What's the DIFFerence?

Question about Diffs for an Imp (impreza)

Q) Is a 12KG Center diff a good upgrade from a 4KG?


Situation :
So currently i have me a GC imp all caged up , and a 98 RS transmission (and rear diff) hanging out in storage (couldn't pass it up for $200). My 2011 plan is to go AWD.

I also currently have an OBX Torsen Front Diff which should fit in the 98 gear box. (i **think, but maybe I think wrong**) So I'll be having this transmission opened up for hopefully the obx installed, and that would be a great time to Install a center diff.

It looks like i've located a 12kg that could be shipped to me for $800 (including purchase price) So far that's a bit cheaper than I've even found a rear diff for!

I plan to Later on get a Rear Diff (2 way clutch? 1.5 way kaaz (cheaper) ) ?

Is this plan solid? do you foresee problems? is the 12kg junk? Will my stage times benefit from an upgraded center (assuming the car is driven well, like petter solberg driving the before and after)

long term I'm thinking
Step1) upgrade power train
Step2) upgrade brakes
step3) upgrade motor NA 2.5l


.. I Imagine right off the bat 120hp with awd doesn't really get wheel spin but when eventually i'm making 220 it could be an issue.

well that's enough of me spinning my wheels ... hehehe thanks for input

12 Kilograms are three times heavier than 4 kilograms. so, If'n you're worried 'bout sprung weight, then the 4KG diff is better.

Or, if you need the extra mass of the gear ring to act as another flywheel to keep up your momentum since there's only 120 horsies, then the 12KG unit would be better.

Or, if you're referring to the breakaway torque of the limited slip plates, then Iunno.

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---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973



Edited 1 time(s). Last edit at 08/05/2010 12:29PM by SgtRauksauff.

It'd be sprung weight

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Welcome to the cult of JVL drink the koolaid or be banned.

SgtRauksauff Wrote:
-------------------------------------------------------
> 12 Kilograms are three times heavier than 4
> kilograms. so, If'n you're worried 'bout unsprung
> weight, then the 4KG diff is better.
>
> Or, if you need the extra mass of the gear ring to
> act as another flywheel to keep up your momentum
> since there's only 120 horsies, then the 12KG unit
> would be better.
>
> Or, if you're referring to the breakaway torque of
> the limited slip plates, then Iunno.


Sh'up! it's Subarus rather unique measure of turning resistance.

Stick the whole fawkin thing in a lathe and set to 60 rpm and put the torque wrench---old bar type---against the bed and switch on the late.
As it turns at 60 rpm it better deflect the needle to 12 kg/m

fawk it, stiff cenetr is OK.
I have 50 n/m in my Xratty center diff.

>
> ---** To be in compliance with the Anarchy **---
> Jorden R. Kleier
> Baraboo, Wisconsin, USA
> 1990 Mazdog Protege 4WD
> 1973

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

To translate into more simple terms the 12Kg has nothing to do with the mass of the unit, it has to do with how much torque the unit will transfer to the other end of the drive line at a given RPM.

So a 12 Kg unit will transmit 3x the torque to the non spinning wheels.

Since all my AWD rally experience is in Audi's and Mazdog's where I had 100% lock up between the front and the rear or no lock I can say I vastly favor the 100% lock up or really stiff diff even in a 120 HP Quantum Syncro Wagon I once owned. I can also add my street Subaru Legacy GT with the stock diff needs way more lock up, I can tell this often when I drive in the rain with vigor.

Though a center diff may be sprung weight, the issue is it is still rotating mass and as such will effect transitional speeds ~3 times the same added mass to the non rotating parts of the car such as the co-drivers belly. Again we have no idea if the mass of the 4kg and the 12kg diffs are different however as the numbers are not about the weight.

I am no Subaru expert so I may have mis stated something in there but the over idea is correct.

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In the long run reality always wins.

Derek the difference is all inside. The"centipose" of the silicon sploogie goo is higher---centipose being diff talk for the viscosity----and/or the tiny little steel round rings--less than 0,5mm/.020"---may be skinnier yet leaving less space between the discs and plates. So it's stiffer, kinda like upping the preload or the ramp angle on a good clutch plate diff.
If you ever come visit I can show you a Cossie rear VC i have that was taken apart. All the VC things are all made by GKN branch called GKN Visco-drive and they are pretty nice units IF one can set them for performance rather than the light set ups everybody does OEM. they do them light so there will be no problems with the functioning of the ABS---can't have the wheels turning at the same rate, eh...

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

The 20kg diff is perfect.

I ran one for years before exploding a 5 spd, and going to a 6 spd with DCCD.

It will take some getting used to, but it completely transforms the car.

---

Chris

speaking of viscous stuff, I remember reading or hearing about some Mazdog team somewhere that drilled holes in the diffs, heated 'em up for a few days to drain the goop, then put in some better goop. Better goop being silicon from breast implants..

Powered by boobies!

--sarge

---

---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973

More info on that Jorden? Where do ya get teh gooo?

What happens after so many miles on a viscous diff and the goo wears out?

---

Michael LeCompte

I'm going to have to dig through my archives when I get home. I kinda dismissed it at the time as neat info, but not applicable to anything that I had at the time.

--Jorden

---

---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973

pikespeakgtx Wrote:
-------------------------------------------------------
> More info on that Jorden? Where do ya get teh
> gooo?
>
> What happens after so many miles on a viscous diff
> and the goo wears out?

Who said it wears out?
HA! Assumption! HA!

It is my understanding that VCs work as set for 100s of thousand miles barring the fluid escaping.
Kinda like DSM ECUs work fine until the smoke inside escapes, then you send them off to get fresh smoke installed...


Hey, weren't you 'spossed to be getting some real suspenders for your carro, mang?
>
> Michael LeCompte

---

John Vanlandingham
Sleezattle, WA, USA

Vive le Prole-le-ralliat

www.rallyrace.net/jvab
CALL +1 206 431-9696
Remember! Pacific Standard Time
is 3 hours behind Eastern Standard Time.

Thanks a bunch.. that clears that up (mostly)


so in a nutshell ya the closer i get to true 100% locking the better? sweet.

I'de opt for the 20Kgm/100rpm unit over the 12Kgm/100rpm unit but the little research i've done so far says that in a "pre 99" transmission only the 12kg fits.
And i have a 98 RS transmission.

A1337STI Wrote:
-------------------------------------------------------
> Thanks a bunch.. that clears that up (mostly)
>
>
> so in a nutshell ya the closer i get to true 100%
> locking the better? sweet.
>
> I'de opt for the 20Kgm/100rpm unit over the
> 12Kgm/100rpm unit but the little research i've
> done so far says that in a "pre 99" transmission
> only the 12kg fits.
> And i have a 98 RS transmission.


Both the 12kg/100rpm & 20kg/100rpm STi center diffs werwe available for the phase 1 trannies (which is what you have.) Not sure what is still available for those now (unless of course you can find a used unit.) You also can bolt the whole center diff case and everything from a later model tranny onto your older tranny, provided you switch it to 1:1 transfer gears if it's from a WRX that had 1.1:1 transfer gears equipped. However, only the 20kg/100rpm was available for the later transfer case.
Both the 12k or 20k diffs are quite good for gravel rally. However, referring to your original post, you are all backwards in the order in which you plan to upgrade the diffs. By all means, upgrade the center diff first. That will make the biggest improvement to the handling. But don't even think of installing a front lsd until you have a rear one, The front you can even consider optional once you get the center and rear upgraded. A front lsd without a rear one will just make the car push even worse than before. And a torsen style diff is the last thing I would choose for a gravel rally car.

pikespeakgtx Wrote:
--------------------
> More info on that Jorden? Where do ya get the gooo?
> What happens after so many miles on a viscous diff and the goo wears out?

I don't know that it was because the goo wore out, I think it was because the goo that they used was thicker/stronger/had a higher centipoise (measure of dynamic viscosity) than the original.

I asked my friend about it again, and he said:
"Eric Carlson told me Archer racing used the silicone stuff for breast implants in the stock viscous center diffs of 1st gen DSMs. Eric and his friend Chris [Czyzio] had one of those diffs in their Eclipse rally car and they said it was awesome and never broke."

So, I was wrong when I said it was in some Mazdog diffs. But the process should be the same, basically. Not sure where to get Boobs in a Can, though. Wait, I thought boobs WERE 'cans'....

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---** To be in compliance with the Anarchy **---
Jorden R. Kleier
Baraboo, Wisconsin, USA
1990 Mazdog Protege 4WD
1973

Yeah I need to get some good suspenders... but it's just LIFE getting in the way.

I put an offer on a house on Monday (and you know they don't come cheap here in the bay area), still waiting to hear on that.

On top of that the girlfriend put me on the hook to buy a 1900 freaking dollar dog this weekend...

Which has to fly with me to Maryland for my good ol' buddy's wedding on saturday. Which I get to loose a week of work to go to. Yippie.

aaaaaaaaaaaand.... that carro in question.... has a tranny leak that spilling 16 dollar a quart fluid all over the ground daily. I replaced the drivers side axle seal and the retaining clip, but that didn't fix it. The fluids comming from above that... It's either where the shifter goes into the housing on top the gearbox or it's the seal around the 5th gear cover. It's so hard to tell cos the body is in the way. Last night after work I pulled the battery, bat tray, and tranny mount to get a look from the top and of course it hadn't leaked.

But this morning I get to work and the case is wet again.

---

Michael LeCompte