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99 UK S-turbo
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Discussion Starter · #1 ·
OK, ignoring the recent icey weather we have had, I'm fed up with the front wheelspin in the foz, and am truying to decide what to do to fix it, my old Ford 4x4 didn't suffer from it (33/67 torque split and much stiffer centre diff) and I want that feeling back!
99MY 5MT s-turbo (circa 220bhp and 230lbft) running 04 Impreza STi suspension

Thoughts
1/ STi 20NM centre diff
+ More traction
- More understeer without extra mods that may or may not get it back to where it is now

2/ ALK replication using spacers to lower the wishbone mounting
+ reduced understeer
- only slightly improved front end traction

3/ DCCD from a classic STi
+ Rear biased torque split
- Expensive (and how) and extra compexity in fitting, also ABS doesn't like it!

Thoughts/ideas?

Simon
 

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2003 XS
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169 Posts
4/ Buy a new car

I'm totally with you, Simon. Having driven rear wheel drive cars most of my life, the front wheel spin drive in the Forester drives me nuts. Swapping in a DCCD would be pretty cool. I have no idea what's required to do that but if you figure it out, let us know.
 

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1999 (UKDM) S-Turbo (LT) 4EAT
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217 Posts
I share your frustration Simon. I miss my torsen equipped '88 Audi 200 Quattro in these conditions. I don't miss it's impossible location of spare parts problems but that mechanical/vacuum system and it's variable 25/75/25 centre diff which locked automatically and the manual rear diff lock... well, it was just so much more predictable than my Foz... and I could switch the ABS off too when required.

It's probably unfair of me to compare a manual torsen quattro with a 4EAT Foz but that damn centre diff just annoys the hell out of me. I want it locked when it needs to be, not when the slow brain decides to lock it, momentarily destabilising the back end. The mechanical system was just so much quicker to react.

I know of the modifications available to the Foz to install manual diff lock and ABS defeat switches, but they would invalidate my insurance so it's a no go.

Sorry to say this on the Foz forum but I'm afraid I prefer the old technology quattro to the chip-controlled modern world.
 

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2009 Outback XT-B 5MT
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10,241 Posts
You'd need the DCCD from a newer 5MT DCCD trans (post-99) if you want that swap to be easier. Finding that setup will be challenging though. As for ABS not liking it, you could easily wire up a relay that opens the DCCD when the ABS engages.

PPG makes a helical center differential, but actually obtaining one is challenging as well. They make a front diff that's readily available, but the center requires a few phone calls and no lack of dollars ready to spend. The helical rear diff is a similar situation to the center. Available if you make the right calls and have the money to spend. That said, 3 helical LSD's drives like a Humvee. Any slippage, just drag the brakes. 3-wheels on ice, one on dry works like a charm!
 

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99 UK S-turbo
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Discussion Starter · #6 ·
I was aiming for the DCCD from a 98-00 Impreza STi, not aware of any DCCD 5speed after that? (all 6 speed). but they sell for at least £350 ($500) so not cheap, installing it physically is easy, adding the electronics is harder (and can be pricey) or I could get it rebuilt as a conventional mechanical plate diff.

I have tried to find an Audi 'Torsen' (Helical) centre diff as its a similar layout (concentric front input/outputs) to ours and may be a cost effective way of doing it (although it may be an impossible job - who knows)

I'm running 19mm front/20mm rear bars, not sure why you ask though as the wheelspin is in a straight line!

Also as both front wheels are spinning, I'm not sure a front LSD will help, also its a much more expensive and time consuming mod than any other!

I have a Cusco rear plate diff, to fit but while its the fronts spinning, there seems little point!

Simon
 

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Discussion Starter · #8 ·
Thanks peter, may be good off road but will cause more understeer than the 20Nm centre which is an easier option!

Simon
 

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2009 Outback XT-B 5MT
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Peter, that is only for the automatic transmission.

Simon, Just because both fronts are spinning, doesn't mean there will be no benefit from a front LSD.

The transfer gears changed in 99, and an old style diff won't fit a new style transfer gear. The easiest thing to do, supposing the DCCD units have 1:1 transfer gears like you do, is to swap the whole back-half of the transmission (so the rear cover and the transfer gears). If you do that, model year doesn't matter.

Finding one cheap will be difficult, but not impossible. I'd keep your eyes open and see if you don't come across a good deal on e-bay. Remember, if you find a 5-speed DCCD box in good shape, the gearset is worth a lot to kids in America who think STi 5MT gears are worlds stronger than what they have. You can probably break even!

The Audi diff is unlikely to work. I'd be shocked if the spline count and bearing surfaces were even close to the same.
 

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I was aiming for the DCCD from a 98-00 Impreza STi, not aware of any DCCD 5speed after that? (all 6 speed). but they sell for at least £350 ($500) so not cheap, installing it physically is easy, adding the electronics is harder (and can be pricey) or I could get it rebuilt as a conventional mechanical plate diff.

I have tried to find an Audi 'Torsen' (Helical) centre diff as its a similar layout (concentric front input/outputs) to ours and may be a cost effective way of doing it (although it may be an impossible job - who knows)

I'm running 19mm front/20mm rear bars, not sure why you ask though as the wheelspin is in a straight line!

Also as both front wheels are spinning, I'm not sure a front LSD will help, also its a much more expensive and time consuming mod than any other!

I have a Cusco rear plate diff, to fit but while its the fronts spinning, there seems little point!

Simon
Ah! Was thinking more along the lines of front wheel spin when exiting a corner. Is there anyway to modify a center diff to change the split like DCCD without the adjustability? Driving other cars, 35/65 seems to be the sweet spot. Wouldn't changing the weight distribution help as well (at least with putting the power down).
 

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^ Cusco tarmac diff would be another option. But as I recall they're a bit difficult to get your hands on and $$$.
 

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1997 S/TB
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245 Posts
Excuse the simpleton's approach, but couldn't you swap the front and rear sections of the centre diff internals to reverse the f/r bias to the opposite of what it currently is? Not sure if it'd give the required effect, or even if it's possible, but worth a thought, even if there's a different way of doing it? I guess what I'm really asking is: would a rear biased diff give a better feel?
 

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2003 XS
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Excuse the simpleton's approach, but couldn't you swap the front and rear sections of the centre diff internals to reverse the f/r bias to the opposite of what it currently is?
Unfortunately, no.

would a rear biased diff give a better feel?
Yes. It would cause the car to oversteer rather than understeer when throttle is applied in a corner. Rear bias allows the driver to steer the car using throttle inputs.
 

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Forgive me for asking, but why is this not possible? Bear in mind I'm a complete novice when it comes to the internals of this type of diff! Thanks
 

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2003 XS
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Foresters equipped with a manual transmission use a viscous center differential. Under normal driving conditions (i.e. no wheels are slipping), the system supplies equal torque split between the front and the back. Only when one end starts slipping does the differential begin to compensate. It's a symmetrical system to start so, if you flip it, in theory, it would work exactly the same.

Think of two plates held together by friction surrounded by fluid. Each plate is connected to an input shaft. One shaft goes to the front wheels and one shaft goes to the rear wheels. When the front tires slip, the plate connected to the front wheel input shaft spins faster than the plate connected to the rear wheel input shaft. This difference in speed shears the liquid inside the diff and heats it up. This makes the liquid thicker, which increases its ability to transfer torque to the slower moving input shaft.
 

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I've got a Cusco tarmac centre diff that I just took out of the Impreza. I'm hoping to trade it for a 12kg viscous centre diff. I know I don't want a 20kg, I'm hoping a 12 will be better than the 4kg but not as solid as welding or 20kg.

Being an open planetary setup....I do not recommend it for slippy surfaces. It took me exactly 1 day in the snow to try it and swap it out. I might as well have been in my 2wd pickup truck with no weight in the back and race slicks for tires.....(well maybe all seasons). Any throttle over bare minimum results in rear end breaking loose and complete loss of AWD goodness.

That said....find me a 12kg center diff and we'll make a trade!
 

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Simon,

If you can hook up a DCCD, it should give you a 35/65 torque split without any electrical/sensor input. This may be a pretty good option considering the alternatives.

Mark
 

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99 UK S-turbo
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Discussion Starter · #18 ·
Jstorm, yes the Cusco centre is open and so not what I want really, nor do I probably want an 'open' DCCD although the post 97 ones are 60:40 so not too bad.

As far as I'm aware the 12Kgm (yes not Nm as I was using doh) centre is only availbale for pre 97, after that its 20 or 4.....

Looks like I'm down to a 20Kgm or a DCCD either electronicly controlled or with rebuilt clutch pack (more preload and a play with ramp angles) to act like a regular plate diff.

Simon
 

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Hey Simon....you can always put a phase I tail section on your trans (if you can score a 12kg diff).

Speaking of which....any trans gurus know if phase I transfer gears will work in the phase II case? I'm working on a potential swap deal and I'll need new teeth should I pick up the parts.

On the DCCD subject, you could always use the "Spiider DCCD controller" or rig up a 12v switch similar to the torqumada lite mod..... 40/60 when you want the tail to be comin 'round, then flip a switch to lock it up to pull you the rest of the way through!!!
 
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