Eggbert

I'm in the middle of removing my transmission as the first step in removing my engine to have bore-scoring fixed. I havn't exposed the flywheel yet to check it, but am wondering about using a single mass flywheel on the street. No doubt Porsche had a good reason for using a dual-mass flywheel,, yet most other cars use a single mass flywheel and are fine.

Nobody seems certain as to what causes bore-scoring, but I've had a thought on why it that is very likely wrong, but it does explain some things. What if the scoring is caused by a failing dual mass flywheel? The vibrations would be worse the closer to the flywheel, so that explains number 6 being a problem first. (Other reasons can contribute.) Also, the vibration damping properties, like a harmonic balancer, are mostly required at low rpms. At high rpms, the flywheel sort of self-straightens and the damping properties arn't needed nearly so much, if at all. That may be why driving like Aunt Martha is worse for the engine as far as bore-scoring is concerned. Also, using a solid mass lightened flywheel for the track would not need damping as the engine is primarily driven at high rpms not to mention the low mileage accumulated compared to a street car. Might not be a valid reason, but as nobody seems to know for sure, I suppose it could have merit. I wonder on bore-scored repaired engines if the flywheel was found to be bad....

Listen to this guy from about 4.50 to 5.00 minutes.


So.... is using a single mass flywheel for the street such a bad idea? I know some of you must have done so.

2swoosh

very interesting! need the experts here to please chime in for validity of his claim, thanks

Caveman Smash

I am considering a single mass flywheel as well, though strictly for weight loss. I am still gathering information on the topic and do not have answers to your questions, but if I were performing the modification now, I would install a clutch/flywheel kit that utilizes a sprung clutch. That should dampen harmonics and provide a nice weight reduction. FVD offers separate kits for 987.1 and 987.2. Not cheap, but worth it if it preserves the motor.

https://www.fvd.net/us-en/FVD116912A...-flywheel.html

Dave in Chicago

Can’t speak to the 987 specifically, but on my 968 club race car, a lightweight flywheel made sense… reduction of rotating mass allowed better rev-matching during very rapid upshifts/downshifts, among other things.

But, it definitely increased driveline vibration. Made the transaxle sound like a bucket of bolts at certain rpms. Kinda unnerving that, but ultimately harmless.

This was in a very well balanced race engine that spent most of its life north of 5500 RPM, under full throttle, and redline elevated.

On a streetcar I would stick with the dual mass stock flywheel all the way. The smart engineers at Porsche have it right, IMHO.

ZuffenZeus

There is no documented correlation between cylinder bore scoring and light weight flywheels. What is well known is that installing one on M9X engines causes harmful harmonics that are not cancelled and misinterpreted by the knock sensors at the least and even a snapped crankshaft at the worst.

Quote from Jake Raby on the subject:

"Installing this flywheel removes ALL harmonic dampening of your engine and transaxle..

One person has recently broken a crank... He didn't listen to me when I told him what caused his issue more than likely, so he reinstalled the same flywheel on his new crate engine. Two events later I got another phone call from him stating that he had broken another crankshaft and he should have listened to me. He is now on engine #3 and is broke, so broke that he is having to make one engine from 3 broken cores..

Another engine (2.7 DE car) had a knock, it was pulled apart and had a cracked and breaking crank.. When I threw the assembly for this engine onto the balancer it was immediately 10 grams out of balance, when the pressure plate was added that went to 19 grams and the flywheel was nearly new and had never been touched..

Both of these are in addition to the X51 engine that snapped a crank in half last year, also using a LWFW... There have been two other instances of similar consequence that people have contacted me about since the new year, but I did not see their parts first hand.

Harmonics have to go somewhere... The dual mass was utilized for a reason-Components that are forced to absorb them won't like it.. And it appears that these harmonics also end up being sensed by the knock sensors as possible detonation, so then the ECU retards timing and that reduces HP. I have gathered data that proves that these harmonics that can't be absorbed are directly related to reductions in net power, as much as 5HP in one instance from my test car.

Balance and harmonics are two different things... Sure an engine thats out of balance will have more harmonics, but even an engine thats perfectly balanced will still have harmonics that need to be absorbed..

The dual mass flywheel and it's dampening characteristics help to absorb these harmonics, the LWFW does not have any dampening capability because it has no second mass separated from the primary mass by absorption material.

Consider the fact that the dual mass flywheels that do fail may be failing because they are actually doing their job!! A flywheel is a wear item, it is a component that is designed to be disposed of after it's job is complete... A crankshaft is not a disposable item and if not absorbed somehow, somewhere these harmonics will find the weakest link and thats when things break.

With the M96 everything is rigid once the dual mass is removed, that means the harmonics from the engine, transaxle, CV joints and even the axle bearings are all going upstream directly to the crankshaft.

When the mass of a dynamic assembly changes as radically as it does when a LWFW replaces a DMFW the plane of balance must be compensated for, that means even if the flywheel that is placed onto the engine is perfectly balanced, when coupled to the rest of the dynamic assembly it will be imbalanced if the plane is not corrected.

The only way to do this is with the engine disassembled in a balance machine like mine. Thats why I refuse to install a LWFW onto any engine unless I am creating it's engine from scratch and can ensure the unit is balanced as a complete dynamic assembly then indexed for reassembly.

FWIW I have yet to see a single LWFW that has ANY balance marks on it brand new out of the box. Every unit I have spun up has had imbalance that exceeds my tolerance threshold...

When the second mass is removed where do those harmonics go?????"

Jake Raby - Flat 6 Innovations

old man neri

Interesting. On a VW TDI it's the other way. Vibrations from the engine go to the transmission and wreak havoc in there.

All I know is if my car came with a DMF from the factor I am leaving it in there.

Dave in Chicago

This kinda sums it up.

On the big 4 cylinder in my 968, Porsche ran two counter-rotating balance shafts, each spinning at twice the engine rpm, to manage that engine config’s inherent 4th-order harmonics.

They added complexity, spun at 14,000 rpm, sapped a few hp, etc. But without them, vibration-induced adventures followed.

Yup, seems that “Harmonics have to go somewhere...”

GLTHFJ60

You can install a rss harmonically dampened crank pulley to compensate. That's my route.

2swoosh

does this mean i have to drive my Cayman hard everyday and visit 5000+ rpms all the time to avoid the potential issues? lol

Eggbert

Yes.... according to many. There have been reports that track cars don't seem to have bore-scoring as much, but then again, they probably have low mileage.

You might find this thread interesting: https://rennlist.com/forums/997-foru...revention.html

I guess I didn't make myself too clear in my original post. It wasn't really a pro/con thing about solid vs. dual mass, although based on some research and from the comments I've read here, I'll stick with the DMF.. What I was really concerned with is that as a DMF has a relatively short lifespan and IF a failed unit can contribute to bore-scoring, then should I go with a single mass flywheel. At this point I feel like a careful inspection of the flywheel at clutch replacement time should suffice.

Another symptom of bore-scoring is a knocking or ticking at idle. Check this video out and you may see what I think may cause the ticking;


This whole thing about a failed DMF causing bore-scoring is just speculative on my part, but the facts do seem to support the theory. Sure would be nice to know how many bore-scored engines also had a failed DMF.

XuTVJet

One thing that seems to get missed about running a single mass flywheel is that one needs to run a spring clutch disc to aid in damping. Porsche uses sprung clutch discs on their cars that don't run DMFs. Most of those cars also run mutli-disc clutch setups.

I think one should be completely fine running a single mass flywheel, sprung clutch disc, and a fluid crank damper.

With that said, I wouldn't run a single mass flywheel on the street as it's annoying. At first, the excitement of the quick revving nature of the motor and ease of rev match downshifts makes you look past stop and go driving. It gets REAL annoying when it's hot out and the engine is heat soaked. The car will feel gutless below 4000rpms in 1st and 2nd.

ZuffenZeus

The OE LuK Dual Mass Flywheels are dynamically balanced to the crankshaft. Changing this delicate balance would be like putting only ONE unbalanced carbon fiber wheel/tire combo on the front of the car and expected the car to move smoothly at high speeds.

IMHO, the light weight flywheel could be used if dynamically balanced with the crankshaft and pulley during a custom engine build.

KrisA

Just out of curiosity, how do we know that flywheels like those from companies like Aasco are NOT dynamically balanced to the crankshaft?