Flywheel-pressure plate balancing question

[hcoles]

Hey Engine/Balancing Experts,

Car is an 89 3.2.

I now have a very noticable vibration after pulling the engine to replace the starting ring - I also put in a modified Wevo trans mount with the black (soft) bushings. So the vibration could be from that ("new" bushing vs. 20 year old), but I don't think so. I drove a 3.2 today with hard mounts and it seemed fairly smooth in comparison.

About 4 years ago I had the stock flywheel and pressure plate balanced. Some metal was drilled out.

Just the other day I had the engine out, replacing a starting ring. I'm pretty sure I put the pressure plate back on in the same location, I remember seeing I had it wrong without the bolts tightened and moved it before tightening the bolts. I wonder what are the chances something went out of balance just replacing the starting ring? The engine felt pretty smooth before the taking the motor out to replace the ring. The compression has very close readings. New rotor, cap, plugs and Magnacor wires installed in the last couple of days.

The idle is as smooth as ever... vibration seems most noticable at 3000 and seems to graduallyl rise peaking at 3000 and then staying about the same.

Vibration goes away when driving and clutch goes in and engine speed down. So I don't think it is wheel/tire or half shaft related.

I wonder if a main or other bearing went out because of low oil at start up.

[lucittm]

I am not sure what Porsche does at the factory but when I received my RSR flywheel-clutch the folks at Patrick Motorsports balanced the whole assembly and provided me with a witness mark to ensure the assembly was bolted together in alignment.

Likely each part is individually balanced at the factory but if you had a shop balance the whole assembly, it has to go back together exactly that way. If not there would be a minor vibration. Installing a "new" transmission mount would not necessarily increase the vibration, but it will make the vibration "feel" different. You are experiencing a dynamic vibration, that's why it occurs at a certain RPM.

I would install the old bushing and see if it returns back to "normal". If not, then it is your flywheel-clutch assembly and you will just have to live with it until you next drop the engine.

Mark

[hcoles]

Mark,
That's about what I was thinking. Re. the Wevo mount, the original G50 mount is modified to become the "new" G50 mount so installing the old mount is not an option.

I'm about 95-99% sure I bolted the pressure plate in the same position it was just before this last drop. The starting ring looks to be a concentric item so little chance to pick up a big dynamic vibration.

Thanks for the comments,
-Henry

[Steve@Rennsport]

Henry,

Flywheels, pressure plates and such are all zero-balanced at the factory (and by competent engine builders). This means they are balanced individually so that discreet components can be replaced as required without upsetting the dynamic balance of the engine and in any position. To do otherwise would require that everything be balanced and replaced at the same time and thats not only impractical but expensive,...

Vibration problems can occur when a pressure plate becomes damaged through improper operation, outright abuse, or long term wear. Further, clutch discs can cause similar things when they are damaged.

I would suggest that you inspect the pressure plate and disc for broken straps, off-center platen, and perhaps a broken or loose clutch hub. Sometimes it takes a sharp eye and some experience to spot a problem.

[hcoles]

Steve,
Thanks that's what others were telling me and it makes sense.
That being said, if the pressure plate or flywheel had metal taken off the result is most likely that it is now out of balance, but it wasn't bad before this last disassambly. I figure any fly by night balancing is not as good as the factory process. I looked over the parts (pressure plate, clutch disk) when apart and didn't notice anything, of course I wasn't looking that hard at the time. I replaced the throw-out bearing, reused the flywheel, pp and clutch disk. Could be that some part is now damaged as you suggest.

I'm also down on power, so I'll probably work on that for awhile before dropping the engine again.

As you may have read, I replaced the Magnacors, plugs, cap, rotor. Didn't change anything. The old Magnacors ohm'ed from 3-9k so they are good, going along with what you and I were discussing the other day.

Thanks very much for the info.
-Henry

[HawgRyder]

Also...check the pilot bearing in the crank.
If that is worn, the vibration may come and go depending on where the front end of the input shaft ends up (could be off-center) each time you use the clutch.
Bob

[Steve@Rennsport]

Originally Posted by hcoles

Steve,
Thanks that's what others were telling me and it makes sense.
That being said, if the pressure plate or flywheel had metal taken off the result is most likely that it is now out of balance, but it wasn't bad before this last disassambly.

The balance won't change unless they did it using a hand file,....

I figure any fly by night balancing is not as good as the factory process.

Fly-by-night, certainly, but most machine shops use a lathe or milling machine.

I looked over the parts (pressure plate, clutch disk) when apart and didn't notice anything, of course I wasn't looking that hard at the time. I replaced the throw-out bearing, reused the flywheel, pp and clutch disk. Could be that some part is now damaged as you suggest.

The PP must be closely inspected to make sure the outer straps aren't broken and the platen is perfectly centered. Same for the disc: look at the disc VERY carefully and make sure the hub and springs are not broken or damaged. If you aren't sure what to look for; get a shop or experienced friend.

I'm also down on power, so I'll probably work on that for awhile before dropping the engine again.

Thats a whole 'nuther deal and in fact, may be related. Remember, unless ALL cylinders are making the same effort, you'll get a vibration. Check fuel delivery at each injector and of course, confirm a nice fat blue spark at each plug.

As you may have read, I replaced the Magnacors, plugs, cap, rotor. Didn't change anything. The old Magnacors ohm'ed from 3-9k so they are good, going along with what you and I were discussing the other day.

Just confirm that each plug is getting a good solid spark. After that, its time to look at injectors and look for vacuum leaks.

[notmytarga]

+1 on Steve's recommendation to check your ignition. It sounds like a cylinder isn't getting spark. This would make things seem unbalanced when asking for power, but less with deceleration. It would also explain why you are "down on power".

Don't drop the engine just yet!! Or pull your cam towers.

[Walt Fricke]

The first time I took parts to a balancer, he said he was going to balance things as a unit. I was OK with doing this with the crank and its various gears and pulley. But talked him out of doing that for the flywheel and pressure plate and ring gear. For the reasons Steve pointed out. He knew a whale of a lot more than I did about balance, and of course I had no clue as to how balancing was done. I am glad I was insistent.

Pressure plate issues can be insidious. There are three thin steel straps which connect the inner plate to the outer housing. Each of these is actually made up of two straps. Guess what: it is possible that only one of these breaks. It is going to stay in position, and only by close inspection can you find this break. But you will sure feel it when it breaks, as the clutch will behave differently. If I recall rightly, it will make decoupling harder - more stroke needed. Or maybe impossible - there always will be at least a slight drag.

Sounds like your problem is off in some other direction, though.

Walt

[lucittm]

Walt,
You crack me up. First you say that he was a "balancer", I assume he makes part of his living doing balancing. Then you say you talked him out of doing it the way he wanted, and that he knew more about balancing that balancing and that you had no clue about it.

If you were an average consumer who would sometime in the future replace your pressure plate without balancing the flywheel, I would agree with you. But you know that you will likely balance all the pieces when your engine comes apart the next time, just to be sure.

Porsche did the individual piece balance, as Steve said "so that discreet components can be replaced as required without upsetting the dynamic balance of the engine". But Porsche also does this for economic reasons in mass quantities. Once the jig is set up, it is faster to balance all the individual pieces available in a lot due to the assembly line process. All the flywheels are likely balanced together and then installed on the engines. The pressure plates are likely all balanced together and then mated to the pre-assembled engines before the transmissions are mated farther down the assembly line/process.

You are only building one engine, it makes sense to balance rotating parts together, as you did with your crank. The reason for this is that the dynamic imbalances are magnified with the greater weight of the total rotating assembly. Since these parts are rigidly mounted together in the engine, they will keep their dynamic relationship. If you then replaced your pressure plate, you could take it to the shop along with the flywheel and they could adjust just the balance of the pressure plate and leave the flywheel as it is but still bolted together as it is in the engine.

Not as important with low RPM rotating assemblies but when you approach 10,000 RPM the most minor dynamic imbalance will be noticeable especially at the harmonic frequencies.

Mark

[hcoles]

Steve, et al. :
Thanks very much for all the excellent thoughts. Here is an update.

Yesterday I "borrowed" a stock 3.2 muffler from a buddy and swaped with the existing one.

My other 3.2 buddy helped with the swap as it is easier with two people. We then took a ride. There seemed to be a very noticeable improvement in power. We tried a few timed runs e.g. 3-6k in 3rd but couldn't get good data, too much traffic. Today we might try side by side runs on the freeway to see how I now compare to another 3.2 with the same cam I have.

Spark - as mentioned new cap/rotor/wires/plugs - during idle I pulled plug connectors, one at a time - there was a very consistent roughness for each one pulled. The idle with all connected is very good, never better. The current conclusion is that I don't have a spark issue. I haven't checked for a strong blue spark, I guess I pull a plug and ground it somehow and look.

Vibration - after the install of the alternate muffler - the vibration is still there but doesn't seem worse. It could be that the Wevo black rubber rebuilt trans mount is causing some frequency to come in that didn't come in before. The pp could have a strap broken, I didn't check that closely when I put things back together, like I now know I should have.
Clutch feel seems to be as before. I tried one experiment, placing my hand on the intake manifold and reving the engine to different rpm's, I didn't notice that the feeling of vibration was worse at different rpm levels.

Maybe someone experiencing the pp strap breakage issue can comment on the rpm and intensity or other memory of the episode. For example - the vibration was definitely felt at 2k rpm and steadily got worse as rpm's rose... or vibration was noticeable even at idle.

Injectors - I guess could be something there, to eliminate that issue I could unplug each injector from the harness and see if the roughness is the same for each cylinder unplugged. If yes, then I would conclude that all injectors are getting the PWM signal. To eliminate an injector issue as a root cause, I guess the next step is to send them to WitchHunter.

I could pull out the old timing light and see what that looks like against the flywheel.

Thanks for all the good info. and help.

[hcoles]

Hey guys, how about this for an idea.

Put the car up on jacks, unbolt the trans mount and support on a jack with towels or something like that. If the cause is an imbalance I'm thinking things will really shake, if the cause is some sort of combustion uneveness, things will not shake so much and will be heard out the exhaust. Right now the exhaust sort of sounds like the unevenness is coming out the exhaust, I may not have mentioned that before. This will also tell me if the vibration I feel is coming through the Wevo trans mount.

There is also an 80% or more chance this is all caused by over sensitivity on my part. I'm guessing many out there are very sensitive to the slightest change in noise or vibration to their Porsches.

[hcoles]

another update - we tested side by side on the freeway - the other car has a SW chip so takes off after 5k or so. The current conclusion is that the muffler change brought back the power. The vibration is still there but not worse. The idle is smoother than ever, almost smooth even with the 964 cams. I'll drive for a week and then maybe check a few more things next weekend.
Thanks for all the comments. Could still have a damaged pp.

[Walt Fricke]

Mark

I have, on a number of occasions, replaced the pressure plate without removing the flywheel. Same with the ring gear. Or replaced the flywheel without replacing the PP.

I recently found my precious RSR ring gear had shed a tooth. Why, I don't know, but it sure left pecker tracks around the inside of the bellhousing. So I replaced it with a previously balanced regular ring gear. So it is important to me to have what one might call external components separately balanced. I don't run my race motor much above 8,000. Can't afford to maintain a 9,000 motor. Does anyone run a 911 over 9 grand?

The stuff internal to the crank doesn't get replaced often. If the case is apart for bearings or something, and the gears are staying, I don't have the balance rechecked. Is it apt to go off just through use? In fact, I wonder if it really matters to have these gears on the crank when balancing - they are all pretty uniformly machined, and the key is awfully close to the axis of rotation. Unlike a crank, which is a machined forging (is it not?), and at some point in its life was balanced to factory production specs, while a racer might want a closer balance.

My point, if there was one, was that I had enough faith in my untutored logic to resist the suggestion of a highly experienced professional. And I was right, or right enough. He favored package balancing because it was closer to his view of excellence in balance. But if I was really all wet I know he would have said take your business elsewhere. He was the kind of shop you had confidence in - one man operation, shop full of various tools, not fishing for business. Alas, he passed away a few years ago, and I don't know where locally I can go. Haven't had to for a while.

Where harmonics come in I don't know. If all the parts are individually in balance, could the assembly nonetheless end up out of balance? I know harmonics can cause issues (like the RSR 70.4 cranks shaking the flywheel off because a 4th order harmonic had a node right at the junction of the two, per the authorities), harmonic balancers can be used to dampen these effects, etc. But I've not seen it suggested that balancing the flywheel/ring gear/PP as a unit would help with problems elsewhere. Can components all be in rotational balance, but the assembly of them not be? Torsional effects at high RPMs?

Walt (with apologies to hcoles and his vibrations)

[hcoles]

I wonder of a vibration analysis would show something. For example if there was one cylinder causing the vibration because of a combustion difference the frequency would be 1/2 the rpm.
If the vibration frequency is equal to the rpm, that should be caused by an imbalance, right? Who's has equipment to check this? There must be a hand held gadget, I don't need the rocket science version.

[lucittm]

Walt,
Excellent post and you have advanced the knowledge of the board, I'm sure. My point was, as you said, more out of perfection of the art of balancing. I would humbly submit that most of the parts that rotate in our engines are pretty close to balanced right out of the box. Aftermarket, less so due to the higher cost of precision balancing vs. dynamic spin balancing.

I'm sorry to here your friend is no longer with us. He sounds like a good guy.

I agree with you that parts like a ring gear would not affect the balance as much as a flywheel or PP due to less mass even though the rotational moment is large.

Harmonics is a whole different cat. The only way to know that one is to use a precision balancer (a big one for a crank) and spin it from 0 to the max RPM using a strobe light to see the harmonic then calculate the natural resonances of that frequency to determine if there are stability issues in the materials. I have often wondered why big V-8s have harmonic balancers but jet engines and race cars don't? Oh yeah, they have the unlimited balancing budget.

Mark

[lucittm]

hcoles,
The precision balancers detect minute displacement forces caused by the weight shift as the object rotates; a running engine would have too much torque and vibration from the valves closing and cylinder firing to see the imbalance in a shaft unless it was huge!

To give you an idea, I used to calculate shaft imbalance in the Navy. At 600RPM the shaft is turning at a frequency of 10Hz (revolutions per second). Multiply this by the number of teeth on the bull gear (the big reduction gear) that sets up the vibration maybe 360 teeth? So you get 10x360=3600Hz. If the bronze propeller blades are resonant at 3600Hz, it may "ring" at the 5th or 10th natural harmonic, which would be 18,000Hz or 36,000Hz. At this frequency, you can't feel it but you sure can hear it and it sounds like a warbling bird.

I imagine our engines would have minor oscillations that cause displacement based on the crank throws. These get added to the gear mesh ratio of the intermediate shaft and sprocket mesh of the camshafts, and further muddied by the cam lobes and rockers striking the valves plus the number of vanes on the alternator. Sheesh, it's just too much.

Mark

[Walt Fricke]

Mark

I am afraid my days of hearing 36,000Hz are long gone. And I was always a classical music guy.

One of the great things about this Forum is you never know when you will run into a guy who, by the way, balanced Navy drive shafts! Power company engineers have explained that they torque huge' diameter bolts by heating them and then snugging the nut, allowing the cooling to get the needed stretch.

Porsche saved weight on the RSR ring gear in two ways: They cut out the flange between the mounting bolts. And they beveled the backside of the gear teeth! The first looks pretty useful. The later, despite being about as far from the center as you can get, strikes everyone who sees it as inherently of small value. But no one has calculated it. If I get really bored sometime, I just thought of how I can do that by figuring the volume of tooth shaved times the number of teeth times some average weight per volume of steel. Then figure what that does to MOI.

I've never seen exact MOI figures for parts like these, because they can only be approximated as annuli. When I quizzed some engineers about how one could figure MOI experimentally, I don't recall anyone having a standard method. We discussed putting the part on a spindle, attaching a weight on a string wound around it, and measuring the time it took for the weight to fall some distance. Nothing came of it.

Walt

[Walt Fricke]

I wonder if shops (Patrick Motorsports, for example) might not favor balancing a bunch of components all together. It works, for one, if not as convenient for some users. But might it not also be quicker/easier for the shop?

Bolt them all together, attach to your crankshaft end jig, and spin 'em up. Do the magic, drill some holes or grind a bit here and there. Aha, done. Punch in witness marks, put on some red paint so they are easy to see, and you are done.

If one went crazy about things, maybe you would make sure to grind all your fasteners so they were within a tenth of a gram of each other. We do that kind of thing (sort of) with reciprocating weights (well, maybe to a couple of grams).

[Walt Fricke]

Situation: owner makes two changes: replaces starter ring gear, and changes engine mounts. Car develops obnoxious vibration rising to max at 3,000 rpm and holding steady. Believes engine related because goes away when coasting at speed.


Ring gear:

Query - why did you replace the ring gear? What went wrong? Where did the replacement come from? Won't they seem to work fine even showing wear from engaging the starter with the motor running over many years?

I agree that these look so symetrical that one would not expect significant imbalance here. For that matter, I'd not expect to see any imbalance to this part show up as low as 3,000 rpm. It is not a heavy part. Using an old fish scale I don't much trust, I make it to weigh about a pound and a half. My sainted RSR ring gear looks to weigh a pound (oh, but that's an important half pound for a high reving motor). Your stock pressure plate (I assume same as for a 915?) weighs 12 pounds. And flywheel what - at least that.

I didn't notice the missing tooth from my ring gear, and that motor spins to 8,000. Found it when motor out for other reasons. Hard to make ring gear imbalance the culprit.

I'd expect a reputable shop, when working on a street motor, would be fine with swapping in a used ring gear in apparent good condition without balancing it.

Drilling holes, by the way, seems a standard way of balancing flywheels and pressure plates. What hole to drill where is the mystery to me, but that's what balancers do. The ring gear, if touched, would probably have a little ground off here or there where it doesn't affect function.

I had Clutchmasters put a new diaphragm spring in one of my RSR PPs. The spring looks awfully symetrical. When done they rebalanced, as there were some new holes.

CVs: vibration only with clutch engaged and engine pulling. Still there if you rev to 5 or 6K and let it slow under compression? Nonetheless, have you retorqued the CV bolts which were unfastened as part of the R&R? At least that is never a bad thing on general CV principles.

The confounding factor here is that various other things (many suggested) can coincidentally go wrong on their own.

Down on power after the modest work done on the motor at this point? I suppose you could have reattached something incorrectly, but I think that's kind of hard to do and have things work as well as yours does. Plus you've done this before, no? And swapping mufflers helped the power? So coincidentally one of the baffles inside the old muffler fell apart and created an obstruction?

Oil pressure: Why do you think you may have wiped a bearing due to low oil on start up? At least with decent oils, when you just pull the engine, don't do anything involving an oiled part, and then put it back, there ought not to be any issues. Unlike a rebuild, where many of us spin it some to build some oil pressure and distribute some oil before we try to start it (not trusting assembly lube entirely), yours had oil where it needed to be. And that pump builds pressure fast. The line from the tank to the pump is full of oil shortly after you refill the tank through gravity.

The mantra I have heard for bad bearings is 1) loss of power, 2) increase in oil temp, 3) decrease in oil pressure, 4) boom. Spacing between events varies.

Engine mounts: owner believes not culprits despite being somewhat (?) stiffer WEVO units, based on riding in other similar cars with these (or much stiffer?) mounts.

Query: since these use some of the old mount, is it possible to mess this up when mixing old and new parts?

Also, is it possible that in the process something moved closer to something else (touching, in fact), so that a vibration always there but not noticeable is now conducted into the chassis or some other sounding board?

I thought the WEVOS were a complete bolt in package like solid mounts, but I see I must be wrong there as I was thinking of the ones for 915s. Were this a '86 or earlier 911, I'd say get a friend to give you some old mounts from his stash of replaced but not discarded parts to check out the mount theory. Any shops or Porsche junkyards nearby which might loan you a G50 mount?

Because I suspect Mark has put his finger on it - despite changing only 2 of 4 vibration dampening mounts, this caused a preexisting condition to reveal itself. Who knows, maybe this is your excuse to go to the hard Wevos?

I seem to recall that at some point Porsche went to some lengths to deal with vibrations conducted into the chassis from the shift rod? Dual mass flywheels? Caused other issues?

Alas, so far no one has chimed in and said he had the same issue, and solved it with strategically placed duct tape or something.

Walt