I've read somewhere where a secondary counter rotating shaft with weight is use to counter harmonics forstraight 4 cylinder engines - patented by Mitsubishi??? but these were basically directly driven via chain. is my understanding that DMFW has some sort of spring/elastomer coupling the two disks.

Question, with direct coupling, is the harmonics cancellation accomplished independent of engine rpm.
Question, with spring between the two, taking out harmonics from 700 rpm to 7000 rpm, 10x range would need spring to become 10x stronger??

I may have found answer to question here

http://www.exedyusa.com/multimedia/specsheets/ClutchFundamentals.pdf

RMS leaks are becoming VERY rare.

this is good news but this is 2000 S 3.2 with only 18k miles on it I am talking about so it has not had enough running time to know

My Aasco lwfw arrives tomorrow morning. I alongside my crankshaft are looking forward to it!

hmmmm so you think it will keep it safe as per my original post?

I stumbled across this and read with great interest ...and also cringed at this misinformation mixed with correct information. I've spent 10y years of my life training people how to use balancing machines, and have been called into many facilities to balance something when no one else could do it. I have also spent another 8 years in the field of vibration analysis.

**) Balance has nothing to do with the total mass of the object, so a lighter flywheel does not change the balance of the the assembly. Unbalance is measured as mass x radius. There is no such thing as "zero balance". Zero balance only means "my balancer isn't accurate enough to measure this".
**) unbalance ALWAYS takes place at the same frequency as the speed of the rotating shaft ...no matter what speed the shaft is spinning.
**) Harmonics always take place at direct integer multiples of the shaft running speed (2x, 3x, 4x etc ...never 1.23x).
**) Unbalance can causes harmonics, but can never cure harmonics.
**) Assuming the shaft is rigid ...and it is safe to say all crankshafts are, (even though I know of 2 that were not), any Unbalance in the crank flywheel assembly can be corrected in two planes, but is best corrected on the crank, then add each component and balance that item in the same balance plane. This is the preferred way ONLY because it allows parts to be replaced without needed disassembly of the entire engine to re-balance it.

Given those statements of fact, a lighter flywheel will NOT change the balance of the system regardless of how much it is overhung away from the rear Bearing. RMS leaks are probably caused by bearing clearance issues and seal design issues, and can not be cured or made worse by any type of flywheel. A lighter flywheel will always make for a quicker reving engine, but some mass is desired to smooth out the pulses of the engine and more importantly to retain enough inertia to make starting from a dead stop easier. The heavier the flywheel, the more accurately it must be placed on the crank. If you mount a light weight and a regular flywheel that both have theoretically zero unbalance, the flywheel and pressureplate that weights 20 lbs and mounted to a flywheel with .0010" of clearance, would have half the residual unbalance as a 40 lb flywheel & pressureplate that have .0010" of clearance!

Now onto harmonic dampers. These are designed to reduce torsional vibrations. A person needs either 2 rotational accelerometers or 2 Polytec torsional vibrometers to measure these vibrations. While it can be done with one unit, it requires certain assumptions to be made. It can NOT be done with a regular accelerometer.
Piston movement turns the crank because the crank's rod journal is off the crank's centerline. The crank's rod journal acts as a lever arm attached to a shaft. The piston and rod push down on the lever arm, which rotates the shaft. That force is resisted by the entire weight and inertia of the car, which the driveline is attempting to push forward. It is impossible to create a perfectly rigid crank, so some of the energy from the piston bends the lever arm and twists the crankshaft.
When the force is reduced enough for the crank to spring back, it doesn't just return to its original shape immediately. Instead, the force of untwisting causes it to shoot past its original shape and twist in the opposite direction. This twisting goes back and forth a number of times until the crank settles down. The number of times this event takes place and its duration depends on the crank's construction (cast, forged, billet), mass, and quality of materials. When the engine moves through its rpm range, the speed at which the power pulses from ignition of the air/fuel mixture will eventually match the natural frequency of the crankshaft. When this happens, the vibrations moving through the crank will build upon each other and gain strength, often with damaging results. Running an engine without protection from these vibrations can cause flexplate or flywheel failure, loosened bolts, broken timing chains, along with erratic timing and valvetrain functions. The crankshaft will fatigue and eventually fail, usually at the snout. The purpose of the harmonic damper is to protect the engine from these damaging vibrations. Most quality harmonic damper designs reduce the vibrations traveling up and down the crankshaft by absorbing the vibrations and converting them to heat energy. Many racers use the words "balancer" and "damper" interchangeably, but since almost all engines are internally balanced, "damper" is the more accurate term.

There is a good reason that the damper is normally attached to the front of the crankshaft. The flywheel and the resistance provided by the rear wheels absorb crankshaft harmonic vibrations at the back of the crank. So if you run an engine without a damper, the crank will show greater twist the farther you get from the flywheel. Since the timing chain is normally at the front of the crank, running without a damper (or even with a poor-quality, inefficient one) reduces crankshaft life, but also causes the ignition and valvetrain to behave erratically. OEM manufacturers have started to place the damper in the flywheel to quiet transmission gear noise. As manufacturers have moved from very strong billet or forged cranks to sintered metal cranks, they are more susceptible to stress cracking due to torsional loads. Sintered metal is less expensive to produce, but the drawback is they break much easier. The harmonic damper (on the nose of the crank or built into the flywheel are both little more than a heavy rotating mass coupled to the crank by a rubber gasket with a specific damping frequency and with a very low Q. As the flywheel ages that rubber deteriorates and the frequency will change as will the Q. Eventually it will be damping the wrong frequencies and will become so compliant that it will break. This is the reason for not reusing a old dual mass flywheel. Unfortunately they are quite expensive, and the stock units are darn heavy and are not well suited to quick reving engines.

If you have made it to this point, you probably now know more that you really ever wanted to know about harmonic dampers, harmonics, and unbalance, (and according to ISO and ANSI it is UNbalance not IMbalance despite the high number of people in the industry that don't know the difference and even dictionaries that get it wrong).

-Gregg

Answers to Sb01box

"The sprung clutch just absorbs some of the initial shock but does nothing for the harmonics" 100% true statement. The sprung clutch helps reduce the shock of clutch engagement from being transferred to the gearbox. A dual mass flywheel makes the sprung clutch redundant. Never use a sprung clutch with a dual mass flywheel and don't use an unsprung clutch with a single mass flywheel.

"I've read somewhere where a secondary counter rotating shaft with weight is use to counter harmonics for straight 4 cylinder engines - patented by Mitsubishi??? but these were basically directly driven via chain." There is inherent unbalance that takes place at 2x the RPM of the crank rpm based on the weight of the rod ends attached to the crank in in-line 4 cylinder engines (not horizontally opposed engines nor V6 or V8 engines ...they have different issues). These 4 cyl engines use the timing chain to also spin two shafts that are intentionally out of balance ...and they spin it at 2x the crank RPM and a little above the centerline of the crank. This unbalance directly offsets the mechanical unbalance that takes place due to the positions of the 4 crank journals.

"is my understanding that DMFW has some sort of spring/elastomer coupling the two disks." Answer: Yes, and that is what acts as the damper or the shock absorber. The elastomer is positioned so it doesn't just allow the outside weighted ring to just bounce unrestrained like a spring, but rather it must overcome some friction like a shock absorber being expanded and compressed. It is less like a spring and more like a shock.

"Question, with direct coupling, is the harmonics cancellation accomplished independent of engine rpm." Answer: with direct coupling there is no harmonics cancellation, regardless of engine rpm.

"Question, with spring between the two, taking out harmonics from 700 rpm to 7000 rpm, 10x range would need spring to become 10x stronger??" No the harmonics are not due to the rotating mass or the RPM of the engine, but rather due to the power generated by each explosion of the firing cycle of the engine. While this does changes in force at different rpms, the damper is "tuned" to do the most damping at the specific frequency that the crank needs the most assistance. (if you ever saw the video of the tacoma narrows bridge, you have seen an example of something reasonably rigid that needed assistance at a specific frequency. Tuning of the harmonic damper is done by adding or subtracting mass from the outside rotating mass or by adding stiffness to removing stiffness from the elastomer ...In both cases this must be done equally 180 degrees opposite each other and with the same amount of weight (or elastomer stiffeners) or unbalance will be added to the crank assembly.

-Gregg

Thanks for the detailed description. I remember harmonics being very interesting in my Physics class experiments. I noticed right away how soft gear-changes were when I bought the Boxster, due to the DMFW, You don't get that tire-chirping jolt with a hard shift.

let's see, horizontally opposed engine with a SEVEN bearing main......and a LWF is gonna break the crank? please.....

if GT3's are breaking cranks, me thinks there is another issue. this occurs when the harmonics flex the crank back & forth until it fatigues (like a paperclip you bend back & forth until it breaks). i'd really like to see a photo of these failures to see if the fracture is indeed metal fatigue or if it's a helical separation, indicating something else all together......

hmm. i may have to eat my words. Raby has some photos on his site of a broken crank from an X51. hard to say from the photos, but it looks like fatigue to me.....

IMO this is very bizarre in a flat6

okay, this is driving me nuts. what would drive a harmonic in this engine that the DMF could actually balance out?

what i'm getting at is that this is a flat 6. it has inherent mechanical balance of 1st and 2nd order. further, the CG stays constant about the crank. the rocking couple of each bank is balanced by the other bank. harmonic balancer or no, this simply shouldn't happen w/ this configuration.

there could be some torsional harmonics, but the 1st order torsional (due to the engine pulses) is inherent to engines in general, since they create torque. IMO, this is just as well damped by a sprung clutch than by a DMF.

finally, the DMF is at the wrong end of the crank to really damp any higher order harmonics that may appear......


HMMMMMM: Jake - did the broken X51 crank come from a car with a solid clutch disk, or a sprung clutch disk?

Well, I'm no expert, but you can have as many main bearings as you want, they do nothing to lessen the torsional forces, because by nature they provide almost no resistance to rotation, they only prevent bending. When tightening a bolt, whether you support the ratchet head or not doesn't change the torque on the bolt, just whether the socket will stay in line with the fastener or not.

If I remember it was from a LWF flywheel, tho I don't remember if it was a sprung disc or not.

I played it safe when I built my motor, and I have a dampened crank pulley on a dynamically balanced engine with a LWF and sprung disc.

I tend to agree with insite. There are numerous torsional forces in any internal combustion engine, the pulsing of the engine upon firing of each cylinder is easily heard as a car idles ...and the mechanics are the same at higher rpms, but not as noticeable to our ears. The absolute need of a mass damper to avoid breaking a crank seems pretty remote, but I guess it is possible - depending on what the crank is made of..

I would expect this would have everything to do with crank manufacturing techniques and engineering. There will always be resonant frequencies that every crank wants to vibrate at, and this is true in the torsional direction too. How compliant the crank is in the torsional direction is what matters, as also at what frequency the crank wants to recoil at when a torque load is released.as speed increases the the rotational forces will certainly cross that frequency and the crank will be excited. A billet or even a forged crank will by it's nature have a higher natural frequency than a sintered metal crank. They are also much stronger because of the way they were manufactured.

Personally I am not a fan of sintered metal cranks, I believe they are more likely to fail in torsional loading scenarios.
Are the cranks that have failed, all been sintered metal cranks?

Unfortunately, all M96/M97 cranks are sintered :(

indeed. still, very bizarre that this happens. cloudsurfer - harmonic balancers are normally tuned to damp the natural frequency of the crank. who designed the damped pulley, & do we think it's tuned?

i wish i could get that broken crank under a microscope.......still, with it being sintered, it would be a lot harder to tell exactly what happened.

Well aware :) The pulley I have came from RSS, and was "designed for the M97 3.8 engine to retain the factory damping characteristics" so honestly, who knows if or how well its tuned, but I figured it was better than having nothing, so it went on my motor.

I, too, would be interested to see the actual failure of that broken crank....

here's the best close-up on flat6innovations.com:



http://i156.photobucket.com/albums/t...rokenCrank.jpg

Thanks for the detailed explanation to my question!

What happened to it insite? It looks, its been in trouble? :confused:

Crank Picture
 

Guys,

Sorry I'm so late to the party. That is a picture of my crank. It was running a Aasco LWFW with a Sachs Sport spring clutch when it broke. During the rebuild Jake balanced both assemblies and I am currently running thr DMFW with a Spec non-sprung plate. Since I am interested in longevity over ultimate 10/10ths performace, I took the safer, but by no means absolute proven route. That said the HP and TQ put out by the Flat 6 3.8 with improved X51 upgrades, makes the advantages of the LWFW a moot point in my opnion. Now if Jake could come up with something to keep my skirt from flying up at 154 at the end of the main straight at Brainerd, I could get into the 40's and be happy!

Lon

Lon,
My Wife goes that fast, actually he highest land speed record is only 153, so you have her slightly beat.. Are you wearing a Nomex skirt? Seemed to work for her. You could always couple your skirt with cloudsurfer's stiletto heels and have a true winning combination.

Anyway, the broken crank issue is being handled.. We finally decided to step up crankshaft production and have units mass produced from true forgings. The testing is going to be long and hard and we can't get it done fase enough. Most cranks we are seeing now are not passing magnaflux tests, even from bone stock street engines with DMFWs. Rebuilding without magnafluxing is like Russian Roulette with these engines. I received another X51 broken crank call last week, it was broken in 4 places, not just two.

Land Speed
 

Jake,

Yes but she does it from a standing start, I comming out of turn 10. Anyway, we'll install a new crank once you get the individual throttle body and new cams worked out! But I'm sure thats another thread.

I am looking forward to all of the tracktime I can get this year. I'll get you a fresh oil sample early May.

I know that this topic is from a few months back, but I was reminded of it while reading some of the other forums.

Are we sure that the rattling sound is coming from the engine? I have read that a lot of other vehicles with similar dual-to-lightweight flywheel conversions have similar rattling and their consensus seems to be that the sound is coming from the gears of the transmission and not the engine itself.

I tend to agree with is hypothesis as it seems to make sense: At idle, clutch out = sound. At idle, clutch in = no sound. So obviously something that changes with the clutch being pressed gets rid of the sound. That means we can narrow the problem down to something that changes when it is pressed.

What changes?

With the clutch in, the transmission and engine not connected. When the clutch is not in, the engine and transmission are connected. I believe that the dual mass flywheel dampens the engine vibrations from going to the transmission. With the light flywheel not dampening these vibrations, they travel to the transmission gears while the clutch is not in and the car is not moving.

The sound does sound like it is echoing from the transmission area and not from the engine itself.

One way to find out for sure: Have someone run an engine with the Aluminum flywheel on it without a transmission connected. :)

Anyone have any thoughts on this?

Has anyone else with the lighter flywheels noticed that the sounds seems to be coming from the transmission, and not the engine?

i never considered that since i figured my sprung clutch disk would settle it down. i'm putting it up in the air Monday to drop the tranny; i'll take a listen with it lifted & see what i think.

i always thought my release bearing was shot or something.....

I now have a 2006 Variocam Plus 3.4L from a Cayman in my 986 with the AASCO / Stage 2 Spec Sprung Clutch. I also replaced the entire arm and throwout bearing when I did the swap, so everything is good shape. Mine makes the same rattle now as it did with my 3.2L.

When I put on the RSS underdriven crank pulley Dampener, it didn't change the sound in any way at all either.

FYI, i was using the LWFW with a spring centered clutch. i'm pretty sure it wasn't a fractured crank, but i haven't opened the motor yet. the engine had 140k miles on it.

The material on the magnetic drain plug that you posted a few days ago look like rod bearing material to me.. This can be from a compromised crank that fractured at the radius of a rod journal and toasted that bearing. This is exactly what happened to Lon's engine, when he sent it to me he thought that it had lost a rod bearing, when it arrived I found the broken crank before I even disassembled it. Who knows, only a teardown will prove what the real cause is.

The sprung center disc only helps with harmonics and frequencies that are sent upstream through the input shaft from the drivetrain. The sprung center disc does nothing to aid in the harmonics generated by the engine that cannot be absorbed by the second mass of the flywheel after it has been removed and replaced with a single mass.

Jake, can the loss of the dual mass flywheel be compensated with an UDP that has a harmonic balancer built into it? like the ones offered by RSS or ATI Racing..?

http://www.atiracing.com/products/dampers/damper_tech.htm your comments will be greatly appreciated.

Gilles, across the street from El Toro base...

.

Nothing makes up for the huge second mass positioned centrally between the engine and transaxle.

Everything helps, but nothing makes up for it completely.

if you were near El Toro in the early-mid nineties I was doing my part keeping you awake at night in those big Helicopters!

Are you referring to chinooks?

CH 46 Sea Knights.. Thats what I crewed and twisted wrenches on at Tustin and El Toro in my USMC days.

Jake,
CH-46, good on ya!
I had the pleasure of flying CH-47 in Australian Army, but we did all our initial & maintenance test flight training in Ft Rucker, AL & Ft Lewis, WA.
I was VERY impressed with US Army guys & their teaching! I'm sure the Marines set just as high a standard.
I wish we had guys like you working on Porsches in Australia.
All the best,
Matt

Newb chiming in... :)

After reading all of this it seems like the safest bet is to stick with an OEM\Factory flywheel but I would still like the assembly a little more performance ready. Jake you seem like the Guru here, have you heard of any problem with the Spec clutches offered or any better brands? I haven't been able to find any other companies offering performance clutches for 2001 986.

I am having a LWFW (and new clutch) installed right now. Should be done by tomorrow.

After reading through this thread (and others), there is no way that removing the DMFW and installing a LWFW can cause crankshaft failure. No how, no way. All production engines are factory balanced to the degree necessary (along with the proper design considerations such as bearing placement and type) to ensure proper and sufficient engine balance.

No engine is in perfect balance, no matter how much time and effort you put into it. Some engines can be in better balance than others but meeting the engine balance criteria to ensure long-term engine reliability is easily met by standard design and production methods.

Harmonics are a red herring in this discusion. Harmonics have very little energy unless a forcing function is driving at one of the harmonic frequencies. Yes, this can happen to a really old bridge in high winds but can't happen in a modern engine because the only forcing function in an engine is the cylinders firing which create the primary or fudamental frequency.

Beyond that, there's nothing in an engine (analogous to the wind in the bridge example) that would drive a harmonic with sufficient energy to cause catastrophic failure of the engine.

The DMFW is meant to reduce minor engine vibration from being transmitted to the body and interior (as part of NVH control), not to keep the engine from self destructing.

What type of clutch are you going with?

You're assuming the aasco unit is balanced. I had mine done this week. The clutch / fw combo were 17g out of balance. 15g of that was the flywheel. That can break a crankshaft.

07proto - I use a sprung spec stage 1

The outer section (approximately 4 pounds (not grams)) of my DWFW was flopping around like a beached whale because the damping material was so worn. That had to create vibrations that were 100x what any LWFW might cause due to out-of-the-box unbalance and its probably been that way for several thousand miles.

Any flywheel that is several grams out of balance won't destroy an engine - or a failing DMFW (with 4 pounds wobbling around on the end of the crankshaft) would be the end of every engine.

I installed a Sachs High Performance Spring Hub Clutch, PN: 88-1861-000-017. Drove it home today, runs and shifts great.

By the way, I skipped replacing the IMS too. If it's gone 90,000 miles, I figure that there is no reason why it won't keep working. Call me an idiot if you'd like, but I'm spending the IMS replacement money on new tires and more track time. :D

How many g's do you think are exerted outward by centripetal force at the edge of a flywheel turning 7300 rpm? 100? 1000?

Try ELEVEN THOUSAND THREE HUNDRED. 11,300g at my redline. That means 17grams becomes 425 lb. Over time, that wobble can absolutely fatigue the crank. It's not forged, it's sintered.