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Dual mass vs single mass flywheel


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I am having the clutch replaced on my 1999 Carrera 2 and the shop doing the work has recommended replacing the dual mass flywheel with a single mass flywheel. Can anyone explain the advantages and disadvantages of this option?

The dual mass flywheel is the only source of harmonic dampening in the engine, removal of that capability can lead to serious issues, like cracked (or worse) crankshafts. These engines are not particularly well internally balanced from the factory, so the dual mass dampening is rather important. More than one leading Porsche engine builder has recommended against using single mass flywheels unless the entire engine components and the flywheel are properly rebalanced as a unit.

A second consideration is how well the single mass itself is balanced; we have seen several that were 10 grams (and more) out as delivered. These flywheels are also difficult to have accurately balanced; only a handful of machine shops can do a proper job. And even after one is correctly rebalanced, they still can be a bit of a pain to drive on the street due to clutch chatter related issues.

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I agree with JFP in PA, but would like to add that when you go to a single mass flywheel you are reducing the weight of the flywheel significantly. This has a benefit of a slightly faster revving engine. If the ECU is not tuned for it, you may have a stalling problem when coming to stops.

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Before you take that shop's advice you might want to read this:

http://www.flat6innovations.com/broken-crank

I believe this is what JFP was referring to. Specifically (and these are Raby's words).......

I feel that this failure was attributed to by a couple of things-1- The engine was "upgraded" to a lightened flywheel. This new flywheel was installed onto the existing stock engine without being balanced to that assembly. This created an imbalance in the rotating mass AND it did away with the factory dual mass flywheel.2- The dual mass flywheel was removed to alow the single mass lightened unit to be installed. This eliminated ALL MEANS OF HARMONIC DAMPENING!! The crankshaft was forced to absorb ALL harmonics from the engine and transaxle when the dual mass unit was removed..So- adding the light weight flywheel was a double negative, not only did it create imbalance, it also eliminated the harmonic dampening of the dual mass arrangement. Due to this I feel that adding a lightweight flywheel to any existing engine is not a wise decision, and that they should only be added when the entire rotating mass can be balanced and indexed to accomodate the lightweight unit. This means engine disassembly, so I'd only add one of these when doing one of our performance upgrades so the entire assembly can be precisely balanced.

Edited by ferrugia
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IIRC the experts say stick with the dual mass even if your engine has been balanced to within very fine tolerances, unless perhaps you are building a race car.

If you've gone so far as to tear the engine down and have it profesionally balanced to within say a gram or less, I would say the benefit of having it balanced (along with the others you probably made while you were in there) are likely to far outweigh the benefit of a lighter flywheel.

Remember there is not normally a harmonic underdrive pulley either.

Are there cars on the road with LWFW that don't have problems, sure, but I guess if you want a better insurance policy stay with the DMFW.

FWIW, my dual mass was off by at least 20 grams from the factory (not to mention crank and rods were a few grams off each). That 20 grams out at 5 or 6 inches at 7000RPM is a huge power zapper! Of course, went with micropolish crank and R&R rods (pre-balanced) and everything came to within a gram. Didn't knife edge or anything though. I bet I could safely raise the rev limit!

Edited by logray
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Speaking as the owner of that particular 2 piece crankshaft (now a 20k doorstop), I would also recommend staying with the dual mass FW. I had the LW balanced as well, but Jake still recommends against using it. My car is strictly track, but the small potential increase in performace does not outweigh the cost of a rebuild. I am concentrating on getting the ROI to a sane level and am at 4k track miles and the engine is performing flawlessly.

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  • 5 years later...
Speaking as the owner of that particular 2 piece crankshaft (now a 20k doorstop), I would also recommend staying with the dual mass FW. I had the LW balanced as well, but Jake still recommends against using it. My car is strictly track, but the small potential increase in performace does not outweigh the cost of a rebuild. I am concentrating on getting the ROI to a sane level and am at 4k track miles and the engine is performing flawlessly.
Finally! I've searched the web over, top to bottom, looking for someone, ANYONE, who had even a single failure caused by a LWFW. Now I've finally found one. ONE mind you, amidst literally hundreds of people confirming they've been running their LWFW for years and hundreds of thousands of miles without issue.

Oh, there are plenty of people saying "I know a guy who installed a lwfw and it broke his crank", but in every instance they credit Jake raby as their only source (except JFP, who also credits several porsche engineers as saying it's a bad idea).

Anyway.....
Thanks for posting here as the owner of that infamous crankshaft. I was beginning to think it was all made-up! Haha

Sent from my SM-G930T using Tapatalk

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  • 3 years later...

I don't understand all this negativity on lightweight flywheels, only because one person thinks it may have been the cause of an engine failure.  I'm with @Maytag on this...

 

1) lightweight flywheels are not any harder to balance than heavy flywheels. 

2) The dual mass is designed to reduce the pulsing forces into the transmission and reduce gear noise, while it also smooths out the pulsing torque forces into the engine that is only a secondary benefit and the highest forces in the engine at the cranks are due to the piston.  The dual mass flywheel would have virtually no impact on these crank forces.

3) The lightweight single mass flywheels were/are used on GT3 RS cars for years. 

4) If you install a lightweight single mass flywheel you should also install a spring friction place to take the place of dual mass flywheel.  For the few failures that have been documented, what type of clutch plate did they install (springer or original)? 

 

Net net, until I see some data backing up the failures, I'll take my chances and go for the increased performance.  

 

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27 minutes ago, Michael Petersen said:

I don't understand all this negativity on lightweight flywheels, only because one person thinks it may have been the cause of an engine failure.  I'm with @Maytag on this...

 

1) lightweight flywheels are not any harder to balance than heavy flywheels. 

2) The dual mass is designed to reduce the pulsing forces into the transmission and reduce gear noise, while it also smooths out the pulsing torque forces into the engine that is only a secondary benefit and the highest forces in the engine at the cranks are due to the piston.  The dual mass flywheel would have virtually no impact on these crank forces.

3) The lightweight single mass flywheels were/are used on GT3 RS cars for years. 

4) If you install a lightweight single mass flywheel you should also install a spring friction place to take the place of dual mass flywheel.  For the few failures that have been documented, what type of clutch plate did they install (springer or original)? 

 

Net net, until I see some data backing up the failures, I'll take my chances and go for the increased performance.  

 

Welcome to RennTech :welcomeani:

 

1.  No one said they were, but many are not balanced out of the box, which is an issue.  And some machine shops simply don't want to add the cost of developing processes and tooling to specifically deal with low volume alloy flywheels they see infrequently.

2. Regardless of what you may think, the dual mass flywheel absorbs torsional harmonics from the crankshaft, which in most Porsche engines is cast, not forged, and the rotating assembly is not very closely internally balanced from the factory. Hence the need for a torsional harmonic absorption device like the DMF or suffer cracking issues.

3. The GT 3 RS engines come with a forged crank, unlike most Porsche engines, and the rotating assembly is internally balanced to a much higher standard than the regular production engines, which are two of many reasons why when new short blocks were still available, the production versions sold for less than $20K, while the GT car short block was well north of $50K.

4. Short answer: both, so while the sprung disk may make the car more drivable, it didn't prevent crank cracking.

 

Net net:  It is your money, and your engine; feel free to go in whatever direction you please.  But when someone asks about using one on the M96/97 engines with a cast crank, I will continue to go with my experience and that of a shop that builds some of the best versions of these engines and recommend only doing so if the entire rotating assembly is balanced as an assembly.

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2 minutes ago, Silver_TT said:

You can lead a horse to water............

 

Yeah, but if you hold his head under long enough, the bubbles stop coming up................................😵

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I love the technical discussions, but I don't approached the immature criticisms, especially coming from a moderator.   "Holding a horse under water, and waiting for the bubbles to stop"?  Are you guys teenagers?  This thread is starting to sound like a twitter rant.  Lets keep automotive forums about our common passions and leave the criticism for other social media accounts. 

 

I also think you may find some of your members know more about cars than you give them credit for.  

 

But, your counter your specific technical points:

 

 

1.  No one said they were, but many are not balanced out of the box, which is an issue.  And some machine shops simply don't want to add the cost of developing processes and tooling to specifically deal with low volume alloy flywheels they see infrequently.  --> I'm installing an AASCO aluminum flywheel.  They are a good brand and I have seen no data that their balance is better or worse than factory porsche flywheels.  It would technically be easier to balance a single mass flywheel than a dual mass, as there are tolerance build ups in the rotating mechanism of the dual mass that may make it actually harder to balance.  Again, I have no data to know which flywheel is better balanced.

 

2. Regardless of what you may think, the dual mass flywheel absorbs torsional harmonics from the crankshaft, which in most Porsche engines is cast, not forged, and the rotating assembly is not very closely internally balanced from the factory. Hence the need for a torsional harmonic absorption device like the DMF or suffer cracking issues.  -->.   Here is a very good article on the physics of the dual clutch, it was writing by LUK during the development of the dual clutch design many years ago, See pages 69-93.  https://www.schaeffler.com/remotemedien/media/_shared_media/08_media_library/01_publications/schaeffler_2/symposia_1/downloads_11/luK_kolloquium_en.pdf.  It is true the dual mass flywheel reduced the vibrational forces on both the engine and the transmission, but we are effectively replacing the dual mass flywheel with a friction plate that has incorporate springs that gives the torsional decoupling of the engine from the transmission.  Under load, the dual mass function moves from the flywheel to the friction plate.  The rotation of the engine has no idea where the reduction of the true torsional peaks comes from, it just knows the peaks are lower.  It is for this specific reason that it is recommended not to use a rigid friction plate with a single mass flywheel.

 

3. The GT 3 RS engines come with a forged crank, unlike most Porsche engines, and the rotating assembly is internally balanced to a much higher standard than the regular production engines, which are two of many reasons why when new short blocks were still available, the production versions sold for less than $20K, while the GT car short block was well north of $50K.  --> I think we strongly agree that better balanced engines are always better in high-performance applications. I also think it's a good point the GT3 RS uses a forged crank versus a cast crank of other variants.  

 

4. Short answer: both, so while the sprung disk may make the car more drivable, it didn't prevent crank cracking.  -->  You may be correct, but I would like to see some data (any data) that backs this up.   There are tens of thousands (maybe hundreds of thousands) of aftermarket lightweight single mass flywheels in the market today, and we do not see widespread complaints of crank failures. I see a lot of opinions, just not many actual proof points.  In the beginning of this thread someone mentioned Jake Raby thought an engine failed do to a single mass flywheel.  Instead, maybe the crank had a large grain boundary diffusion when it was manufactured and it was destined to fail, and it just happened to fail with a customer with single mass flywheel.  We will never know.  But we should be able to look at the quantity of field failures and make an education assessment. 

 

Im installing a single mass flywheel with a spring friction plate from Sachs; it will have hundreds of track hours per year and I PROMISE to report back here if I have an issue so other can benefit from my learnings (or mistake!) 

Edited by Michael Petersen
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Thanks for the clarification Silver TT... I appreciate it.   

 

To be clear, Im not a snowflake, Im far far from it.  I just like to see these blogs about cars (which we all share a love for) to be a step above the the average junk I find on mainstream social media.  Let's debate the problem, but not attack a persons IQ. 

 

 

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