Macster

Maybe with the newer models (2003 and up) this is the case.

Some few years ago I was at a Porsche dealership and came upon a 986 Boxster in for a new clutch. The old clutch disc wore right down to the rivets. The car was brought in because the clutch was grabbing.

The tech said the rivets wore a circular depression in the flywheel. I asked to see it and he said it was out to be resurfaced. We talked and there didn't seem to be any official objection or caution towards resurfacing a DMF. He sent DMF out once in a while to be resurfaced.

Before i replaced an otherwise reusable DMF -- even if all it needed was a resurface -- I'd want something from a pretty official Porsche source the DMF was not resurfaceable.

insite

a few things:

first, i do believe it is possible to resurface a DMF, HOWEVER, the tech must have the correct tool to lock it during the procedure. that being said, if i were going through the motions of replacing the clutch, i would just use a new flywheel.

concerning single mass flywheels & damping, Spec Clutch makes a clutch for the boxster with a center-sprung disk instead of the OEM solid disk. personally, i love my Aasco. it does make a little bit of noise at idle, but the car is a lot more fun to drive with it in place.

Macster

While the Spec Clutch has a center sprung disc to help cushion clutch engagement shock the flywheel being single mass doesn't offer the crankshaft vibration dampening feature the stock DMF offers. At least one respected Porsche engine rebuilding expert has come out against using anything other than the DMF on these engines for this reason.

insite

i would agree that the elastomer on the DMF would do a better job in this situation. still love my SMF!

Schnell Gelb

When the DMF vs LWF questions come up the same pros/cons are reviewed. One fundamental question has never been answered(??).
Why do some very credible Rennlisters have no c/s failures with a conventional flywheel - yet others predict/had disasters ?
I ask because although careful balancing of all the major rotating engine components may help reduce imbalance, you'll never eliminate it. And w/o a harmonic balancer .......
Is it a case of avoiding a specific rev range/duration?

Macster

I think it can best be explained by the natural variability/differences between two otherwise "identical" systems (engines in this case).

We see this all the time. Water pumps fail at different times. Fuel pumps. You name it while it fails -- at least over a large population of systems -- it fails at a time of its own choosing, so to speak.

The same is true of this DMF/LWFW thing.

While not every LWFW installation will end in doom there's an increased risk.

One is relying upon the amount of margin the engine has.

Some engines come out of the factory with optimum, or just better balance and smoother running due to other factors. A LWFW in one of these engines might run hundreds of thousand of miles with no problem.

Some engines though come out of the factory with less than ideal balance -- though still within commercially acceptable limits -- and other contributing factors less optimum as well, and a LWFW in one of these engines might not run very long at all before a problem arises.

Or the engine might fall victim to something else before the LWFW has a chance to exact its toll.

These cars have a pretty light flywheel to begin with. I'm not sure how much real benefit there is to be had from running a LWFW. I dare say the benefit is probably more psychological rather than physical.

But people will continue to fit a LWFW and that's their choice.

Schnell Gelb

Thank you.
So the longevity of an M96 fitted with a LWFW instead of the stock DMF is a question of statistical probability. It depends of where on the bell curve of balance/weight tolerances a specific engine falls.
Since few of us would measure dynamic and harmonic balance before deciding to fit a LWFW, the outcome is dumb luck.
That may concern some who have the LWFW fitted?
Particularly when you notice that even the people with access to the latest in balancing technology still do not recommend a LWFW vs. DMF
http://www.aircooledtechnology.com/i...amic-balancing

insite

i disagree with this part. the stock flywheel is pushing 50lb (massive!!). consider an acceleration pull from 4k RPM to 7300 RPM in five seconds. for that to happen, you have to add about 70kJ of energy to the flywheel. this equates to a parasitic loss of almost 19HP. do the same math with a 15lb flywheel and the parasitic loss is about 5.5HP. the delta, about 13HP, becomes useable. if we assume 15% drivetrain loss, that is an 11HP gain at the wheels. this amount varies under different conditions, but the gain is real.

beyond reducing parasitic loss, the RPMs drop faster when you let off the throttle, allowing for quicker shifts. overall, drivability is greatly improved for the enthusiastic driver and maybe reduced a bit for a gentle driver (harder to get the car going from a stop).

another note on the DMF/LWF damping issue: i think most of the issues people have when they swap to a LWF are related to the balance of the parts. if they are out of balance even a bit, the crankshaft can fatigue over time. in my case, the FW/clutch combo were out 17g. that is a lot at 7200RPM.

Schnell Gelb

My stock DMF was also out of balance by a surprising amount.The PP also.
When I weighed and balanced the pistons and rods, they too had surprising variations.

Macster

Assuming your numbers are real, I haven't the time to confirm them, an ''11hp" gain frankly is very little compared to the total engine output. And to gain this at the cost -- unknown but it could be very high -- to the loss of the cushioning between the engine and the drivetrain, plus the loss of the dampening of crankshaft, is a trade off of questionable value.

While the flywheel/engine hardware may not be optimally balanced unless one does disassemble the engine and balance it to a finer degree as part of the installation of a LWFW the situation is made worse with the fitting of the LWFW. The loss of dampening then becomes even more serious.

The crankshaft dampening not only reduces stress on the crankshaft, but because the crank is coupled to the IMS and this to the exhaust cams which in turn drive the intake cams (in the 5 chain engines, in the 3 chain the IMS drives both the exhaust/intake cams directly) this dampening reduces the amount of stress at the IMSB and to that possibly transmitted to the other cam drive hardware.

So to repeat, to fit a LWFW delivers -- using your numbers -- delivers marginally more HP to the rear wheels it does so while putting the engine at risk of serious/terminal damage. It increases stress at an already questionable area of the engine, the crankshaft to IMS coupling and the IMSB.

IOWs, fitting a LWFW to one of these engines is like switching from a 6 shot pistol to a 5 shot pistol in a game of Russian Roulette.

Schnell Gelb

One interesting issue not covered is the effect of a worn out DMF on C/S & main bearings ,
The life expectancy of a DMF may be as short as 80k miles(mine).
Until recently the info on checking for wear was not easily obtainable. A worn DMF would produce huge shock loadings on the bearings and c/s.

insite

i think you may be overstating the risk a bit. i wouldn't say it puts the engine at serious risk of terminal damage. there is a reason this engine doesn't have a harmonic balancer at the front of the crank. it doesn't need it. it really doesn't need one at the back of the crank either; the drivetrain does that job just fine. you DO want some damping between the engine & trans to smooth clutch engagement & quiet things down, but in this configuration motor, the risk here is quite low. there are other motor configurations where this is a serious consideration; there are rocking couples & natural harmonic conditions that just aren't an issue in a flat 6.

WRT the IMS bearing, main bearings, etc, the key to those is balance of the assembly. at high RPM, a small amount of weight imbalance on a relatively large spinning disk is a bigger issue. if the harmonics are right (or wrong depending on perspective), it's possible to induce a condition that can hammer the rear main bearing flatter over.