300guy

Given this information... perhaps this is the reason why 968s rattle like hell with a light weight Fidanza flywheel!

It would be a neat experiment to see if the RPM at which this rattle happens changes if the slot plugged or welded.

mattipuh

Who's willing to test? I'm interested in results too.

FRporscheman

I don't think plugging would affect it, but welding might.

With the slot welded, would the frequency change, or would the amplitude change (decrease)?

And I don't think I mentioned it but I figured out why my S2 setup was vibrating more than the 968 setup. Lady Luck and Miss Forture conspired against me, and they made my BS belt skip a few teeth on the lower BS at almost the same time I did the swap. I figured this out a while back and reset the BS belt and shafts, and now the car is as smooth with the S2 setup as it was with the DMF. If there is a difference it is negligible. Maybe if I obtain a slotted TT it would get better but I don't think I need to bother.

mikew968

What was the original objective? This sounds like a ton of work. If you wanted to get away from the 968 dmf the 17 or 18lbs smf that Pete from RS barn sells would have done the trick while preserving all the original components of the 968. I have one in the race car and it is great without the amount of noise and vibration of the 13 lbs fidanza.

FRporscheman

The original objective was to install a 951 torque tube in lieu of my 951 engine installation, but keep it driveable while still having the 968 engine (while I build the 951 engine). The S2 flywheel was the only solution and I'm very happy with it anyway. I love the lighter PP.

Plus, I prefer to use Porsche parts.

savvas944

i am not sure if i followed this correctly , but it seems that the whole issue could be the fault of the wrong BS timing on the first place, or worn torque bearings .?

havent tried the Fidanza flywheel, but got an aluminium one 7.2 kg versus the 13.2 kg of the DMF from LR, fixed straight on using the same clutch plate and presure plate with new 12.9 socket head bolts no vibrations at all from it , with the engine revving all the way to 6,700 rpm much faster, and spinning
the rear 10jx18 ,285/30 pirellis n4 , even with the LSD fixed at 80%.
My car has the slotted torque tube as well.

300guy

968 engine/bellhousing/torque tube?

Where did you get the 7.2 flywheel - is it a Fidanza?

PorscheDude1

That 7.2 is in kilograms.

FRporscheman

Yes, all of my vibrations after switching to the S2 flywheel were probably from the misaligned lower balance shaft. I believe it skipped a few teeth right around the same time I swapped in the new setup, so I was fooled into thinking it was from the flywheel.

I don't know if not having the balancer pulley would still be fairly smooth, but since I have it now anyway I'm going to keep it. And like they say, Porsche put it there for a reason. Once the 951 engine goes in I won't have a use for it, so I might sell it.

savvas944

the 7.2 kg flywheel was purchased from Lindsey Racing in Oklahoma, and is a SPECK flywheel.
very good workmaship.Came complete with the starter and timing gear,.Direct bolt on at 675 usd.

300guy

The harmonics in my car are load dependent. During normal deceleration it happens at about 3200-3600 rpm. However, this can shift a bit depending on the gear and load (uphill/downhill/brakes).

My torque tube has 200,000 miles so its probably due for a rebuild... but everyone else that I've heard of with a Fidanza 14lb flywheel has this harmonic/vibration problem.

I've done some reading on harmonics and 'single propeller shafts' and it seems that for a given shaft with a load there will be a natural frequency that will cause torsional imbalances. The variables include length, material, support (bearings, etc) and load. There must be some differences between the 968 and 944 shafts. Its now known that the torque tube housing is slotted on the 968 to reduce torsional rigidity and the 968's is shorter. It seems that Porsche eliminated the symptom by having a ridiculously heavy mass on one end (DMF) and having it sprung.

RajDatta

Actually, when they went to a DMF, the disk was no longer sprung. Its a solid disk.
Raj

FRporscheman

I have always wondered if it would help (or hurt) to add more bearings to the TT. Once you have it out and apart to replace the bearings, it would be a cinch to just install extra bearings.

It's important to put the bearings in at the right positions too, or else the driveshaft could be allowed to vibrate in different modes.

PorscheDude1

That's a good idea Arash.
If the resonance could be located, an
extra bearing could be installed near that
location.

300guy

Thats not a bad idea to install an extra bearing... probably put them closer to the back since thats where the vibration is coming from.

968 Turbo S - the DMF is sprung... effectively the same as the sprung clutch disk.