Page 1 of 1
How do MT center diffs work?
Posted: Fri Feb 13, 2009 1:44 am
by James614
From what I understand, the Subaru 5spd and 6spd uses a viscous center coupling. I've read about how they work, and it was described identically to VLSD. But that can't entirely be the case, because VLSD diffs are generally said to suck balls for any activities involving wheelspin, and overheating the fluid will break it down and result in an open diff.
If that is the case with the center diff, does that mean all goes guys doing pavement donuts with their Rexs are gonna be spinning only their back wheels come winter?
Posted: Fri Feb 13, 2009 4:55 am
by 93forestpearl
Well, the viscous center operates on the same principle and any other viscous unit. With enough abuse, they will cook themselves. Some JDM 6mt's had viscous centers, but everything USDM STi was a DCCD unit. I believe the Legacy Spec B 6 speed is a viscous unit.
The DCCD is completely different than a viscous diff. Actually, it is quite similar to the transfer clutch on an automatic, except it is actuated by an electromagnet instead of hydraulic pressure.
Posted: Fri Feb 13, 2009 4:49 pm
by James614
So how is positive is the locking between the front and bcack axles on a 5spd? If the back wheels were buried in 3ft of snow, could the fronts pull it out?
And what about 6spds without DCCD?
Posted: Fri Feb 13, 2009 5:37 pm
by skid542
A viscous LSD will work to maintain equal RPM between the two sides. This means that in the case of having the rear buried in snows, the center diff will try to turn the fronts at the same rate as the rears - effectively sending more torque to the front. You may still get some slip at the rear but the fronts will be getting torque applied to them.
*Guys, correct me if I'm wrong.
All 6spds come with a DCCD. If you do not have a controller to control it, then it will act as open diff when the front or rear starts to slip, i.e. it'll send power to the slipping wheels.
Posted: Sat Feb 14, 2009 3:55 am
by AWD_addict
Not all 6 speeds come with DCCD. Go look at a Legacy SpecB, as 93forestpearl mentioned it does not have a DCCD like WRX STi models.
Yeah Lee, that's how a VLSD behaves.
The center VLSD works due to differential speeds. So if the rear wheels are on ice and spin a little, the VLSD begins to lock up; sending torque to the front and rear. It never fully locks, unless it's overheated and the plates melt together. In which case, it's no longer a "differential."
Posted: Sat Feb 14, 2009 11:28 am
by James614
But my understanding of VLSD is that if it overheats, the fluid breaks down and becomes useless.
My scenario of the back wheels buried in the snow was supposed to imply the vehicle's ass stuck in a snow bank, needing much more than standard force to pull the vehicle out. I would imagine the VLSD would overheat and slip too much to supply adequate torque.
Posted: Sat Feb 14, 2009 8:44 pm
by skid542
*sigh*, I failed observation 101. Correct, the Spec B's do not have the DCCD.
However, James, you are incorrect in believing that you will kill the center diff in the snow. Maybe if you sat there for 15 minutes with your foot on the floor just waiting for something to happen then it'd overheat. The reality is, if you stick your car in a snow bank to where Subaru's AWD won't get you out, then no amount of power alone will. You have to get out and dig and work it.
Subaru's OWN in the snow. Take it from me, in northern Idaho, and all the other members on the BBS that live in the great snowing northern part of the US
.
Or am I completely missing the point of your question?
Posted: Sat Feb 14, 2009 9:08 pm
by James614
I dunno what came over me... I've never doubted Subaru AWD before... But hearing compared to VLSD rear diffs which I wouldn't trust withheavy sustained torque transfer just got me thinking.
My 4EAT has some serious tractive force at both axles, wouldn't male sense for the MTs to be less effective.
Posted: Sun Feb 15, 2009 9:51 pm
by tahoe.ss
James614 wrote:I dunno what came over me... I've never doubted Subaru AWD before... But hearing compared to VLSD rear diffs which I wouldn't trust withheavy sustained torque transfer just got me thinking.
My 4EAT has some serious tractive force at both axles, wouldn't male sense for the MTs to be less effective.
wow with the 4eat center diff lock switch mod i have to say i'm am also EXTREMELY impressed with suby all wheel drive. i also would be concerned about the manual tran's center diff. what are the options for a locked center diff in a manual without dccd? can suby's get a locked spool style and leave the rear lsd and the front open? i see no problem in it except torque bind but would that allow one to remove front axles for summer driving
Posted: Sun Feb 15, 2009 11:50 pm
by Legacy777
