Tech guide · Drivetrain
Gearboxes, diffs and DCCD
The Impreza’s four-wheel-drive system is strong, but the parts are not as interchangeable as they look. Gearboxes, centre diffs and rear diffs changed between markets and years, and mixing the wrong combination can destroy a transmission. Read this before you swap anything.
At a glance
Match the ratios
Front and rear gearing must match, or the centre diff will overheat and fail.
R160 or R180
The STI’s bigger R180 rear diff brings bigger axles and handbrake drum.
Ask first
Get the code off the casing and ask in the forum before buying a swap.
Gearboxes
| Car | Gearbox | Notes |
|---|---|---|
| Classic Turbo / WRX / STI | 5-speed manual | The later version (from 1999) has a larger casing and is stronger. Hard use and big power can strip 2nd and 3rd gear |
| New Age WRX (2000–07) | 5-speed manual | A known weak point under launches or with a lot more torque: watch for grinding in 2nd and 3rd |
| New Age STI onward | 6-speed manual | Much stronger. Check 5th and 6th synchros on used cars |
Centre differential and DCCD
- Standard cars use a viscous centre differential.
- DCCD (Driver Controlled Centre Differential) lets the driver adjust how locked the centre diff is. Japanese STI Type RA, Type R and later STI models had it; in the UK it arrived in 2004 (the WR1 was the first UK car to have it).
- From the 2005 Hawkeye, DCCD gained a mechanical limited-slip element and steering-angle sensing.
- A DCCD conversion needs the gearbox, the controller, the switch and the correct wiring: it is not just a gearbox swap.
Rear differentials: R160 and R180
| Diff | Found on | Notes |
|---|---|---|
| R160 (smaller) | Classic Turbo and WRX, and New Age WRX | Smaller handbrake drum and axles |
| R180 (bigger) | STI 6-speed cars, and many Japanese classic STI Type R and Type RA models from Version IV. Check the diff on any classic STI before ordering parts | Bigger axles and handbrake drum; usually with a limited-slip diff |
The big warning: matching ratios
Every four-wheel-drive Subaru must have the same overall gearing at the front and rear wheels. If the front and rear final-drive ratios do not match, the centre differential is forced to slip constantly, and it will overheat and fail, sometimes within a few miles.
- Japanese 6-speed STI gearboxes and UK/European/US 6-speed gearboxes use different transfer and rear-diff ratios. A JDM gearbox needs the matching JDM rear diff, and the other way around.
- The same applies when swapping a 5-speed for a 6-speed: the rear diff, prop shaft, axles, hubs and sometimes the handbrake all have to match.
- A ratio that “looks the same” on paper does not guarantee the parts fit. Axle splines, hub sizes, prop-shaft length and flanges also differ.
- Before buying a gearbox or diff, get the code stamped on the casing or tag and ask in the forum. Someone has done the same swap before.
More guides
Impreza wheel fitment guide
Impreza injectors and turbos explained
Buying a used WRX or STI
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