[miked801]

Anyone know for sure what type of rear diff the STi will have? I know it's mechanical, but I'm wondering if it's Clutch-type or Gear-type. Also, if it's Gear-type, no chance that it's a Quaife is there? I know that some of the current model lineup use "double-planetary gear type variable torque distribution" for center diffs; maybe they'll use that for the rear?

 

[jbhebert]

The STi will most likely have a standard clutch-type limitted slip in the rear, like previous cars, and torque-biasing diffs in the center and front diff. These are fluid dispersing diffs very similar to the Quaife.

 

[miked801]

That's a little disappointing. I suppose that's what you get with a car originally intended for rally use... those guys only have to last three days on one diff. Any ideas what the average replacement age on those is?

 

[jbhebert]

HIN?

I've run clutch-type limitted slip differentials in a number of vehicles and have not had any problems with them. Keep the fluid clean and they should last the life of the car. I'd rather have a cluth-style limitted slip in the rear since it is "always on". Most fluid dispersing diffs like the Quaife require time to move the fluid around to correct for wheel spin. Keep the Quaife in the front (better for steering) and the clutch pack in the rear.

 

[miked801]

thanks for your informative reply. my understanding was that clutch-type diffs typically needed maintenace more frequently than that. i stand corrected.
onto another (related) question- now that i've had some folks here explain to me why dropping the clutch is so hard on awd cars, it sounds to me like the clutch-type diff would most likely handle it the best. is that true?

 

[jbhebert]

The two items are unrelated; "dropping the clutch" refers to rapid engagement of the clutch from a standstill. This hurts the entire driveline because of the sudden shock it receives. A clutch-type limitted slip differential is essentially a standard open differential that has friction disks on the spider gears. These disks allow some slippage for cornering and the like, but limit the amount of traction loss when one tire experiences less traction than the other. Typically, these differentials are rated in a percentage. Most are 25% locked, which is good for most street use. I've seen them as high as 40% locked, such as in my Alpina. Much higher than that and you're looking at a hard locker that uses gears instead of clutches.

Bottom line, on an AWD car the rear differential has little to do with easing the pain from a dropped clutch.

 

[miked801]

Ok that makes sense. My reasoning was that all that power slamming the diff at once would be a tiny bit less destructive on a clutch-type diff since it would slip, whereas a gear-type wouldn't.

 

[jbhebert]

This is potentially true in a situation where one of the rear tires has significantly less traction than the other. However, both types of diffs will allow slipage at first. The clutch type should actually allow less slippage immediately, whereas the fluid disersing diff will slip more at first until it has a chance to bias the torque to the other wheel. So neither one really has a distinct advantage in this area.

 

[mikeofpa]

Praxis System Air Ride Suspension

I read that the rear diff is a "180", the WRX is a 160. The 180 is larger, and contains more fluid for cooling.

 

[super_bean21]

any of you guys experience a break in period with the rear lsd? in my wrx wagon, just changed the rear diff fluid with mobil synthetic gear oil, (75-somethin') the front fluid still looked clean and amber, rear was clouded grey already after 12900 miles. any similar stories? this wouldn't be some sort of strange additive would it? is it customary to break it in similar to the engine?

 

[jbhebert]

On the standard WRX, the front differential is open and the rear is limitted slip. The likelyhood is that the rear fluid will get cloudy much quicker because the clutches wear as they rub against each other. I'd say it's probably pretty normal.

 

[91TB78]

I've heard we get the rear mechanical LSD. In the brochure that SOA sent out it says on the back "Mechanical Limited Slip Differential."

 

[jbhebert]

That's what I heard, as well. The front will be viscous for less unpleasant feedback through the steering.

 

[91TB78]

I think the drivetrain is spot on in terms of diffs. I'm glad they chose a mech diff in the rear..I would imagine they are going to be quicker reacting than say a viscous type.

 

[Chin]

Argh, Please give advice.

'00, '01 RS => Viscous rear, open front
WRX => Viscous rear, open front
Sti => Mechanical rear (not sure what type), Suretrak front

Seems to be a lot of confusion on what is on what.

Chin