[s-cars] torsen rear diff; first impressions

[Kirby Smith]

I had a torsen rear diff installed last week.  I obtained it from Bob
Pastore who recently informed the list that he had a new shipment.  I
believe these were acquired in Germany by Keith Maddock and come from
V8s.  Last night I tried to test the diff's behavior in light snow.  I
was interested in how much "bite" the rear end would get.  Note that a
controlled experiment would be difficult, so this is annecdotal
evidence.  The snow was about 5 mm on the pavement, relatively untracked
as it was midnight, and the temperature was -3F.  The only salt on the
road was whatever was residual, and it wasn't going to be very effective
at -3F.  Tires are Hakka Q's 205/60-15 with maybe 15 kmi on them.

Normally in slippery conditions with open diffs at each end of the car
and a torsen at the center, one tire at each end of the car will receive
power.  With a locked rear, three tires are powered, but unless one
wants to operate with mauual locking and override his rear differential
lock controller, locking is automatically released above 15 mph.  With a
torsen rear diff, torque is applied to both rear tires similarly to
their being locked, but with proportioning according to slip.  And this
behavior is available at all speeds.

Anyway, I had noticed before this transplant that I could get a rear
tire to slip under various conditions.  Last night I followed a newish
(and very clean) Corvette into Derry.  It seemed to have its wide wheels
on (?!) and was tiptoeing thru town more than would normally be required
by pedestrian risk and police supervision.  At a light where he was
intending to turn left I took the r.h. lane.  When the light changed I
accelerated at a level that he presumably could beat in dry conditions,
but surely knew he couldn't manage on the snow.  Obviously we weren't
racing as he was going a different way.  However, for me, besides
demonstrating to him the vast superiority of a quattro in the snow, I
wanted to check out wheel slip.  I didn't detect any.  Perhaps in dry
conditions I could have put more power to the pavement, but I was
impressed.  Little steering correction was needed.

The diff installation occurred along with new rear bushings all around,
Koni shocks and 92 springs, aluminum crossover pipe, and various
miscellaneous tasks.  The driveshaft was repacked and reinstalled using
the special alignment tool.  Afterward, my mechanic noticed a vibration
at 55 mph that he spent some time trying to chase down.  Because it was
possibly the tires, as two are out of round but otherwise all are
balanced, I took the car back so I could see what the vibration was
like, and because I might need to wait until spring for my SO-2s to go
back on the be sure it isn't the tires.  

There is a noticeable vibration.  I would like to understand its source
so it can be addressed efficiently rather than changing cv joints and
whatever else comes to mind just in case.  This vibration could have
preexisted the transplant and wasn't noticed, but I doubt it.  It is
quite noticeable at 80 mph in 5th, and in the 50s mph in fourth if a
light touch is put on the shift knob.  This vibration seems to he a
higher frequency than I would expect from the tires, perhaps even higher
than the driveshaft.  I am trying to confirm if it is rpm related, and
not just speed related.     

Has anyone who changed his flywheel because his two-piece OEM unit had
too much slop noticed a reduction in vibration afterward?

kirby