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wow.* that's horrible.* all because of the balancer?
Well Paul, I do not know but the brain trust say so. The story about the engine which I found in Germany is a smart mechanic who dropped a M8 nut into one of the cylinders and when the engine was started the cylinder tower cracked. I could not find anything else being wrong with the engine except a mark at the top of piston and in the combustion chamber. The bearings looked like on the photo.
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
Those bearings look pretty good. The thrust bearings are barely worn though the first layer. I would have guessed that the thrust bearings had less than 20,000 miles on them.
The front main bearing is very typical and does "show" the mileage/time. This wear is very typical. The tension from the alternator belt and the A/C belt pull the crankshaft through the oil film and allow it to touch the main bearing for a few seconds...until there is oil pressure.
Much of the 928 front main bearing wear occurs from changing the oil...and not filling the new oil filter with oil.
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
Those bearings look pretty good. The thrust bearings are barely worn though the first layer. I would have guessed that the thrust bearings had less than 20,000 miles on them.
The front main bearing is very typical and does "show" the mileage/time. This wear is very typical. The tension from the alternator belt and the A/C belt pull the crankshaft through the oil film and allow it to touch the main bearing for a few seconds...until there is oil pressure.
Much of the 928 front main bearing wear occurs from changing the oil...and not filling the new oil filter with oil.
Mr. Brown, I was thinking of the scattered marks on the outside of the bushing, marks I have never seen in any other engine.
Åke
The tension from the alternator belt and the A/C belt pull the crankshaft through the oil film and allow it to touch the main bearing for a few seconds...until there is oil pressure.
Much of the 928 front main bearing wear occurs from changing the oil...and not filling the new oil filter with oil.
This is exacerbated by removing the AC and not using the AC replacement pulley? Too much pull in that one direction?
Mr. Brown, I was thinking of the scattered marks on the outside of the bushing, marks I have never seen in any other engine.
Åke
That outside marking is completely normal and common. Started seeing this in engines starting in the late 1980's. Even saw this/see this on brand new bearings. Something that occurs in the plating process.
This is exacerbated by removing the AC and not using the AC replacement pulley? Too much pull in that one direction?
Definitely.
The A/C belt "counteracts" the pull from the alternator belt, which pulls on the nose of the crankshaft very hard.
The custom high output alternator I make has a "free-wheeling" pulley, which requires dramatically less belt tension. (Most belt wear and stretching occurs on shut-down, when the belt stops almost instantly, requiring the rotating mass of the alternator to also stop almost instantly. A "free wheeling" pulley allows the alternator to spin down, once the belt stops turning, reducing the belt stretch and reducing the tension required.)
My first experience about harmonic dampers was on my 1986 SAAB in the signature. Currently, we are up to about 250.000 miles. In about 2006, it was literally ringing when decelerating coming off the petal above 3500 rpm. After some time, I changed the damper and bingo... Everything quieted up. I did not think about the potential bearing damage I have to admit. Anyway, everything is on the car is still original: alternator, head gaskets, starter, turbo, etc... I will have to pull the motor sometime.
Coming back to the OPs question. This answer pretty much sums it up:
Originally Posted by GregBBRD
It's probably all a moot point, now that the rubber between the two layers is rock hard. The vast majority of the dampers quit "absorbing" harmonics years ago.
However, there has been a lot of talk about damping lately and all the efforts Porsche supposedly did to make the 928 smooth: harmonic balancer, motor mounts and torque tubes. The big problem in this discussion is the applicability to the current status of the production of such parts. These rubber materials are organic and have very large material property tolerances. My understanding is that even some "high end" rubber has huge variations in outgassing rates and coefficients of damping for the same material of up to 20%. I am not sure if this was all carefully considered in the purchasing spec that Porsche gave to its suppliers back then. Further, the suppliers likely have no record of the material data for that material at that time now, so that a reproduction of a Porsche harmonic balancer is probably hit and miss.
Based on this line of thinking, I have no trouble buying:
1.) Super bearings from Constantine
2.) ATI dampers from Greg (I have purchased 2 to date)
3.) Volvo motor mounts (although I admit that I bought Porsche for my last project)
Experience will be the best teacher on this topic. I am hoping that now in modern times that the suppliers to both Constantine and Greg have sufficiently specified the traceability of the damper material in use, so that the gained experience in the R&D and usage of the products up to now remains applicable. In other words, I trust more a new harmonic balancer from Greg that has testing and roadmiles behind than a new one from Porsche, even if the Porsche damper were to be a new production run at this stage.
My first experience about harmonic dampers was on my 1986 SAAB in the signature. Currently, we are up to about 250.000 miles. In about 2006, it was literally ringing when decelerating coming off the petal above 3500 rpm. After some time, I changed the damper and bingo... Everything quieted up. I did not think about the potential bearing damage I have to admit. Anyway, everything is on the car is still original: alternator, head gaskets, starter, turbo, etc... I will have to pull the motor sometime.
Coming back to the OPs question. This answer pretty much sums it up:
However, there has been a lot of talk about damping lately and all the efforts Porsche supposedly did to make the 928 smooth: harmonic balancer, motor mounts and torque tubes. The big problem in this discussion is the applicability to the current status of the production of such parts. These rubber materials are organic and have very large material property tolerances. My understanding is that even some "high end" rubber has huge variations in outgassing rates and coefficients of damping for the same material of up to 20%. I am not sure if this was all carefully considered in the purchasing spec that Porsche gave to its suppliers back then. Further, the suppliers likely have no record of the material data for that material at that time now, so that a reproduction of a Porsche harmonic balancer is probably hit and miss.
Based on this line of thinking, I have no trouble buying:
1.) Super bearings from Constantine
2.) ATI dampers from Greg (I have purchased 2 to date)
3.) Volvo motor mounts (although I admit that I bought Porsche for my last project)
Experience will be the best teacher on this topic. I am hoping that now in modern times that the suppliers to both Constantine and Greg have sufficiently specified the traceability of the damper material in use, so that the gained experience in the R&D and usage of the products up to now remains applicable. In other words, I trust more a new harmonic balancer from Greg that has testing and roadmiles behind than a new one from Porsche, even if the Porsche damper were to be a new production run at this stage.
The major difference between a Porsche damper with perfect rubber and on of my custom ATI dampers is more than just a better design damper.
Porsche made the damper a "slip fit" on the crankshaft. In order to do the best job to absorb the harmonics, the damper needs to be tight on the crankshaft. My damper hubs are a .0006" to .0008" interference fit.....they must be heated to install.
That outside marking is completely normal and common. Started seeing this in engines starting in the late 1980's. Even saw this/see this on brand new bearings. Something that occurs in the plating process.
Mr. Brown, I checked another front bush bearing which has seen a severe thrust bearing failure, it did not show any scattered marks on the outside. I also checked the two new main bearing bushings I have in stock and they have no marks. The markings cannot come from the plating process. A long time ago someone on this forum mentioned the scattering marks may come from a defective Harmonic Balancer. I do not know, however it make some sense.
Åke
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03-17-2019, 02:17 PM
