dropkick_muppet

iTrader: (4)

has anyone here with a WRX changed out the center differential's limited slip unit for either an aftermarket or STi unit? while looking for some suspension goodies on the Mauro Motorsports website, i noticed that they sell STi front, center and rear limited slip units. they reccomend starting with a center limited slip if you're going to start swapping them out. i'm pretty sure that the STi piece they sell is a clutch type limited slip, versus the stock viscous unit. i'm thinking that be adjusting the clutch packs, you could approximate the STi's 35/65 torque balance, which, unless i'm grossly mistaken, should help with the WRX's severe understeer.

at about $800 for the parts, and another $400 for installation labor, it's not a cheap modification to make. however, if it endowed the WRX with handling closer to that of the STi, i'd think it'd be a pretty worthwhile investment. if anyone out there has done this, please, share your experience!

dropkick_muppet

iTrader: (4)

bump -- c'mon! someone out there has to have at least thought about this for a couple of minutes...

go go go

iTrader: (2)

ok, two things you are misunderstanding.

Center diff is not clutch type. (aftermarket)STi center diff is still viscous.
It is much stronger one though.
you can get completely locked up one too.

stronger center diff will give you more understeer on tarmac
The DCCD on STi is set no LSD when it is 35:65.
It is completely free. When it is 50:50 it is locked up.
New STi DCCD has a computer that automatically adjust the locking % between the two extreme.

Cusco sells 35:65 split center diff called tarmac gear.
This one is not LSD, it's completely free and you must have Front and Rear LSD to install Cusco tarmac gear

dropkick_muppet

iTrader: (4)

you're right, i realized my mistake after doing some more research into the LSD unit.

i'm assuming that by 'stronger'' you mean a diff that will allow less slip, correct? i don't understand exactly how that would create more understeer.

limited slips diff will never fully 'lock' that's what locking diffs are for. a limited slip will only allow one wheel to turn a cerrtain ammount more than the other, which is defined by either the clutch plates or the plates and fluid used.

a completely locking diff in the center would force a 50/50 torque split between the front and rear wheels, just as a locking front or rear diff would force a 50/50 torque split between the two wheels that the diff turns.

i was looking for this on google, and wasn't able to find much in the way of information in english. i don't suppose you'd be able to point me to some information (in english) on this, would you? also, why would you need front and rear limited slips to make use a center differential that split the power 35/65 front to rear? the stock WRX has a rear limited slip, so the worst case scenario is that you have 65% of the engine's torque split as some ratio between the rear wheels, and 35% of the engine's torque being put through one front wheel. with a limited slip center, more than that could easily be put through an open diff in the front under the right conditions.

don't get me wrong, i'm not trying to argue about this, i'm just trying to understand the mechanical aspects of this.

dropkick_muppet

iTrader: (4)

CUSCO
TARMAC GEAR FOR 6MT
Torque/ Front 35 : Rear 65, Suitable with Center diff with viscous LSD, Unsuitable with Driver's control center diff GDB for 6MT
147,000 ( -->US$1193.93 )


TARMAC GEAR
Torque/ Front 35 : Rear 65
GDA 147,000 ( -->US$1193.93 )

this is the best listing i could find with google, and it mentions nothing about needing a front and rear limited slip to use the Cusco Tarmac diff. it came from http://www.japanparts.com/SubaruPart...ansmission.htm

go go go

iTrader: (2)

>>>>
i'm assuming that by 'stronger'' you mean a diff that will allow less slip, correct? i don't understand exactly how that would create more understeer.
yes, aftermarket STi center diff has more viscousity and it creates less slip.
When you turn, front wheels are making more travel than rear wheels, especially on tight turns. The more you limit the slip in the center, it'll be harder to turn in.
Stronger LSD in the center is mainly for gravel and snow competition where it is hard to get traction and torque moves back and forth due to the wheel spin on either wheels. Also it is good for launch because it can spread the power more evenly on all four wheels.
On STi, electrical center diff, the center diff is set up to free while the car is turning and grabs it when it is under acceleration.

>>>>
limited slips diff will never fully 'lock' that's what locking diffs are for. a limited slip will only allow one wheel to turn a cerrtain ammount more than the other, which is defined by either the clutch plates or the plates and fluid used.
You are basically, right.... but there is another type of LSD. It's the electronic one on stock STi typeRA and recent STi, it is called DCCD and the locking % can be set at 100% lock.
Also STi sells fully locked center diff. No slip differencial. It's just a piece of metal.

>>>>
also, why would you need front and rear limited slips to make use a center differential that split the power 35/65 front to rear?

All the STi equipped with DCCD has both front and rear LSD. It is because the center diff is open when the torque split is 35:65. When the center diff is completely open, the traction can go back and forth without regulation when wheel spin occurs.
Let's say you replace stock viscous center LSD to Cusco Tarmac gear and right front wheel starts wheelspin, maybe because it goes off the ground or maybe because of wet road. When right wheel starts wheelspin, left front will get no power because the front diff is open. This is same situation as FWD car with open diff.
Now, when the front is having wheelspin and rear is not, all the power will be sent to front if the center diff is open. Car won't move forward till the right front regains traction.
That's why you need both front and rear LSD when you put open diff in center. Stock center is LSD for a reason.
It does say you need front and rear LSD to use Cusco Tarmac gear 35:65 center diff, in Japanese catalog.

dropkick_muppet

iTrader: (4)

okay, so what you're saying is that the Cusco 'Tarmac' gear, is actually not a limited slip unit, correct? if it's not a limited slip, then it's either open or locked, right? there really aren't any other possibilities other than limited slip, locked/solid/welded, and open. anything that splits toque in a fixed manner can't be an open diff, at least in my understanding. it could be locked in such a way that it would force an uneven torque split, but i'm not exactly sure how you'd accomplish that from a mechanical perspective.

for the situation you described (all power to one wheel, i.e. right front assuming a stock WRX drivetrain with Cusco Tarmac gear), the center diff would have to go from 35:65 to 100:0, thereby completely taking the limited slip rear out of the equation and only sending power to the front open diff. that's only possible with an open center diff. i don't understand how the tarmac gear could suddenly change from 35:65 to 100:0.

i'm still of the opinion that the worst case scenario is 35% of the engine's power to one front wheel, while the other 65% is split between the two rear wheels. given that situation, it sounds like a front limited slip would improve the performance of the 'tarmac' diff, as you'd wind up with all wheels being driven at some percentage of the engine's output, rather than just three wheels. that said, you can easily get three driven wheels with the stock WRX's center LSD, rear LSD and open front.

does anyone know the percentages of the stock WRX's center and rear LSD units? my guess is that the center is probably very close to 50/50, possibly with a front drive bias, and that the rear probably maxes out as 75:25 so as not to be intrusive into every day driving.

go go go

iTrader: (2)

maybe how I described was confusing...

Cusco tarmac gear is open diff and it is 35:65 fixed split.
It doesn't do 50:50 and it is not LSD

The DCCD is the one that changes any ratio between 35:65 to 50:50. when it is 35:65 it is free. when it is 50:50, it is locked.
It is electro-magnetic LSD.

Other example of variable torque bias LSD is ATTESA system on GTR and ACD on Evo. Evo also has variable rear diff AYC.
ACD and AYC didn't come to US though.

On Subaru, there is no center diff that does 0:100 torque split.
GTR's ATTESA does however, and when it is 0:100, it is completely RWD of course.

The rear viscous LSD on stock WRX is very weak. You can't even notice even when you are turning in the parking lot, can you?

dropkick_muppet

iTrader: (4)

how can an open diff do that? there must be something that i'm not getting...

true, that's the whole point of a viscous unit. they're supposed to be as unobtrusive as possible.

dropkick_muppet

iTrader: (4)

okay, i've got an email into the guys at JapanParts.com, so i'll be sure to post whatever information that they're able to provide.

dropkick_muppet

iTrader: (4)

here is the word from the guys at japanparts.com -- funny that they refer to i-club!

Quote:

According to CUSCO, TARMAC GEAR fits on JDM Impreza WRX(GDA) w/ 5M/T and it changes Frt.:35/Rear:65 torque. And TARMAC GEAR requires the LSD on both Frt. & Rear.(exp. CUSCO LSD Type RS) and car does not have the Driver's Controll Center Diff. It will fit on w/o Frt. or Rear LSD but will not work well. JDM Impreza GDA_has_the center diff. with biscus LSD, Frt. Open Diff. and Rear Mechanical LDS as standard. We advise you that you'd better buy the TARMAC GEAR first and if you_are not happy enough, get the Frt./Rear Mechanical LSD.
You'd better visit and ask someone at the ower's club follow,https://www.i-club.com/forums/

Z1 Performance

iTrader: (7)

Rather than messing with the center diff, the front is the one to do - a front LSD completely changes the character of the car for the better.

The rear visvous on the WRX is not weak at all.....in fact, it can withstand much more than most WRX's can dish out.

Adam

Steppin Razor

iTrader: (5)

thank god someone said it.

sigma pi

iTrader: (6)

if you get the center diff can you pull the ebrake up and turn?