Why doesn't anyone make a near-normal weight single mass flywheel?

[JoeFromPA]

Hey all,

Just wondering if this has ever come up:

Seems like the stock dual-mass flywheel is a poor fit for these cars. It wears out faster than it's lifespan due to the low-mileage these cars receive (for the most part) and the fact that it's the damping material that's wearing out.

Since the clutch is sprung, it seems to me like the best aftermarket solution would be for someone to make a near-normal weight single-mass flywheel. Normal weight being close to the dual-mass flywheel (you wouldn't need it to be identical to maintain the same level of drivability, since some of the DM flywheel's weight comes from the damping packs)

This is not a super complicated design, wouldn't require any additional fabrication, and should be a high-profit item since the stock flywheel is near $800 and we're talking about replacing it with a piece of (high-quality, tight tolerance) machined metal.

Thanks all,

Joe

[JFP in PA]

Very simple: Like light weight single mass flywheels, a single mass standard weight flywheel would probably lead to problems elsewhere, most likely internally to the engine. The dual mass flywheel is not a substitute for the springs in the clutch; it absorbs and dampens vibration and harmonics inside the engine, mostly the crankshaft. The springs in the clutch disc absorbs shocks in the drive line (transmission, axels). They are not doing the same job, and removing the dual mass flywheel without thoroughly rebalancing the engine internals could lead to serious issues…………….

[JoeFromPA]

I understand the concepts; they are blanket statements thrown over a situation. Let me provide some examples:

...

In 2005-2006, Subaru used a dual-mass flywheel on the same year Legacy GT models in 5-speed format. In 2007-2009, they switched to a single-mass.

This is a straight bolt-on application, albeit with a different clutch to fit the face of the new flywheel. They did not re-balance the internals, and the single-mass was a few pounds lighter.

How do I know? Because Subaru offers the 07+ single mass flywheel as a bolt-on replacement choice (with the 07+ clutch) for this engine/transmission combo.

...

Single-mass flywheels are used in a huge variety of automotive applications known as "smooth". They are commonplace. The concern with vibrations and harmonics when switching is:

1. When someone goes to a racing-style application of a lightweight flywheel.

2. If something is poorly engineered.

Single-mass flywheels by themselves, especially when made "heavy" (usually 25+ pounds), dampen vibrations and harmonics just fine.

...

Last example: A flywheel's mass and location of mass is not a precision instrument to perfectly balance internal harmonics. How do I know?

1. Because flywheels, by their nature, receive wear over time. As long as their surfaces remain smooth and true, they balance out fine.

2. Single-mass flywheels routinely receive resurfacing, which involves the removal of mass/weight. If a shift of a few ounces mattered, this would not be a suitable action.


...

I could go on. I'm not an engine builder nor do I develop parts for cars. My knowledge of cars tells me that a single mass flywheel of similar weight and with proper design would be a simple solution to allow Porsche owners to stop worrying about dual-mass flywheels wearing out.

[JFP in PA]

Judging by the number of torsional stress related failures observed on otherwise stock M96 engines by going to single mass flywheels, I would say it was poor value for the money…..

[JoeFromPA]

JFP - Can you elaborate or provide a link? I did about 10 minutes of searching and found nothing (except issues on chatter, which is normal with LW flywheels).

Also, since oneof the boxster's issues is supposedly due to a heavy-ass flywheel hung off one side and not enough support on the other, I'd think that a bit less mass would be preferable....

[JFP in PA]

Originally Posted by JoeFromPA

JFP - Can you elaborate or provide a link? I did about 10 minutes of searching and found nothing (except issues on chatter, which is normal with LW flywheels).

Also, since oneof the boxster's issues is supposedly due to a heavy-ass flywheel hung off one side and not enough support on the other, I'd think that a bit less mass would be preferable....

In general, reducing the flywheel mass would offer some benefits (e.g.: quicker revving). Problem is that the dual mass unit acts as a harmonics dampener, absorbing some of the torsional forces on the crankshaft, reducing cracking tendencies. We have encountered problems on M96’s running single mass wheels with out any additional dampening (adding the GT3 dampened front pulley, complete harmonic re-balance of the engine, etc.) The magazine “European Car” did an article about DMF setups about 3 years back, but (if memory serves) the individual with the most data on the subject is Jake Raby (Flat Six Innovations), who probably knows more about the M96 than anyone out there. From one of his postings (concerning using a light weight single mass flywheel on the M96):

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?????

Raby comment

[JoeFromPA]

Originally Posted by sppmo

Joe - maybe it has to do with the flat-six engine design? I don't think anyone here would have a better answer than the folks at Stuttgart. I wonder if you can mail them the question? I've have never tried to me mail Porsche. Has anyone else?

I doubt it. I know of no reason why that would effect it, and Porsche also used single-mass flywheels up through ~1989ish....not sure exactly when they switched...