Также если кому будет интересна инфа с насиока...
the wrx uses a 1.1:1 reduction in the rear diff, which gives it the tail-happy split under load, so be aware of that if you're planning on changing to the WRX rear VC unit. I hesitate to call it an LSD, since the VC units respond only to a delta in speed, not torque like every other "LSD". Yes, it does technically limit slipping, but.... I guess I'll let you decide.
yeah late model subarus have the VC center, but for competition road use, I would recommend first doing suspension (springs, struts, bushings, etc) and then, if you still want to do the rear LSD, think about changing
center diff. Several center diffs are better for "hard" driving that the VC unit, like the Cusco Tarmac Gear, which splits 35%/65% (front/rear). Then you could match that to a helical, torsen, or clutch-type rear, depending on your purpose. rallispec.com has a little bit of info on differentials
http://www.rallispec.com/product-diff.htm) and the different options that are out there.
Simply putting in a rear diff will be risky, because you may over-work the center CD unit. Going away from a VC-type center would allow you to "smoke" the rears through a turn without putting undo stress on the
relatively weak stock center diff. There's also the option of upgrading the stock center from an 4 kgf/100rpm to a 10 or 20-rated VC, but I would
recommend against it.
You're looking at some hefty expense for all this. The cusco tarmac center gear is $1464 and the STi SureTrack Rear LSD is $890. That's $2500 of diffs, before you even start installation. $2500 will get you much more if you spend it on suspension.
In summary, do suspension first. Then plan the drivetrain mods. Per dollar, you'll get much more out of suspension mods.
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Where would be the best place to start ? LSD @front or rear for road use ?
Sorry, just re-read the post and realized I never answered your question as such...
The front will help turn-in and traction where wheel-spin-induced understeer would normally occur.
The center should be kept "weak" VC, 1-way LSD, or open, unless you're planning on rallying or need lots of snow traction.
I think the Cusco Tarmac Gear, even though it's 36/64 F/R, is an open diff... I need to do some more research there but info is hard to find in english. An adjustable, normally-open center diff is used on the STi, both here and overseas, for track use. The "thumb wheel" on the STi only lets you select the lockup of the center diff, from 0% to 100%. The F/R bias stays 36/64 regardless. Every good racer seems to recommend leaving the center diff open during dry conditions, and using only partial lock for wet conditions. Full lock would be for maybe drag racing or heavy snow/ice.
I've seen the stock VC unit(s) overheat fairly easily under rough driving. I drove the Dragon (
http://www.tailofthedragon.com/) up, and down, and started to go back up, and noticed that mid-corner handling was basically crap. This was on good sticky tires, dry roads, and hard cornering. I let the car sit for about