Wisconsin Joe

You are kidding, right?

A Torque Tube isn't an impact. The impact gives short, sharp impacts (that's where it gets it's name) on a fastener. If there's "give" in the extension, some of the "impact" is absorbed. The force is released in between impacts.

The TT rotates. It spins. No matter how much it twists, all of the force is transmitted (unless it goes all bendy and stuff). It does 'absorb" some of the force as it twists, but once the shaft reaches max twist, all the force is transmitted. The time spend absorbing the force is miniscule, compared to the force transmitted.

the flyin' scotsman

hi Fred,

I have had a noise from the TT/TC for a while that Ive been procrastinating with as Im very fortunate to drive the GTS in the nicer weather months and the Cayenne as a a daily driver.

I did a few checks this summer; disconnect front flex plate clamp and back off the coupler and measure the distance between front of TT to 1st TT brg (from engine).

Findings:
with engine running and front flex plate disconnected there was no noise.
the front brg has migrated to the rear (dont recall distance but significant)
with flex plate reconnected noise is loudest at torque converter/rear TT

Conclusion:
the crica 30 YO brgs are all past there sell buy date and as Im in there Im doing it right with all best available components so my son who will probably inherit the car can find this thread and report

A few years back I rebuilt the GTS TT (theres a thread on RL) and used Constantine brgs married to Gregs TT shaft and GB short (clutch shaft).....its perfect

Constantine

I used the word "pinned" to describe how the manual transmission drive shafts of the 924/944/968 and 928 are held in place at the back of the crankshaft. What you describe is correct, the drive shaft, or the short shaft, depending on which model Porsche you are referring too, has an end nub which is inserted into a bearing which is inserted into the end of the crankshaft, thereby restricting its movement or "pinning" it down. This system differs in the 928 automatic drive shaft configuration where the end of the drive shaft is held by the front flex plate clamp. If you don't restrict the movement of the drive shaft end somehow, the drive line harmonics will vibrate the unsupported drive shaft end.

I used "pinned" as in the definition found on the website www.thefreedictionary.com "to hold fast, immobilize," not in the literal sense of where a thin metal rod pierces through an object holding it in place. Sorry for any confusion.

SeanR

You might want to look up what a torque stick is.

hacker-pschorr

Joe is talking about the difference between an impact wrench & a regular drill. An engine is a regular drill, doesn't operate by "impacts".

If you connect a torque stick to a regular drill with a higher torque capacity than the stick, the torque stick will reach maximum flex immediately and deliver full torque of the drill to the fastener.

The action of the torque stick is to "absorb" the torque over it's rating and relaxes when the hammer goes around for another blow. Engines do not operate this way.

76FJ55

Maybe, until the trans shifts, then you get a spike well above engine torque.

hacker-pschorr

That's where the torque converter comes in, another variable in this comparison which makes this comparison not very accurate.

GregBBRD

I seldom kid.....

An engine fires a cylinder and then coasts until the next cylinder fires. Identical to how an impact works. Yes, it is very fast, but whatever is attacked to an engine has to absorb these pulses.

An electric motor delivers constant torque.

The two are completely different.

76FJ55

Torque converter or not, there is torque required beyond just what the engine is generating from combustion, required to bring the engine down to its new gears dictated RPM. This torque is a result of the inertia of the engines rotating assembly.

hacker-pschorr

That still doesn't mean power is "lost" due to the twisting of the drive shift like a torque stick connected to an impact wrench.

Interesting, yea...didn't think of it that way.

Along the lines of the debate asking if the piston in a 20,000rpm F1 car ever really come to a 100% full stop before changing direction.

BUT - if the "torque stick" drive shaft has a limit of say..... 50ft-lbs we would then need to discuss if the momentary lapse of power between cylinder firing drops below this mark. Otherwise as long as the delivered torque is always above said range, it never has a chance to "relax".

I've tried those torque sticks with various different impact wrenches - they don't work. They all allowed the fastener to far exceed the rating.

FredR

Malcolm,

That the bearing carriers are tired and on the move should be no surprise. If they are not where they are supposed to be then it is not hard to imaging other nefarious things going on. When you pull the old shaft it would be interesting to investigate to see what [if any] detriment it has suffered but given what you are splashing out I can well understand reluctance to further investigate. Have you used any mitigations to stop shaft migration through the flex plate clamp or has it held solid throughout your ownership?

Wisconsin Joe

I know what they are (more or less).

But the brief "let off" of an impact allows it to relax.

While there is a (very) brief pause between power impulses with a motor, there's no "relax" on the crank. It's a constant turning.

H-P mentioned that you can easily over-torque with them, even using an impact.
How about with a steady pull on a torque wrench? Could you set a TQ wrench well above (say 10%) and reach it with a slow, steady pull? I would think that unless the Tq stick deformed to the point of damage, you could easily reach torque levels well above the rating.

Edit to add:

After a bit of thought, even the impulses of a piston engine are fairly smooth.
#1 - It's not a "fire and then coast", it's a smooth push all the way down on the firing stroke. If it's a sharp firing impulse, then there's detonation (knocking).
#2 - There are 8 cylinders. 4 are going down at any given time, 2 being powered. The power strokes overlap enough that there isn't any "coasting".
#3 - Even on a "one lunger", there's a flywheel that smooths out the impulses. v

It's a lot closer to an electric motor than an impact.

Jerry Feather

Interesting analysis, Joe, and might even be taken a bit further.

For each revolution of the crankshaft in the 928 V8 there are four power pulses, each, as you describe more of a smooth burn that any kind of impact, and no coasting during the power half of a given piston power stroke. Then, while each one is making its power stroke there are three other pistons doing the same thing, each evenly spaced around the given crank stroke.

When compared to an electric motor, running at say 3200 rpm, the electric motor, on 120 volts AC, is getting individual power pulses at the rate of 60 per second and each with a pause in between equal in length to the pulse. Coasting as it were. The V8, at the same 3200 rpm is getting 4 smooth power pulses per revolution which totals 12800 per minute or 213 per second, over three times as many as the electric motor.

That seems very smooth to me, and much more so than the electric motor, and without even taking into consideration the flywheel.

No impacts involved.

the flyin' scotsman

Fred,
No surprise at all given they are 'only' 30 years old and to Gregs point, used well
I have the shaft out of the GTS when I rebuilt its TT using Constanine brgs and Gregs shaft; I could look at it but it is a 5 speed so perhaps not relevant to the auto discussion.
As far as mitigations: periodic check/release of front flex plate; replacement, loctite and careful torque of front and rear pinch bolts; Porkens clamp added to front clamp. I did these over the past ~15 years of ownership and approx. 100k kms

FredR

Malcolm,

Does the S4 have the 28mm shaft [tapered] or the 25mm earlier design?

Rgds

Fred