Re: Synthetic Tranny Oils

[Heather & Jeff <hjr@buffnet.net>]

Here's a simple explanation how the "spin-up" phenomena works with
synchros: (sorry in advance for B/W)

Your tranny is filled with shafts & gear clusters. Picture you are in
first gear, and attempting to shift into second. As you push in the
clutch, your input shaft to the tranny is disconnected from engine
power. As you move from 1st thru neutral towards second, you have now
disconnected the input section from the "back".  The input side of gears
is now freewheeling by itself, neither driven by the engine nor loaded
by the rear end.

As you attempt to shift into second, the freewheeling input section is
spinning faster than it would if it were engaged to the 2nd gear
section.  This causes the clash of gears common in "unsychronized" or
worn trannys.  In old pre syncho trannys, (and still used in some heavy
trucks) it was common to "double clutch" shift. (push in clutch, let off
gas, shift to neutral, let out clutch, push in clutch, shift into
2nd,.....)  The idea is, get the free wheeling input shaft+ clutch disc
assembly spinning at the same speed as the next gear section.  Synchros
achieve the same result.  Synchros are nothing more than miniature
clutches (actually nothing more than metal cones or rings) attached to
the trailing edge of the shift collar assembly. As you slide the shift
collar assembly to engage 2nd gear, the synchro is pressed against the
faster spinning input section.  If a synchro is not worn, it will not
let you engage the gear until the shift collar and engagement part of
the next gear are spinning equally.  The synchro relies on friction to
get the next section spinning. 

This is the reason for the "synthetic causes tranny shift problems"
not-so-urban-legend.  It is actually true, with a disclaimer.  Trannys
that are designed for dino oils will shift harder with synthetics with
no "friction improver" additives.  Most of the newer Audi trannys came
factory equipped with synthetic (so I'm told by this list) so running
synthetic is proper.

I seem to recall that most synthetic tranny oils have additives to
"improve friction"  (seems kinda ironic??)  The same types of additives
are added to "limited-slip-differential" oils, since these rear-ends
have clutch packs that rely on friction to work properly.

Jeff

88 80q