Quoting H05TYL:
I was under the impression the cusco tarmac centre diff was intended to be used with the stock vcu,
this way you'll never get zero forward motion even if the rear wheels are off the ground.
note it still has the extended output shafts and circlip groove...
also found this on evolutionm - explains my understanding of things quite well.
Quote:
Now, what you're suggesting is to leave the VCU out, but just as has been mentioned, that would make for just an open centre differential. I don't think that's a good thing.
The one person I have talked to who has a Tarmac gear left the VCU on and says that since the VCU takes a finite amount of time to lock up, the Tarmac gear has plenty of time to exercise its rear biased characteristics. Perhaps more importantly, the VCU does not redistribute torque per-se. All it does is link the two output shafts together so that in the event one end slips, the other is dragged along with it. Actually, I should mention that the w5m33's VCU resides in the rear output shaft and the differential body, but it does the same thing in that the short of it is that its action is slip dependent. It is worth remembering too that the Viscous Coupling doesn't really solidly lock up until a remarkable amount speed differential is detected. In fact, it still only transfers a limited amount of torque. I've heard that the w5m33 stock center viscous is rated at 16kgf per 100rpm? Not sure, but it's something along those lines.
The Cusco has assymetrical torque split due to the leverages of its internal gearing.
-the VCU does not redistribute torque until a speed differential is detected, leaving torque distribution alone until one end is spinning more than the other.
-and even when spin is detected, the VCU takes a while to react and only gives a limited amount of torque transfer. In short, it keeps you from getting too crossed up.
Since I was the one quoted on the above post, I feel I must share a new finding I came upon regarding the subject.
The Cusco Tarmac gear for the Impreza is indeed an Open differential. For Mitsubishi's W5M33 however, we have the option of either keeping the stock VCU or leaving it out and running a vcu eliminator. Note that this eliminator is different from how most Talon guys understand it as their eliminator effectively "welds" the center diff together; the tarmac gear's VCU eliminator is just meant to keep something from getting pulled out or something.
Personally, I would run the VCU even for tight autocross use. But I drive the car in the Winter herein Alberta, Canada. Japanese guys on the other hand who do Gymkhana, and love yanking the Ebrake leave the VCU out.
