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I've seen a huge increase in main bearing wear and movement/fretting of the cradle on the block, over the years, which I attribute to a decrease in harmonic absorbtion. I used to take a look at rod bearing wear and be assured that the main bearings would still be nearly perfect. Not any longer. I'm seeing some engines with very nice looking rod bearings and badly damaged main bearings....which I can only attribute to wear from increased harmonics.
I've also cured several chronic engine vibrations which nothing else anyone tried could cure, by changing the damper.
I believe that damage from unabsorbed harmonics is probably a progressive long term problem, not something that will kill an engine or cause drivetrain damage in a few days (or months) of normal driving. However, I also believe that if one was to load and hold an engine at the peak of its harmonics (like they certainly did when testing) destructive damage certainly would occur at a quicker rate.
I also question the sanity of the current trend to remove the harmonic absorbtion device in torque tubes. That is, for the majority of vehicles, a different topic. Howwever, for the vehicles that were not equipped with this device (like the GT models), I believe that a front damper in very good condition is paramount.
And who really knows?
Perhaps the different colored GT front dampers (which this thread was about) were different because of the removal of the torque tube harmonic damper......
Greg,
Many thanks for responding to my post. Having read earlier posts from you on the state of these things it kind of worried me knowing that over here rubbery things in general tend to suffer more because of our "excessive" heat. Generally speaking I try to nurse the motor during the very hot season weather by either taking it more easy or running in the evening time. That being said my expectation is that the damper has a finite life and over here that number will be less that what you experience unless you have customers from the hotter climes as in Arizona. My problem is knowing when to call it a day. I do have spare damper sat in my spare parts bin for the last 10 years that I think came from the original TBF'd GTS motor. Whether this can be used on my S4 motor I do not know at the moment. All I can say was that when I removed the damper during my TB/WP job that I did I noted that the damper came off the crank relatively easy and that left me wondering what exactly it did. I know the drive shaft acts as a transmission damper given it winds up during loading and I know that crankshafts twist as firing pulses are hammered into them. What you are saying is entirely logical and more to the point, extremely disconcerting if something is not done pre-emptively so trust others are thinking about this.
Regarding the drive shaft damper I seem to remember reading in the Porsche 928 book that it was fitted as an after thought to dampen noise in the automatics that had only two bearings fitted. As I understand Constantine recommends three of his shaft bearings to mitigate elimination of the damper.
Much appreciate advice as to whether the GTS damper can be fitted to my S4 motor assuming it has some viable shelf life. Hopefully this is relevant background/contributory to the thread intent- if not could start a separate dialogue on the subject matter if the OP prefers.
I've pretty much narrowed my driveline vibration to this also.
It would take some really serious crank flexing/ringing for this to become something you'd feel as a driveline vibration. The dampener on the driveshaft in the rear of the torque tube (hopefully) kills any ringing (resonant torsional flexing) transmitted from the crankshaft. The driveshaft itself twists and un-twists with cylinder pulses, hence the use of that torsion dampener there. Beyond that, the torque converter acts as a giant torsion "shock absorber". In between, the torque-tube bearings see any ringing; if you cut a failed torque-tube bearing apart, you'll likely see spalling in the races if the ringing is severe/unchecked. Constantine's Super Bearings include a much more 'forgiving' rubber support sleeve that cuts down on this bearing damage, but can't eliminate it.
In cars that have seen extended operation under load in four-cylinder mode, the extra vibrations and loading/unloading will eventually fatique and fracture the driveshaft, typically at the turndown area just to the rear of where the front clamp connects.
Next time you have a crankshaft loose but sitting in bearing saddles in the block, tap sideways (in the direction of normal rotation) on one of the throws near the front while barely touching the nose with the back of your fingers. That slight torsional ringing is what the "harmonic balancer" damper is absorbing/killing.
Hi, recycling a thread here to avoid hijacking a discussion about main bearing wear..
I am wondering how to obtain the rebuilt Greg Brown harmonic balancer, please?
Do we order/request straight from Doc Brown, or does 928sRus have these in stock?
Thanks ! ! ! ! ! ! !
I'm right in the middle of having another run of hubs made. I'm going by my machinist on Monday to encourage him to move forward.
He's a bit independent....
I've been selling these direct, to keep the price point as low as possible, so 928 owners can rationalize buying a new damper. There's probably not enough profit margin for Roger to be interested.
The damper from our GT, with some indication that the rubber is no longer very elastic..
The #1 main bearing, worn in an odd way...
And some fretting between block and girdle:
This was a 240K engine so wear was expected, but I did not expect the wear patterns on the outside of the #1 main nor the fretting between girdle and block. To my very untrained eye it sure looked like harmonic resonance of some sort.
bump; referring to OP about balancer differences: quoting from the 1991 service information book, which lists model year differences:
"Torsional vibration damper in 928 GT
The M 28/47 engine is being fitted with a torsional vibration damper more precisely matched to the engine characteristics, on the front end of the crankshaft. For better identification the front is being marked with a "GT" "
PET lists 1990> as 928 102 120 23 fitted to both manual (GT) and automatic (S4) cars . 1993> damper is 928 102 120 24 with no distinction for manual v. automatic cars as well.
my AC pulley is covering any area where there should be a GT stamp. anyone know if this was a one year part, what made it "better," and what the part number might be?
FWIW, yours is 30yrs old...and works nowhere _near_ as designed. And a "new one" is also 30yrs old.
Best course, call Greg Brown, get one of his.
I lost a LOT of minor vibration after that.
certain you are correct, plus new ones are long NLA. is GB a direct bolt on replacement? cost? my question was more about what I found in the 1991 book, and what the difference in that part is "suppose" to be? can't find any evidence that it existed; green M&M?
sounds like it made a significant difference for you?
These main bearings comes from a 1987 S4 which I acquired and dismantled in the year of 2000. If the odd marks on the outside of the bush bearing comes from a bad HB, which is indicated in this thread (post #23), the HBs started to go bad much earlier than expected. Think the engine had 140.000 km or 88.000 miles on it.
Åke
These main bearings comes from a 1987 S4 which I acquired and dismantled in the year of 2000. If the odd marks on the outside of the bush bearing comes from a bad HB, which is indicated in this thread (post #23), the HBs started to go bad much earlier than expected. Think the engine had 140.000 km or 88.000 miles on it.
Åke
wow.* that's horrible.* all because of the balancer?
03-21-2016, 05:10 PM
