scottzg wrote:is there a way to have a rear biased drivetrain and not spend the 8k for a 6 speed?
Oh, and not swap to an open center diff.
The vicous center diff isn't, technically, an open diff. It's a limited slip, working exactly as a vicous rear diff (or front) works. With the center diff, there are a few things going on. To start, you must know how it is configured, and how the transmission works. There are two shafts in the transmission. The main shaft, and a secondary shaft (the output shaft). The mainshaft is what the clutch resides on. First gear, Second gear, and Reverse are machined onto this shaft, they are all one peice. Third and Fourth gears ride on the input shaft but spin freely on needle bearings. Third and fourth gear are actually all one peice, and are pressed onto the output shaft (effectively becoming one peice with the output shaft). The shift collar locks the input shaft side of third and fourth, to the input shaft. In neutral, clutch pedal released, 1st and 2nd gear are both spinning on the input shaft and the output shaft (the mating gears ride on needle bearings on the output shaft). When 1st gear is engaged, the shift fork moves the engaging collar onto the engagement teeth, which locks 1st gear to the output shaft, spinning the output shaft with the power of the engine. When in 1st or 2nd gear, with the clutch out (pedal depressed), and the vehicle not moving, is the only time nothing is moving inside of the transmission. So the output shaft begins turning when the clutch pedal is released. The output shaft has two ends. One is the pinion part of the front diff's ring and pinion. The other end of the output shaft, goes to the center diff. The center diff is a sealed vicious unit. There is a shaft coming out of the center diff, with a gear on it. There is a shaft with a gear on it "idling" next to the center diff, meaning it's riding on bearings and the only power applied to this shaft is from the center diff's output gear. This shaft is splined, and those splines meet with the drive shaft to supply power to the rear end of the car.
So, your in third gear, and everything in the vehicle is properly warmed up. Your cruising along on a slippery road, and you nail it. The front tires attempt to slip upon acceleration (the front wheels being the path of least resistance for torque to exit the vehicle. Note: Some wheel slip is required in both the MPT center diff, and the Vicous center diff. Otherwise, there will be no change in transmission output speed, and nothing for either center diff to equalize). Engine RPM's increase, and the output shaft of the transmission increases in speed as well. Now, the inside of the center diff, I am slightly confused about. But I would imagine that there are plates suspended on bearings with silicone fluid floating between them. As the fluid heats, it expands. There are two sets of plates, one from the output shaft of the transmission, and one set attached to the shaft with the gear on it, that would drive the rear wheels. How it's setup, I'm not completely clear, it could use vanes like a windmill or it could be far more complicated than anything I've said so far. But the basic function is as follows. One side of the center diff (the plates from the tranny output), are spinning faster than the output plates to the rear wheels. The input plates cause thermal expansion of the silicone, and it attempts to force the output plates to spin at the same speed as the input plates. The center diff only sees these forces. Nothing other than the plates/shafts, is designed to move, nothing is designed to actually touch, the only "wear" item is the fluid. So the tolerances are incredibly close. If I knew more about the setup, I'd imagine that I'd be able to get you near exact tolerances to speed, temperature, etc. on what the center diff requires to operate. Realize, also, that there is an incredible amount of torque going through the transmission at any given time, ESPICALLY at lower gears. Remember, there are actually thousands of foot-pounds of torque at the wheels, not the hundreds shown by dyno's (those readings are estimated flywheel readings).
Thanks to all of these wonderful mechanical concoctions, the time for the center diff to lock the plates together, is almost non existant. Since the input side of the center diff is always spinning, and the output side of the center diff is always spinning at a slightly slower speed (or tries to, thanks to drivetrain frictions and the rear wheels essentially "dragging") the center diff is almost always near the verge of locking. In fact, take a quick drive around the neighborhood and then lift the car into the air. I bet when you try to spin one of the wheels, another one at another end of the car will try to spin also. Better yet, jack up a "cold" car and spin one of the front wheels. Fairly easy to spin. Yank the E-Brake (rear wheels only, dontcha-know), and the front wheel suddenly becomes slightly more difficult to spin. Turn on the car, put it in 1st gear and let out the clutch. All 4 wheels (or at least one at each end) should spin. Yet again, try to stop just one of those rear wheels. Even cold, it's difficult. Quickly yank the e-brake as the engine is "idling" in 1st gear, and the car will likely stall. With a clutch type center diff, you always risk burning out the clutch packs in the diff. Clutch packs are like any other friction surface, they wear fastest when they are slipping. Off the show room floor, the first time duty-solenoid-C engages any of the clutches, they are wearing out, and becoming less and less effective as the time goes on. While I will admit, a clutch type center diff IS, in fact, a split second faster acting than a vicous type, a vicous type is far more consistent. I doubt there is any notable performance advantage to either, espically since any Subaru with a clutch type center diff, has a transmission weighing significantly more (The 6-speed in the STi weighs a staggering 80 pounds more than the 5-speed in the WRX).
I realize that this was an INCREDIBLY long post, and if you want the cliff-notes, I'll state it simply. The "lag" of the vicous center differential, is almost non existant. In my own personal experience, I've found that I will immediately spin one front and one rear when I'm driving on an extremely low-grip surface. Furthermore, short of locked, the vicous center differential is the closest you will come to a true power variable unit. As far as I know, Torsen style differentials require load variation in order to function properly, so on ice, for example, the vehicle with a torsen style center diff (a.la. VW), only the front wheels will spin. At least with the vicous (and to it's credit, the MPT Clutch diff), two wheels are always spinning.
Again, sorry about the lengthy post, I hope at least some information was gained about the Subaru Manual Transmission. I could be completely incorrect, and that likelihood is entirely possible, but at least I might have opened eyes to the confusing mess of gears that lies below the shifters in our cars.
Oh, and Subaru offers center differentials through STi that have different tolerances, allowing more power to be shifted between ends of the car. I believe stock in the WRX is 8Kg, and STi offers 12Kg, 16Kg, and 20Kg. What that all refers to, I'm not sure. It may relate to the effort required before the fluid shears, and allows the front wheels to spin free towards oblivion.
If I am wrong, which again is totally possible, please correct me.
A sole final note, both the MPT clutch system and the Vicous clutch system will also direct power forwards to the front wheels if the rear wheels slip. It works both ways. Just incase that was cloudy to anyone. Both center diffs also help greatly on deceleration to maintain traction (as you come barreling into a turn, you slam the brakes, weight shifts forward, front wheels would like to lock, braking force of the front wheels is diverted slightly to the rear wheels, and helps maintain balance and traction). Usually, you won't just lock the front wheels, but all four of them.
