New problem with 3.6 case

[Tyson Schmidt]

What doesn't make sense to me if the DMF is for dampening the crankshaft for durability, then why was it eliminated in all the racing versions of the 964 engine, and all the performance versions like the 964 Euro Carrera RS and USA 964 Cup cars? The latter of which are basically identical to the regular production 964 engines internally.

When you swap out the DMF on a 3.6 for a light weight set-up, you get rotational noise from the trans with the clutch released in neutral. The G-50 trannies are noisy in general compared to a 915. This is the main reason for the DMF on these cars. It allowed them to eliminate the dreaded rubber-centered clutch discs.

I'm not saying that a DMF isn't used for that purpose in other cars. It would surely help on a 964 motor. But clearly the front pulley is enough to take care of things without the DMF.

[Henry Schmidt]

Originally posted by Tyson Schmidt
What doesn't make sense to me if the DMF is for dampening the crankshaft for durability.........

Who said durability ?

When you swap out the DMF on a 3.6 for a light weight set-up, you get rotational noise from the trans with the clutch released in neutral. The G-50 trannies are noisy in general compared to a 915. This is the main reason for the DMF on these cars.

You are concentrating on effect not cause. Reducing vibration is the reason less noise is the result.

It allowed them to eliminate the dreaded rubber-centered clutch discs.

Eliminate the "dreaded rubber-centered disc" and replaced it with the "dreaded rubber-centered flywheel".

I'm not saying that a DMF isn't used for that purpose in other cars. It would surely help on a 964 motor. But clearly the front pulley is enough to take care of things without the DMF.

If it is "clearly enough", why the excess vibration causing trans noise?

[2.7RACER]

This discussion is the reason I keep reading and following this site. Not only learning about important details, but the hows and whys from different points of view.
Thanks

[JeremyD]

good info here - so if you are rebuilding a 3.6 - seems it would really make sense to shuffle pin the case... Don't skimp in that area.

[J P Stein]

That crack is familar, but odd. It is familar in that it is in the thinest cross section of the web, but odd in that one would think that it would caused by longitudinal force. Applying such force would seem difficult without leaving some telltale mark...a harmonic vibration would seem plausable. The
most likely candidate (for me) is that the crack was there from
the get go....at the machine processing.....which is why we (aircraft industry) PT parts after machining.

It is also familar in having a nice sharp edge from which to propagate. Increasing the strength (or crack ressistance) could be accomplished by radiusing these edges, then shotpeening the whole works.....but cleaning the used shot would be a he!! of a challenge, but just a .03-06 radius would help.

[Henry Schmidt]

Originally posted by J P Stein
.......The most likely candidate (for me) is that the crack was there from the get go....at the machine processing.....
.

The pictures we are looking at are two different cracks. We have two cases with two cracked webs per case totaling four cracks per case. Same webs #5, #6.
Each web is cracked at the top and bottom case through bolt.

Not likely that both cases were machined with this type of flaw.

[Speedster94]

i just read on the LUK and Sachs Website ,the reason and Function for the DMF , it is in German so i try :
The mainreason to install a DMF is to dampen the noise of thr Gearbox created by the resonance of the Crankshaft in very low speeds .
The second reason is to allow the Engine to idle in lower RPMs for lower Emissions .
Third for creating Torque in lower RPMs and keep the Engine speed while shifting .
It has nothing to do with the Torsion of the Crank but to overcome the Pauses between the Firing Cylinders in low RPMs .
I have a close Friend at Freudenberg ( producer of the DMF in 964 ) he told me the same .
And again like Tyson says , all High performance Porsches dont have DMF , the Cup Cars have 450 HP at 8500 RPMs , once a Year the overhaul the Engine and they almost never have to change the Crank or Case , the cases you show had other Problems ,who knows what happened to them .
Harald
Harald

[KobaltBlau]

ok so say the DMF is not involved; why does this not happen in heavily raced/modified 3.2s? torque and stroke, or a fundamental case design or manufacturing difference?

[J P Stein]

Originally posted by Henry Schmidt


Not likely that both cases were machined with this type of flaw.

On the contrary, there is a good probability. The design & the lack of hand finish lend themselves a weakness in that area.
The only thing I can see that would tend to discount my theory is that it is a casting and durring machining, it is not subject to near the built in stresses of a billet or forging.
I'm not adamant about this, but if forced to bet, that's the way I'd go. I've been in inspection for 30 years, in NDT & out....seen more cracks than a porn .....er, nevermind.


We make thousands of complex machine parts each year.
Defects come and go in a random fashion in certain areas of a given part.....which is why we do NDT on every part.
We don't do this to waste money but to insure the reliability of a part...even then, things get by.

[Henry Schmidt]

I just spoke with the smartest ,most experienced Porsche engine builder I know and he tells me two things.
1)993 Cup cars came with harmonic balancer and they raced with them. Some teams that removed them had cracking problems. {I tried to verify this but have not gotten a call back}
2)He has seen many of these cracks and his theory is that the crack is caused by engine vibration. Quote " these cracks are most assuredly caused by hi RPM harmonic vibration"
Andial before they were Motorsports, came up with a modification to prolong the life of the case. As of yet I don't know what that mod is.

BTW, he also said he saw these same cracks in 962s.
The factory never ran a harmonic balancer on the 962. Perhaps they should.

[Henry Schmidt]

Originally posted by KobaltBlau
ok so say the DMF is not involved; why does this not happen in heavily raced/modified 3.2s? torque and stroke, or a fundamental case design or manufacturing difference?

Apparently, if you run high enough RPMs and HP you will see this type of cracking.

[KobaltBlau]

Originally posted by Henry Schmidt
Apparently, if you run high enough RPMs and HP you will see this type of cracking.

same engine builder's experience with the 3.2s? Thanks for all this, Henry, it is AWESOME info.

[Speedster94]

hallo
very true , i looked it up in the Specs from my old Cup car , as you see it says at Nr. 4 :
" Schwingungstilger " in Riemenscheibe integriert
harmonic Balancer in V-belt disc integrated
here is a scan .
It is strange but the normal 993 RS didnt have one .
Harald

[blue82coupe]

Henry.. have you seen this sort of near failure in smaller displacement engines ..3.0 or smaller?. Dan

[blue82coupe]

Originally posted by blue82coupe
Henry.. have you seen this sort of near failure in smaller displacement engines ..3.0 or smaller?. Dan

Duh... read your original post.. that answered my question. Dan

[Tinker]

Henry,

Would you venture to guess a what RPM or horsepower level one would expect cracks to occur?

Tinker

[HawgRyder]

Henry...after looking at the pix of the crack....is there a possibility that there was too much end play in the crank?
It appears that the crank may have "walked" back and forth to put pressure on the webbing.
If it did...the crack would be just where it is.
In other words...the crank "worked" the metal inside the bolt perimeter to create a time related crack.
Does this web hold the end play bearing?
Bob

[Henry Schmidt]

Crankshaft end play has in no way contributed to the cracks we are experiencing in these cases.
The number one main bearing is used to determin crank end play. These cracks are being seen in the number five and six bearing webs.

[Wayne 962]

A couple of points:

Dual mass flywheels are primarily used to isolate vibration from various sources. On the BMW side of things, the E36 318 used a dual-mass flywheel - but only on the cars with Air Conditioning installed. Presumably, the DMF helped to isolate vibration caused at low RPM from the higher compression four cylinder engine, combined with an idle load on the engine when the AC was on. Not a performance addition at all, but merely a comfort. I would think that all race cars would not have a DMF, as they are complicated, and quite heavy.

As for shufflepinning the 3.6 case, you may want to ask around about that. I remember hearing (probably from Walt at Competition Engineering) that the early aluminum cases are susceptible to cracking when they are shufflepinned. The theory behind this is that the case itself becomes too stiff, and can't relieve the stresses from both the power and the heat output. The result is that the actual aluminum metal cracks to relieve the stress. The magnesium "loosey-goosey" cases don't have this problem because they naturally bend all over the place.

I'm not sure what caused this particular crack in these 3.6 cases, but indeed perhaps the design is one of too much stiffness, rather than not enough actual strength.

-Wayne

[soupbone]

Assuming high RPM usage (over 7k) would knife edge, lighting up, and balancing a 964 3.6 crank help workaround this problem when NOT using the harmonic Balancer?