If you're dead set on a full RWD setup, then you pretty much gonna have to go R180. R160 wont last long under the abuse I assume you're gonna give it.

However DCCD is less punishing on the rear diff so R160 is fine unless you've got some serious power under the bonnet. Cheapest solution for a rear diff is to get a plated STI diff from an early STI-RA and reuse your crown wheel and pinion to match up to your current front diff. A second hand early RA R160 plated diff can usually be had for $400 or under. Look on yahoo auctions Japan for great deals. If you can find a OE RA plated diff. OE RA plated diffs require different shafts too. which is a bitch as RA shafts are a pain to get. You can however modify the standard VLSD shafts to fit a RA plated centre. Don't ask me how, I was explained to me once, and I have forgetten.

Yahoo japan also usually has plenty of aftermarket R160 clutch pack diffs. They will probably be a little more expensive, but are generally newer, tighter and in better condition. A brand new clutch diff is usually around the $1400+ mark. A 2-way clutch pack LSD is what you want for predictable and easier to control powerslides. Just remember, plated/clutch diffs are noisy driving around normally. Clunk, Clunk, Clunk.

The DCCD centre diff is a straight swap into a 5 speed. you'll need the DCCD centre diff, and controller.

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RS Liberty - Because WRX's are only good for parts...