Why do lightened flywheels rattle?

Chuforia · 10-08-2010, 08:23 AM

I was out in my buddies 996 last night. He just got it back from the shop after all kinds of fun work, new bigger turbos, changed gearing, etc etc. Anyway when he idles the lightened flywheel sounds absolutely terrible. Rattles like crazy. Our friend with a supercharged 997, his does the same.

I'm a pretty technical guy and have rebuilt all kinds of different engines, know the fundamentals and parts, etc, but I don't understand this. I'm sure someone here has the answer..please enlighten me.

EBS_Don · 10-08-2010, 01:45 PM

The original dual mass flywheel does a great job of dampening all the noise from the transmission as well as adding torsional dampening between the crank and trans input shaft. The lightweight aluminum versions seem almost to amplify the noise coming from back there!

Chuforia · 10-08-2010, 03:55 PM

So that's what it is, just transmission noise coming through that was normally dampened?
It sounds terrible. lol. He stalled a couple times too. lol Between the lightened flywheel not keeping as much inertia and the taller first gear it drives like a different car. I don't know how much boost the thing is putting out now, but it goes like stink. I feel like my Lotus is a relatively fast car, but the low end grunt of that thing is other worldly. He would blow me away all day long. (I can still take him in the curves though

)

johnsjmc · 10-11-2010, 09:39 AM

I,ve been told the rattle is the LWF clutch disc which has springs on it. The dual mass clutch disc is solid without the small coil springs. The rattle goes away as soon as the clutch is engaged and takes up the play on the torsion springs.

Chuforia · 10-11-2010, 09:56 AM

Indeed it does. As soon as the clutch in engaged, no rattle.

cattman · 10-15-2010, 04:48 AM

Quote:

Originally Posted by johnsjmc

I,ve been told the rattle is the LWF clutch disc which has springs on it. The dual mass clutch disc is solid without the small coil springs. The rattle goes away as soon as the clutch is engaged and takes up the play on the torsion springs.

The spring-loaded clutch pressure plates are stage 3, or race plates. This is not the source of the clatter Keith is referring to, although they may contribute to some other sounds.

I am the guy referenced above with the Supercharged 997 and LWFW, and I am running a Sachs Stage 2 clutch disk and pressure plate, which does not have those springs, and my LWFW still clatters. That's how I know that those springs are not the cause.

However, I don't mind the sound of it in my car at all. Plus, people who know what the clatter is smile when they hear it- they know it means business.

BTW, from outside the car, the clatter is more noticeable on the passenger side. My Porsche tech said he thinks it may be because the tranny case is thinner on that side- odd, eh?
CATTMAN