbox side of drive shafts, this will save you replacing them if you choose the correct style six speed.
• Diff Ratio: most likely replacing the rear diff ratio is a must however the 06 sti six speed has a 3.54 rear just like the 99 aus delivered wrx, so replacement is an option then.
• DCCD: A great factor on the later models, however not a must for wiring in , can be left disconnected and the transmission still perform correctly only with more bias to the rear wheels!

The fitment should be carried out by a qualified technician that has experience in field. This information should only be used as a guide only.