964 crank pulley / harmonic dampener ?

[safe]

I've read that the crank pully is critical to the 964 crank, is that really so?

I want to replace it with a serpentine belt pully, maybe from Clewett or something else if you have suggestions.
Is there a problem with this?

[Henry Schmidt]

Originally Posted by safe

I've read that the crank pully is critical to the 964 crank, is that really so?

I want to replace it with a serpentine belt pully, maybe from Clewett or something else if you have suggestions.
Is there a problem with this?

There is only a problem if you're looking to run the engine above 7,000 RPM consistently.

[safe]

Originally Posted by Henry Schmidt

There is only a problem if you're looking to run the engine above 7,000 RPM consistently.

Thank you Henry!

I've got the revlimit set at 6800.

[winders]

According to this information from Neil Harvey, it appears that high RPM running is not the problem:

Porsche 6 Cyl Harmonic Engine Failure – performancedevelopments.com

Of course, I could be reading it wrong....maybe Neil will chime in......

[safe]

Originally Posted by winders

According to this information from Neil Harvey, it appears that high RPM running is not the problem:

Porsche 6 Cyl Harmonic Engine Failure – performancedevelopments.com

Of course, I could be reading it wrong....maybe Neil will chime in......

He talks about the GT3 crank there.
I figure that each type of crank will have different harmonic characteristics, even the same crank with different rods and flywheels.
I've gotten from other sources that it should not be an issue with the 964 due to the relatively low rpms.
If I "need" to add another belt pulley maybe I can mount it outside the original (maybe not possible), but would it make it better or worse?

[winders]

A lot of race cars use 964 and 993 cranks at higher RPMs with no harmonic balancers. Many of them probably have solid engine and transmission mounts. I have not heard of a lot of problems with those cranks breaking.

GT3 cranks are used in a lot of race cars too. Again, at higher RPMs. I have not heard of issues with these cranks breaking either.

I wonder how the solid engine mounts affect the harmonics.

Oh, I really do like the Clewett serpentine belt system. So much better than the stock v-belt setup!

[safe]

Hotrodded 964s has been a thing here for a few years as a track day weapon, never heard of any crank issues either, don't know what pulleys they use.
As far as GT3s the only issue I've hear is that the 996 ones can loose a rod bearing, almost always the same (number 5 I think).

I'll give the Clewett a try, hopefully a winter upgrade. Some other stuff that's actual problems, to fix first.

[Henry Schmidt]

Originally Posted by Winders

A lot of race cars use 964 and 993 cranks at higher RPMs with no harmonic balancers. Many of them probably have solid engine and transmission mounts. I have not heard of a lot of problems with those cranks breaking.

GT3 cranks are used in a lot of race cars too. Again, at higher RPMs. I have not heard of issues with these cranks breaking either.

I wonder how the solid engine mounts affect the harmonics.

Oh, I really do like the Clewett serpentine belt system. So much better than the stock v-belt setup!

The issue with the 964 when running without a balancer is not crank failure. The 964 crank has limited flyweights and giant rod journals. This creates vibrations at higher RPMs that transfer into the case. Specifically, # 7 main web. At higher RPM the #7 main web cracks rendering the case unusable. [ask William about cracked #7 main webs]
The GT3 crank has larger flyweights and smaller rod journals. When combined with longer stroke the journal overlap [the area of overlap between main and rod journal] is greatly reduced creating a weak cross section. Add to that harmonic vibrations and broken cranks are expected.

As for solid mounts: generally speaking, solid mounts are a nightmare. They crack chassis', interfere with carburetor function and fatigue drivers. Driver fatigue costs more lap time than hp power can restore.

[safe]

Originally Posted by Henry Schmidt

The issue with the 964 when running without a balancer is not crank failure. The 964 crank has limited flyweights and giant rod journals. This creates vibrations at higher RPMs that transfer into the case. Specifically, # 7 main web. At higher RPM the #7 main web cracks rendering the case unusable. [ask William about cracked #7 main webs]
The GT3 crank has larger flyweights and smaller rod journals. When combined with longer stroke the journal overlap [the area of overlap between main and rod journal] is greatly reduced creating a weak cross section. Add to that harmonic vibrations and broken cranks are expected.

Would you recommend changing out the 964 crank against a 993 or a GT3? I guess both alternatives requires new rods. Not sure what a 993 crank goes for but a new GT3 isn't that expensive.
At the moment my power peaks at 6350, so redline above 7000 isn't needed really.

[Henry Schmidt]

Originally Posted by safe

Would you recommend changing out the 964 crank against a 993 or a GT3? I guess both alternatives requires new rods. Not sure what a 993 crank goes for but a new GT3 isn't that expensive.
At the moment my power peaks at 6350, so redline above 7000 isn't needed really.

I would use what you have. Keep the RPM low [as you have indicated your intention] and enjoy.

[nickd]

Fantastic info from Henry. I was under the impression that the gt3 crank allowed for a lot of rev headroom in an aircooled 3.6. Are broken gt3 cranks a race-motor-only thing, or a more general issue? Sounds related to a chosen rod length / piston pin height?

Coincidentally, I just got a copy of the 1990 964 C4 Service Technik. Lots of interesting nuggets in these manuals. This is what the factory had to say at the time-

[reclino]

I can't remember where I read that in the 964 with the damper it reduced torsional vibrations enough that it allowed Porsche to more accurately measure crank position, and this allowed for more agressive ignition timing.

[Bill Verburg]

A harmonic balancer is used even w/ 997 4.0 RS cranks

[75 911s]

Aren't they like $1500? Yowsa.

[mikedsilva]

Originally Posted by Bill Verburg

A harmonic balancer is used even w/ 997 4.0 RS cranks

Beautiful motor.
What's the black finish on the fan strap?

[Bill Verburg]

Originally Posted by mikedsilva

Beautiful motor.
What's the black finish on the fan strap?

Dunno, either paint or anodizing
here are a couple of installed pics, it's a 400+hp Rothsport 4.0, the fan is a bespoke high flow item

[ian c2]

Looks like a Gunther fan , and Niel Harvey damper

[Bill Verburg]

Originally Posted by ian c2

Looks like a Gunther fan , and Niel Harvey damper

Rothsport builds the engines for Gunther
the fan is an ATI Superdamper

[ian c2]

I believe Niel designed the porsche application .
If you look close , you should see the performance developments logo on it .

[Bill Verburg]

Originally Posted by ian c2

I believe Niel designed the porsche application .
If you look close , you should see the performance developments logo on it .