Psi and Dif locks comparo.

Fri Oct 4 23:53:41 EDT 2002

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In a message dated 10/4/02 9:07:27 PM Central Daylight Time, SuffolkD at

>Facinated by this:
>I (many years ago) met a wholesaler in MASS who loved to lock down his
>differentials.  He said it was best for the track.  But in walking around
>bone yard I saw at least three Q trannys on the ground he mentioned as
>"junk."  He also mentioned they failed HIM.

Can't relate.  The only q transmission that failed that sits in my shop is
from Bob D doing a tight auto-x with the LT1Q.  FYI, locking diffs is common
quattro trait.  Rear locking diffs (unmodified from rally spec thru 1994)
just don't fail.  If you read thru the available audi literature during the
rally days in the early 80's, the front pinion fails from too much torque
(hello LT1).  It's really because the front trans size is too small.  The
rears just don't fail.

>Now with both (olderstyle 85/87 vintage 4K) (200's unlock the second at
>I understand) difs locked the 4000Q car crab walks in parking spots.
>Wouldn't the two diffs upset turn in and shorten the life of the tranny?

Depends.  Remember the rear diff hasn't changed in design or construction,
200's unlock because the diffs became electropneumatic in operation with a
ground out from the ECU at 15mph.  Snip the blue white wire and it will stay
locked until you turn it off.  Does it upset turn in?  Again depends.  Every
audi track car (mit lockers) I know runs center locked, you take the
understeer as more predictable and accept the ideal brake force distribution
(Jeff Goggin and Glen Powell like to lock the rear only - me, anderson,
audisport, sport wheels, galati go for center locked always on track).  Rear
locked, really depends on venue.  Ideally, you would want to lock the rear as
you enter the apex of a turn to help power out, without the U of going in,
btdt, read my steamboat reports.  At Gingermann, or any hi cf environ, that's
not quite so easy to do.  My method is to try with and without rear locked.
In the case of Gingermann, I won better time (10+ seconds a lap) and handling
on all but 2 turns.   Yes you accept some increased understeer, but it's
predictable, and you are back on the turbo sooner.

>Not doubting your expertise, your reputation preceeds you here as a guru,
>in reading your story, is it more likey the 15 to 22 psi which frustrated
>BMW than  the second Dif lock?

Scott, reread that, I *didn't* add the extra 7psi back in, I only locked the
rear diff.  If that didn't work, I was ready to go for broke (literally) to
defend the marque.  What frustrated the BMW was how the same setup with 1
rotation of a switch made the difference so dramatic.  I changed nothing

>If I lock both difs won't I be replacing trannies often for street or track
>Just curious -Scott in BOSTON

I've been locking diffs for 10years, including a (overheat) operations check
of the electronic diff lock mod in my 83 urq.  after 16hours straight running
of the center diff locked, it stayed locked and I've run a few track events
since.  In the case of the rear diff, I've never seen a failure, ever (the
seals may leak).  I haven't read of one, and our group A rally car runs all
three diffs locked ALL the time (no shutoff function at all), and we've never
had a failure at 400+HP.

You certainly could claim that tire wear can increase....


Scott Justusson
audi v bimmer victorious emeritus

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