Aasco flywheel is here. - 986 Forum - The Community for Porsche Boxster & Cayman Owners

greggp · 09-27-2010, 10:49 PM

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

stephen wilson · 09-28-2010, 04:22 AM

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.

insite · 09-28-2010, 08:22 AM

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......

insite · 09-28-2010, 10:10 AM

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

insite · 09-28-2010, 10:49 AM

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?

stephen wilson · 09-28-2010, 03:55 PM

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.

Cloudsurfer · 09-28-2010, 06:08 PM

Quote:

Originally Posted by insite

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?

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.

greggp · 09-28-2010, 08:47 PM

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?

mattatk · 05-30-2011, 01:42 AM

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

sb01box · 10-05-2010, 08:56 PM

Quote:

Originally Posted by greggp

Answers to Sb01box

"The sprung clutch .....................t (or elastomer stiffeners) or unbalance will be added to the crank assembly.

-Gregg

Thanks for the detailed explanation to my question!

rightnwrong · 11-30-2010, 09:54 PM

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

ltusler · 02-27-2011, 10:34 AM

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

Jake Raby · 02-27-2011, 10:55 AM

Quote:

Originally Posted by ltusler

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.

ltusler · 02-27-2011, 11:21 AM

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.

cheetah · 05-23-2011, 11:07 AM

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?

Jake Raby · 06-11-2011, 07:56 AM

Insite,
we see that with EVERY LWFW that we balance, yours was no different.

Sorry about not having time to talk to you about it.. we were a little tied up with a car that was stolen for over a week and just showed up.. I had to Commandeer a Mini Cooper driven by a Priest to chase the Boxster down!

Long story :-)

AndyA6 · 06-11-2011, 06:32 PM

Quote:

Originally Posted by Jake Raby

Insite,
we see that with EVERY LWFW that we balance, yours was no different.

Sorry about not having time to talk to you about it.. we were a little tied up with a car that was stolen for over a week and just showed up.. I had to Commandeer a Mini Cooper driven by a Priest to chase the Boxster down!

Long story :-)

Please DO TELL!

Sounds worthwhile