From: Fred Rourke [rourkefg@omantel.net.om]
Sent: Wednesday, December 06, 2000 4:23 PM
To: 928
Subject: [928] Re: CS Thrust bearing failure

Dear Tony,

Nice to hear from you.

Perhaps I can enlighten you on what we think is going on. When a shaft is
under torsion it naturally wants to shorten in length. The torque tube is
given this name beause its principal function is to transmit torque from the
engine to the gearbox and in the case of the automatic, via the torque
converter. I would presume that in the case of the automatic transmission,
the drive train requires a solid link simply because of the amount of torque
being transmitted. Porsche probably felt this would be more reliable than a
floating collar which would tend to wear.

Consequently, the flex plate acts as its name suggests, as a flexible power
coupling. Nothing unusual with that. Flexible couplings are normally used to
take up any slight misalignment and hence stop any transverse loading from
knocking out the main bearings of the crank. In the case of the 928 drive
train the thing that is fundamentally different from most conventional cars
is that the drive shaft is spinning at engine speed al the time. So far so
good I trust.

When Porsche designed the 928 they wanted perfect weight balance hence they
stuck the gearbox in the rear. No big deal eh? Well not exactly. Up until
the 928 was introduced the only marque I know of that had that configuration
was the Alfa Sud although maybe there have been more. But I am pretty sure
that no manufacturer ever attempted to put that amount of power and torque
into that configuration, remember we are talking about a design that
commenced in around 1975.

When the 928 S4 was intoduced the torque derived was increased quite
significantly. The torque tube, besides transmitting power, also acts as a
transmission shock absorber and it does this by winding itself up to some
extent. Hence as torque is applied the drive shaft will try to shorten
itself and the flex plate does its stuff.

Having established that logic, imagine now that for some reason the pinch
bolt holding the flex plate hub onto the torque tube splines loses its grip
slightly. You are doing 60 mph and you floor it. If your car is running
properly, that kickdown will instantaneously throw your head back into the
headrest  such is the shock load impulse. Now if our perceived scenario is
correct, this pulls out the hub ever so slightly. You then take your foot
off the throttle but that does not lurch your head forward. Hence you can
reasonably assume that there is not a comparable reaction in the other
direction. Thus the spline takes up a new position in the hub until the next
time it is yanked forward. Bit by bit the total displacement increases until
it reaches a point where the thrust on the crank causes the main thrust
bearing to give up the ghost and then you need a new engine and a healthy
bank account. To this day Porsche have not issued issued a service bulletin
telling their dealers to check the flex plate position on automatics but
interestingly they very quickly proposed a solution to a problem that does
not exist. Hmmmm!

Unfortunately to date Porsche do not seem willing to agree with me that a
problem exists on the automatic.

It may well be possible to design a joint that slides but I presume the
extreme load conditions in these cars ruled this out. I would like to see
how GM have tackled this one on the new Corvettes now that they have slipped
the gearbox to the rear of the car 25 years after Porsche thought of the
idea.

With regard to the vibration I have asked the listers several times if
anyone else has experienced the vibration that I did but not much response.
My car is very smooth up to this engine speed [3050 rpm] and very smooth
above it. My instincts tell me that while my flex plate is positioned
correctly the vibration is very much attenuated to the point where it is
hardly noticeable. I think the S4 engine has a resonant frequency period at
this speed hence the great lengths they went to in designing the engine
mounts. I recently drove a 928 GTS that vibrated like a dog at this speed.
Whether it is related to the mounts I do not know but mine were replaced
just before I bought the car 3 years ago. Other listers report bad mounts
cause a lot of vibration at tickover up to around 2800 rpm. Your guess is a
good as mine.

Hope you find that little lot of interest.

Cheers

Fred Rourke
1990 S4 auto-
Oman