76FJ55

I too have wandered why they did this. It makes sense to lock it down at one end to locate the shaft fore and aft. At rear in the case of the 928 makes sense since the torque converter bearings should effectively keep the aft end of the shaft in a fixed position. But why try to retain the front? As previously mentioned all manuals have the shaft float inside the clutch disk.
I have my own theories on why the coupler slips and one of these days I'll try to put my ideas in writing, but until then we'll just chalk it up to another unexplained 928 phenomenon.

G Man

Millions of cars on the road for years with drivelines that slip into the back of the tranny. There has got to be a way to do this.

Tim Murphy

I like what Porken did with his. In my opinion, it "should" solve the problem.

This is how I see it (correct me if I'm wrong):

Torque on the drive shaft during hard acceleration twists the drive shaft, thus shortening it with a tremendous amount of pulling force on the flex plate/clamp.

In stock form, the drive shaft has enough force to pull out of the clamp and when it relaxes it is now pushing on the crank shaft with the spring force of the flex plate untill someone manually goes in there and releases it. Without releasing it, the flex plate is trying to push the crank shaft out the front of the motor and over time, TBF.

Although with a tightened clamp, the drive shaft is trying to pull the crank shaft out of the motor with the spring force of the flex plate during hard acceleration. I have never heard of this causing any type of failure, other than snapping the drive shaft but that's a different situation.

Keep it clamped up and problem solved. My opinion.

hacker-pschorr

Why did Porsche design it like this?

I'm going to chalk it up to mid 70's technology / engineering. It made sense to Wolfgang on a bar napkin back in 73

PorKen

If the coupler doesn't move, then I think the plate only applies pressure momentarily during high torque events.



If you really want to increase flexplate modulus, remove 1 of the 2 stainless steel(?) plates.

Black Sea RD

We looked at it from a lot of different angles and what we found is what Tim states above for why forward pressure sets up in the first place.

We went down the road of trying to make a slip fit at the front flywheel for the drive shaft but it needed more parts and was more expensive to do correctly than what our Super Clamps cost.

Or just design a better clamp than the one originally used by Porsche, which we did after testing different clamping methods.

As far as the rubber 944 automatic flywheel coupler, they come apart after time and are not inexpensive to replace. The 944 engine also does not have as much torque as our engines do so would not suggest using it for 928s even if they can be made to fit.

I really don't understand the point of this thread. With the health of the 928 engine riding on this, why re-hash something with other alternatives that has already been given a sound fix?

Just my .02 cents worth,

Black Sea RD

I would also add it is a pretty cheap and easy solution too.

S4ordie

Constantine - there is nothing 'cheap' about your clamp. Inexpensive is the better term

Black Sea RD

Hi Dan,

Thanks for the compliment!

But I was really adding to Hacker's musing on why Porsche decided to clamp the drive shaft at the front flywheel in the first place.

Kind regards,

S4ordie

Two sharks, two Super Clamps.... Math is easy 2+2= No Worries.

Tails

Wally P,

Interesting thought.

In the marine game, the single collar thrust bearing was unsuccessful, as the axial forward thrust of the propeller increased due to the increase in BHP, it eventually caused bearing failure of the white metal, because the lubricating film broke down, causing boundary lubrication with momentary fusion and fracture of the asperities, which increased the temperature, reduced the viscosity of the oil and the lubricating system went into a degenerative cycle until thrust bearing failure. More collars were added with more white metal bearings, but this type of thrust bearing also failed as the forward thrust increased as ship size increased, speed increased, BHP increased and forward thrust increased.

This problem was solved with the invention of the Michell Thrust Bearing. The design of these bearing is that the bearing is made up of pivotal white metal thrust pads, so that when the collar rotates the pad cants in its fulcrum point and an oil wedge is formed and there is no boundary lubrication. When the shaft rotates in reverse the pads cant in the other direction again forming a oil wedge. This type of bearing can contain the forwards axial force from a single propeller absorbing over 80,000 PS (metric horsepower).

Back to the 928. If the flexplate is made softer in the forward and aft direction and there is sufficient cross sectional area to absorbe the maximum thrust with a safety margin, this would decrease the force generated in the axial direction under hard acceleration that in some cases was great enough to overcome the frictional resistance betweent the clamp splines and the drive shaft splines and the clamp migrated in the forward direction.

If your thought is correct the affect would have a double benefit:
1. It woiuld decrease the momentary maximum force applied on the thrust bearing bearing forward face; and
2. The existing clamping force could be great enough to prevent the existing clamp from migrating thereby deleting the loading force that can be permanently maintained on the thrust bearing aft face by this migration. (The above of course would be dependent on the extent of wear on the splines of the clamping sleeve and the drive shaft splines and that the existing clamp could overcome this wear to obtain sufficient friction force on the drive shaft splines to prevent movement).

Both these would have the affect of reducing the possibility of breakdown of the lubricating oil film to bounday lubricating and the failure of the thrust bearing.

This degenerative sprial is what I believe happens when the 928 thrust bearing fails.

The super clamp, the PorKlamp and locktite prevent this migration and the thrust bearing only experiences the maxium force momentarialy during the hard acceleration and not the migration force continuously from the flex plate.

Tails 1990 928S4 Auto

inactiveuser1

If the old style used a clip,could the shaft itself just have a slight groove in it before the stock clamp and just use a large C-clip or won't stand up to the stress?On the older cars what did it look like as the clip setup?

Earl Gillstrom

Wally,

I would like to understand what problem you are trying to solve. As far as I can determine there has NEVER been a thrust bearing failure on any 928 that has had one of the three known fixes for the problem. 1) LOCTITE, 2) PKclamp, 3) Constantines clamp.

It seems that if the clamp can't move, then the thrust bearing can get enough lube to not wear and the flex plate has enough flex.

Most cars that have had 1 of the above fixes installed have probably had other good procedures applied also like the torque converter bearings replaced.

The only thrust bearing failure that I worked on had so many things wrong with the drive line that it was impossible to determine the cause of failure. It would be interesting to know if Porsche's assessment was actually correct. (Mechanic induced)

FredR

Chaps,

Just to be clear here, I do not think Wally is trying to challenge existing thinking - rather trying to open minds to alternate possibilities something Herr Ferdinand's crew should have done but clearly didn't in this car.

The 928 is a somewhat enigmatic car with lots of great innovation and some downright stupid features like operating the flappy off the same pathetic vacuum system that operates the HVAC system. The stock clamp came out of the ark- just take a look at the clamp that holds the kick start to the splined shaft of a pre-war British motorcycle. Doubtless Hermann the German saw this and copied it on the auto models.

Constantine's solution is mechanical nirvana, Ken's is excellent lateral thinking and hopefully both very effective. Me, being a cheap SOB, prefer the loctite solution because it is really cheap [6 cents for a few drops?] and works [nearly 10 years for me].

Personally, I prefer to save my light sabre for use on DV.

Regards

Fred

Vilhuer

0) Factory circlip & washers setup. Only real problem was inability of mechanics to do things right way and result was TBF warranty claims caused by wrong preload. If old style long shaft were used with washers up to last GTS there would be lot less problems. Original design was sound but Porsche messed it up in '85 MY when washers were deleted.