Kevin in Atlanta

Specifically, what are the differences between the 90- and let's say the 87. Besides the color change? The 90 I am putting back together came with an orange one and wonder if I need to find a black/gray one.

Rob Edwards

Black is GT only, AFAIK. Go with the orange one.

Doug_B_928

Early cars also have black HBs (at least my 79 does).

Kevin in Atlanta

Interesting. The 90 also came with the smaller diameter a/c pulley.

GregBBRD

I think it was so they could identify the engines on the upper storage racks, without climbing around and looking at the paperwork.....

Seriously, the GT dampers are stamped differently. Perhaps the balance was held to a tighter tolerance, because of the higher rpms?

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.

I consider all of them to be equal junk, at this point in time.

GregBBRD

I believe you will find out that the automatics came with the bigger diameter A/C pulley, because they "search out" high gear before they get across the intersection and thus need to operate the A/C at lower rpms.

Daniel5691

Is there an updated HB available, one that still does some dampening?

Thanks,
Dan

Rob Edwards

Best bet is an ATI with a custom hub- Greg has 'em for his strokers.

JWise

I need to check with Greg and see if he could spec one for my mildly modded 90GT (Motorsport X, Bosch 24lb injectors, Sharktuned by Colin - future plans include Greg's headers/exhaust). Would be happy to purchase from him and have Sean do the install.

GregBBRD

I do keep custom hubs and ATI dampers in inventory.

FredR

If by now non of them work as they were originally intended to what should we be looking for in terms of symptoms indicating such?

Given they dampen harmonic vibrations presumably there will be a specific rpm point where such vibrations will manifest themselves noticeably so given all engines of a specific type are the same. Whether S4's, GT's and GTS's have a different resonant frequency would be interesting to know.

Rgds

Fred

928S MN

Very interested in the outcome of this post as I'm close to pulling the HB on my 1985 928S for the TB/WP job.

Anyone have an approximate price range for a "new" HB?

If the harmonics are not absorbed, how does that show up in terms of engine damage? Lost performance?

Sapientoni

Steel Rubber Products does restoration of motor mounts etc and would probably restore it

GregBBRD

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......

dr bob

Greg is spot-on (as usual...). The damper limits the 'ringing' that happens due to the different twisting forces on the crank as different cylinders compress and expand. Failure mode from that 'ringing' includes whorling of the oil film on main bearings and susequent wear from the failed oil-film dam. Longer-term, you'll discover the fatigue life of the crankshaft itself.