<VV> Re: 4 spd syn oil 2-3rd gear

[N. Joseph Potts]

Jim -
GOOD post! Upon reading the earlier report, the answer that occurred to me
was trying to be the answer you gave. But I just didn't apply the thought
even to clarify the matter in my own mind, much less to express it to
someone else.
     I think your concern about super-slippery lubricants overcoming the
function of the synchro cones is correct, too. I believe that Positraction
additives can impair the functioning of synchros, too, although not by much
in the concentrations required to have the desired effect on the
differential.

Joe Potts
Miami, Florida USA
1966 Corsa coupe 140hp 4-speed with A/C

-----Original Message-----
From: virtualvairs-admin@corvair.org
[mailto:virtualvairs-admin@corvair.org]On Behalf Of Jim Simpson
Sent: Wednesday, May 05, 2004 7:34 PM
To: Chuck Sadek
Cc: virtualvairs@corvair.org
Subject: <VV> Re: 4 spd syn oil 2-3rd gear


Hi Chuck.  Not sure if I can really give you any good answers, but I've
just been all through a late 4-speed and have learned more than I really
wanted to know about it.

The synchros on all four forward gears are identical in design.  The only
reason I can come up with for it being harder to shift into lower gears is
that you are dealing with more rotating moment of inertia with the lower
gears.  Keep in mind that all the gears are constantly meshed, only the
synchronizers hubs are moved to engage a gear.  When you shift gears, one
of the two synchronizer assemblies is moved forward or aft to speed up or
slow down the INPUT rotating mass to match the OUTPUT shaft.  (The output
is always hooked to the rear wheels via the differential and is spinning at
whatever the car speed dictates.)

That input rotating mass includes all of the mainshaft gears, the
countershaft, the input shaft, and the clutch disk.  When you go for a
lower gear, you have to accelerate all of this a higher speed than with a
higher gear.  When the transmission is cold, the oil slows things down very
quickly, probably to about standstill even on a normal shift.  So 1st to
2nd with a cold transmission requires spinning everything up to about twice
as fast (2.20:1 2nd gear) than a 1st to 3rd shift (1.47:1 ratio in
3rd).  That's twice the amount of energy needed, plus you have all the drag
of the cold transmission oil on everything.

Once things warm up, at least the drag of the transmission oil is greatly
reduced.  This would argue that synthetic lubricants with their better low
temperature flow characteristics would allow easier cold shifts.  But I
don't know how super slippery lubricants affect the "drag" needed by the
synchronizers to do the speed matching.

I suppose if you could do a very well timed shift from 1st to 2nd on a cold
transmission you might find it easy as you caught the rotating mass
spinning down, but I think that would be very quick shift indeed and the
cold transmission oil would keep that syncho assembly from moving easily.

Anyway, that's my theory and I'm sticking with it unless someone comes up
with something more plausible.

Jim Simpson, Group Corvair, '66 Corsa turbo 4-speed.