all wheel drive

You're right, but you have a center differential, as do the high-end automatics (VTD), but not the Imprezas, Foresters and regular Legacy automatics.

And I really meant that it wouldn't be easy to turn the thing, with one wheel or two dragging. Now if it pulsates (and interesting notion, which as far as I know isn't mentioned in any service manual) that's a bit different, you'll only be dragging wheels a part of the time.

If you would read the service manual, Volume 1, "General Information", Chapter "Specifications", Section "D. Transmission" you would see that it is stated for the 4AT :

"Reduction gear (Front drive)

1st reduction 1.000 Final reduction 4.111

Reduction gear (Rear drive)

Transfer reduction "--" (N/A) Final reduction 4.111 "

To me, this means that there is no difference between front and rear shaft speeds.

Now, if you look at a 2 litre turbo (WRX) 5MT manual transmission you'll find :

"Reduction gear (Front drive)

1st reduction "--" Final reduction 3.900

Reduction gear (Rear drive)

Transfer reduction 1.100 Final reduction 3.545 "

This may very well give you the 40/60 split you and your sources mention.

If you read the Subaru documentation carefully, it is quite clear that whenever they sing the praise of their system, they are talking either about their manual transmissions, or their VTD auto transmissions. They deal with the MPT transmissions mainly by omission.

The STi only comes with a manual transmission. This thread was discussing the automatic transmission of a Forester.

The service manual states for the STi 6MT :

"Reduction gear (Front drive)

1st reduction "--" Final reduction 3.900

Reduction gear (Rear drive)

Transfer reduction 1.000 Final reduction 3.900 "

So the STi is split 50/50.

This is not correct. The STi has the same final drive ratio for front and rear diffs, but this is because they have to be the same. The only reason the EAT is different is because it's a viscous coupling. In order to allow a more progressive lockup, rather than an abrupt transfer of torque when the fluid is suddenly heated up, they use slightly different final drive ratios, to cause a constant amount of "slip" in the center diff, allowing some marginal transfer of power to the rear wheels before wheel slip actually occurs.

The torque split in a mechanical diff is determined by the diff's own internal gears, but by the final drive ratio of the two axles.

Paul !!!! It may be that the 60/40 front to rear split (gear set) that is in the 4EAT depends on a certain amount of torque difference favoring the lighter weight rear of the car. This would mean that the default ratio 60/40 may be very close to 50/50 in normal driving which would result in very low slippage on the continuously variable clutch pack Sound reasonable Paul? Just a guess on my part. I did read that the transfer gearset is reversed to

40/60 for the Legacy GT (2000 model) to "give the driver a more sporting feel" I do have the factory service manual for my 2000 Forester (section

Actually I was being a bit facitious - especially since my VCD in my 95 Brighton is toast and tends to lock up after a bit of hwy driving. I tried to back into a parking spot the other day and stalled it twice because of binding!

-- Dominic Richens | snipped-for-privacy@alumni.uottawa.ca "If you're not *outraged*, you're not paying attention!"

Actually, the tech manual does mention the 'pulsating' in a sense... it's referred to a 'duty cycles': "Transfer Clutch Assembly (AWD). The transfer unit consists of a hydraulic multi-plate clutch and a hydraulic control system incorporating a duty solenoid.... Duty solenoid "C" regulates the MPT clutch. It is controlled by the TCU, which determines the degree of AWD by altering the duty ration. As the duty ration increases the amount of AWD decreases. The clutch itself features friction discs that are designed to slip. This eliminates torque binding during tight cornering. In order to get power to the front wheels, the reduction gear powers the reduction driven gear, which is attached to the drive pinion shaft. For the rear wheels, power goes from the reduction drive shaft to the MPT clutch hub..... Beginning in the 1990 model year a new transfer piston was added. This improved torque split control, preventing the MPT clutch from further applying during high speed driving."

So much for some of the other 'theories' out there.... John

Thx!??? That's *it*?? All of that arguing you've caused and all you say is Thx???!!! For shame!

John

you have just gone in a circle with your conclusion. probably just a minor oversight. by your own reasoning, the torque is not a function of the ground or anything other than the f/r ratio in the center diff.

there are two modes, when there is no wheelspin, and when there is wheelspin at one end and/or the other. when there is no wheelspin the distribution remains 64/36, regardless of the DCCD setting, all it does is increase drag (friction) in the system as it approches lock. when there is wheelspin, if the diff is in "open" the torque will be sent to the wheels with the least traction, and when the diff is in "lock" the torque will again be

64/36. 50/50 cannot be achieved under any circumstances.

comments please?

John Emdall wrote: when there is wheelspin,

A minor nit...

When there is wheelspin and the diff is locked, for all practical purposes the torque split is determined by which axle has the traction and which doesn't, not the the diff or anything else in the drivetrain.

Imagine the car sitting with it's front wheels on dry pavement and the rears on glare ice. Just for fun, the car is hitched to a big trailer frozen to the ice. You put it in gear and press on the gas. The rears will spin until the diff locks. Now the axles both turn together. Since the rear continues to spin, there is zero torque (and I'm using the word torque in the physics sense) coupled to the ground. On the other hand, the front axle, as long as the front tires don't break traction, couple 100% of the engine's torque to the ground.

A minor correction to my minor nit...

The car moves forward. The front axle is supplying 100% of the torque because it has 100% of the traction. The rear axle, even though it is not spinning, contributes 0% of the torque because it has 0% of the traction, since it's on the ice.

********* disregard *********************

Since the rear continues

I'd be interested in knowing where you get the 64/36 figure front. The service manual states that the gearing is identical front and back, so I don't see how it can be anything other that 50/50 (nominally).

There's a planetary center differential, which divides the torque unequally from front to rear. The transmission output shaft connects to the planet gear carrier, the front axle connects to the ring gear, and the rear axle to the sun gear. The ratio between the diameter of the ring gear and the sun gear is the ratio of torque split from front to rear. This works because the planet gears divide the force on the gear teeth equally between the sun and ring gear (because otherwise the planet gears would rotate), and the torque on the output shafts depends on the force on the teeth multiplied by the radius of the gear. You can get any torque split you want by changing the dimensions of the center differential.

Back to my original question: Does anyone know if the STi AWD system is the same as the other AWD Subaru systems? Other than of course it being a 6-Speed and having the DCCD.

BlueSTi "Scary-Fast"

The most detail would probably be found at

or

It is my understanding thet the STi uses an electromagnetic center diff.

Carl

1 Lucky Texan

BlueSTi wrote:

No, it's different. It uses a planetary transfer case providing a fixed

64/36 torque split, with an computer-controlled electromagnetic limited slip coupling. The WRX uses a viscous center limited slip diff that isn't nearly as good at putting the power where it's needed.

I understand that you can get any split you want through the center differential. But wouldn't this show up in the gear reductions stated in the service manual ? The WRX's specs show different gearing front and back (balanced by the center differential) but the Sti's specs are equal front and back.