8 cylinder front engine iconic vehicle
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By Geza-aka-Zombo
#26439
At the risk of giving my engineering secrets away, let me speak to ATI damper hub materials. Steel is a good choice as it has good vibration transfer properties (low internal damping), is inexpensive, and is conducive to tapping (for mounting hardware) with little concern for striping threads. Of course corrosion is an issue, so proper finishing is required. Because steel is 3X stiffer than aluminum, less an interference fit between the hub and crank is required, as steel clamps more than aluminum for the same interference. This actually turns out to be a minus as I’ll describe later. Another drawback of steel is that it is a poor conductor of heat (low Kt) and has a low CTE (coefficient of thermal expansion) – these 2 properties make it necessary to excessively heat a steel hub for a long period of time to insure it expands sufficiently to slide on the crank. Cooling the crank (with ice, for example) does not help – putting the crank in the freezer for several hours, or liquid nitrogen is another story, obviously not practical.

Aluminum has a high CTE and Kt, so it will heat more quickly and expand more with a lower temperature increase. Both good properties, especially since the aluminum hub needs to expand more given it requires a larger interference fit to compete with steel’s clamping force. Being corrosion resistant, the 6061-T6 aluminum doesn’t even need a finish; though the anodize I specify brings it up a notch. Tapping directly into the aluminum (without an insert) was a concern I ultimately set aside, as the threads are quite large (especially compared to some of the things screwed into the bare aluminum 928 block casting) and the bolt torque is fairly low. ATI makes hubs using steel and aluminum, so both work fine.

As alluded to above, the specifics of the interference fit is a major consideration and the reason why I selected aluminum. Because the steel requires a smaller interference fit (i.e. 0.0005 to 0.0010”), it is much more difficult to pull off than aluminum, which has a larger interference requirement (i.e. 0.0015 to 0.0025”). Without getting into too much detail, the crank nose diameter has a manufacturing tolerance (assume +/- 0.0002”) – the hub bore also has a similar tolerance. The lower the amount of interference between the 2, the more tightly tolerance the hub bore needs to be to meet the interference fit requirements across all possible combinations of crank nose and hub bore sizes. In a nutshell, the aluminum hub has more leeway because it has more of an interference. Since those designing these hubs don’t have access to Porsche crankshaft machining drawing (at least I don’t), there is too much of an unknown as to the actual crank nose diameter and tolerance – I went the more conservative way – guaranteeing there would always be a sufficient interference, regardless of what the 2 parts measured. ATI, having access to more definitive data on the cranks they design their hubs around, is in a much better position to engineer the lower interference steel design.
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By milrad
#28208
I'm very interested in this offering, but to be honest I'm a bit confused. Reading endless threads on the topic hasn't really helped.

My motor is almost fully apart for countless WYAIT items. I'd read about the dampers and started poking around. At this point, anything "better" is going back on the car. But in a long back and forth with Greg B on a number of potential upgrades, some of which I bought, when it came to the topic of dampers he told me to reuse my stock one rather than buy one of his. He had me send him pictures. Mine is pristine, as far as 30 year old cars go, with no noticeable drying, cracks, or other deterioration in the rubber. He told me he only changes them when the rubber is damaged. He certainly has no shortage of opinions, but I do respect they are largely born of experience.

So I'm back and forth between "it's old, change it," "these new ones are better, do the upgrade while you have it apart," and "yours is perfectly fine, spend the money on some other clear upgrade." Has anyone come across any empirical data that a non-deteriorated but old stock damper is not doing it's job anymore? I understand the "the rubber is hard" logic, but having not ever handled a new one I can't say it hasn't always been that way either.

Reassembly is starting...so I need to make some decisions. Opinions or facts to add?
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By Geza-aka-Zombo
#28293
milrad wrote: Sun Oct 25, 2020 9:46 pm I'm very interested in this offering, but to be honest I'm a bit confused. Reading endless threads on the topic hasn't really helped.

My motor is almost fully apart for countless WYAIT items. I'd read about the dampers and started poking around. At this point, anything "better" is going back on the car. But in a long back and forth with Greg B on a number of potential upgrades, some of which I bought, when it came to the topic of dampers he told me to reuse my stock one rather than buy one of his. He had me send him pictures. Mine is pristine, as far as 30 year old cars go, with no noticeable drying, cracks, or other deterioration in the rubber. He told me he only changes them when the rubber is damaged. He certainly has no shortage of opinions, but I do respect they are largely born of experience.

So I'm back and forth between "it's old, change it," "these new ones are better, do the upgrade while you have it apart," and "yours is perfectly fine, spend the money on some other clear upgrade." Has anyone come across any empirical data that a non-deteriorated but old stock damper is not doing it's job anymore? I understand the "the rubber is hard" logic, but having not ever handled a new one I can't say it hasn't always been that way either.

Reassembly is starting...so I need to make some decisions. Opinions or facts to add?
You raise very interesting points. Like you, I have never handled a new/fresh OEM 928 TVD, so can't compare one to what I pulled off my car, which appears in good shape. However, on mine the rubber is so hard, I couldn't prick or even dent it with an xacto knife. It's more like hard plastic than rubber. Perhaps check yours in this manner.

Understanding that the rubber component on the TVD is the critical spring and damper element in the system and does all the work, compliance in the rubber is very much necessary for it to function properly. Changes to the rubber properties over time, due to age, temperature and fatigue, most certainly impact its performance. However, like you implied, without side by side empirical data from testing, no one can know for sure the extent of the performance degradation. I would point out that, facing the same issues, ATI suggests their street driven dampers be rebuilt every 10 years to replace the rubber (they offer rebuild kits - track driven engines require more frequent rebuild - down to yearly). Just a data point.

Considering how age and temperature degrade other things on the vehicle, such as fuel lines, hoses, bushings, etc. it is not unreasonable to conclude that the TVD rubber is also impacted by these factors. After much thought, my engineering assessment is, unless there is ample evidence to show otherwise (i.e. notable compliance of the rubber), OEM TVDs are most likely shot at this point, regardless of how they look. I felt strongly enough about it that I designed my own hub for my engine. Best of luck with your engine build.
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By worf
#28361
Of what rubber material is the ATI damper composed such that it just feels like a hunk of metal in your hand?
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By Geza-aka-Zombo
#28427
worf wrote: Mon Oct 26, 2020 1:03 pm Of what rubber material is the ATI damper composed such that it just feels like a hunk of metal in your hand?
Couldn't say what elastomer they use, but know they vary durometer and the inertia weight to tune for different applications. The ATI917020 damper is what they call a 2 ring design, which suspends the inertia weight on 6 O-rings, securing it both radially and axially. The weight itself is somewhat buried, so one can't get their hands on it to feel the level of compliance in the design.
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By Geza-aka-Zombo
#28440
Geza-aka-Zombo wrote: Mon Oct 26, 2020 3:41 pm
worf wrote: Mon Oct 26, 2020 1:03 pm Of what rubber material is the ATI damper composed such that it just feels like a hunk of metal in your hand?
Couldn't say what elastomer they use, but know they vary durometer and the inertia weight to tune for different applications. The ATI917020 damper is what they call a 2 ring design, which suspends the inertia weight on 6 O-rings, securing it both radially and axially. The weight itself is somewhat buried, so one can't get their hands on it to feel the level of compliance in the design.
Can't figure out how to edit to add this picture I made.
You do not have the required permissions to view the files attached to this post.
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By worf
#28455
Geza-aka-Zombo wrote: Mon Oct 26, 2020 4:15 pm Can't figure out how to edit to add this picture I made.
Looks like you did?

In any case, that picture makes sense. The 'rubber' part is well-buried inside two surrounding pieces.

I don't have one on the shelf here to look at.
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By milrad
#28545
Geza, sent you a PM...
By Shifted
#28656
   
Last edited by Shifted on Fri Feb 12, 2021 4:14 pm, edited 1 time in total.
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By Sazerac
#31479
Geeza

So, one question I could never get answer on is how do we know this ATI damper matches the 928 engine needs? And why did Porsche have different dampers for the 928 motors? Intuitively, I know the answer: different harmonic frequencies. But, I am looking for more insights into this topic.

I just replaced my dampers on both my cars with GB's, but I would consider upgrading in 10 years or so and might buy your hardware in advance of the change (now) without the damper to be ready. It looks good what i have seen. But how do you know the Super Damper really fits the 928?
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By maddog2020
#31590
Sazerac wrote: Thu Nov 05, 2020 4:17 pm Geeza

So, one question I could never get answer on is how do we know this ATI damper matches the 928 engine needs? And why did Porsche have different dampers for the 928 motors? Intuitively, I know the answer: different harmonic frequencies. But, I am looking for more insights into this topic.

I just replaced my dampers on both my cars with GB's, but I would consider upgrading in 10 years or so and might buy your hardware in advance of the change (now) without the damper to be ready. It looks good what i have seen. But how do you know the Super Damper really fits the 928?
Nobody really knows. ATI makes balancers that dampen vibrations. It really doesn't matter because factory Porsche Dampers have been NLA for years, and are old tech & ATI are the only game available.
By Shifted
#31668
   
Last edited by Shifted on Fri Feb 12, 2021 4:13 pm, edited 1 time in total.
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By Geza-aka-Zombo
#31709
Sazerac wrote: Thu Nov 05, 2020 4:17 pm Geeza

So, one question I could never get answer on is how do we know this ATI damper matches the 928 engine needs? And why did Porsche have different dampers for the 928 motors? Intuitively, I know the answer: different harmonic frequencies. But, I am looking for more insights into this topic.

I just replaced my dampers on both my cars with GB's, but I would consider upgrading in 10 years or so and might buy your hardware in advance of the change (now) without the damper to be ready. It looks good what i have seen. But how do you know the Super Damper really fits the 928?
This is a very good question - one that I was curious about myself and questioned as I investigated the purchase of Brown's product. After deciding to develop my own solution, I delved into the subject and offer the following analysis:

* Both the OEM and ATI damper, since they are a single spring/mass system, have a single resonant frequency (Fn) which should be somewhat tuned to the crankshaft Fn.
* Although it might be ideal that both the damper and crank Fn be matched, it is not necessary. In fact, it is not practical/possible due to factors such as manufacturing tolerances and changes in rubber properties with temperature.
* Vibration energy in one item can be transferred to another even if the Fns are not matched. In my business, to prevent transfer, our goal is to maintain minimum 2 to 1 frequency separation (100%) between critical parts, if practical, to prevent one resonating object from impacting it's neighbor. The ATI is most likely matched within ~20% or less.
* If you look at the ATI damper application tables, you will note that the same ATI917020 damper is used for GM, Ford and Chrysler V-8 engines. Why is this? Consider that the crank Fn = square-root [Crank Torsional Stiffness/Rotational Mass]. Since these similar V-8 cranks share a lot in common, in terms of size, weight, material, etc., one can conclude that both the crank stiffness and rotational mass are all about the same. Even if one crank is slightly stiffer than another, it's most likely a bit heavier, too. Take the square root of this dividend, and the difference is even less. Hence, that is why a single TVD design can be used across multiple engines.
* My final though is that even being slightly off in terms of frequency match, it's got to be a lot better than the non-functioning, rubber hard as plastic, OEM damper I replaced. Hope this helps.

Regarding why Porsche had different versions? I don't know. I understand they had different colors, and perhaps the total weight was different, but this doesn't mean that the Fn was different. Just speculation, but perhaps they developed a heavier duty version (same Fn, with more or less weight and differing rubber properties for increased vibration energy absorbing capability) for higher performance engine applications. Without detailed review and testing, one can't know the details regarding the different OEM TVD versions.
User avatar
By maddog2020
#31829
I had several different factory dampers on hand years ago and couldn't find any discernable difference. GB reasoned that it helped Porsche keep the engines straight in the parts system as someone could easily look at a complete engine and know what it was. At one point all the parts superseded to one that was grey. Yes I did buy one of the last original stock factory dampers. I also purchased one of ATI's 928 damper's and yes ATI did make the hubs. To date no one has gotten them to fit. There were only 5 made. I also purchased one of GB's. It was also a very tight fit. I also had a fluid damper on my 1st stroker engine and it leaked so I ditched it when I built my 2nd stroker. So I had the following:

Stock 1990
Fluid filled
superseded factory part
ATI made Damper with hub (didn't fit)
GB ATI damper

Yep 5 different ones.
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By Sazerac
#31841
Geza-aka-Zombo wrote: Fri Nov 06, 2020 10:53 am
Sazerac wrote: Thu Nov 05, 2020 4:17 pm Geeza

So, one question I could never get answer on is how do we know this ATI damper matches the 928 engine needs? And why did Porsche have different dampers for the 928 motors? Intuitively, I know the answer: different harmonic frequencies. But, I am looking for more insights into this topic.

I just replaced my dampers on both my cars with GB's, but I would consider upgrading in 10 years or so and might buy your hardware in advance of the change (now) without the damper to be ready. It looks good what i have seen. But how do you know the Super Damper really fits the 928?
This is a very good question - one that I was curious about myself and questioned as I investigated the purchase of Brown's product. After deciding to develop my own solution, I delved into the subject and offer the following analysis:

* Both the OEM and ATI damper, since they are a single spring/mass system, have a single resonant frequency (Fn) which should be somewhat tuned to the crankshaft Fn.
* Although it might be ideal that both the damper and crank Fn be matched, it is not necessary. In fact, it is not practical/possible due to factors such as manufacturing tolerances and changes in rubber properties with temperature.
* Vibration energy in one item can be transferred to another even if the Fns are not matched. In my business, to prevent transfer, our goal is to maintain minimum 2 to 1 frequency separation (100%) between critical parts, if practical, to prevent one resonating object from impacting it's neighbor. The ATI is most likely matched within ~20% or less.
* If you look at the ATI damper application tables, you will note that the same ATI917020 damper is used for GM, Ford and Chrysler V-8 engines. Why is this? Consider that the crank Fn = square-root [Crank Torsional Stiffness/Rotational Mass]. Since these similar V-8 cranks share a lot in common, in terms of size, weight, material, etc., one can conclude that both the crank stiffness and rotational mass are all about the same. Even if one crank is slightly stiffer than another, it's most likely a bit heavier, too. Take the square root of this dividend, and the difference is even less. Hence, that is why a single TVD design can be used across multiple engines.
* My final though is that even being slightly off in terms of frequency match, it's got to be a lot better than the non-functioning, rubber hard as plastic, OEM damper I replaced. Hope this helps.

Regarding why Porsche had different versions? I don't know. I understand they had different colors, and perhaps the total weight was different, but this doesn't mean that the Fn was different. Just speculation, but perhaps they developed a heavier duty version (same Fn, with more or less weight and differing rubber properties for increased vibration energy absorbing capability) for higher performance engine applications. Without detailed review and testing, one can't know the details regarding the different OEM TVD versions.
Geeza

Thanks for explaining this.

I see now that not coming from the motor building business I had a kind of wrong understanding of the goal of these harmonic dampers. I was thinking that they would complete a kind of tuned mass damper system to completely eliminate certain peak harmonics "at system level." Based on your comments, it seems that these harmonic dampers are employed to just absorb some of the biggest peaks of vibrations thus preventing major damage. In other words, getting 80% is good enough and it doesn't really need to be well tuned. **If my understanding is correct,** your conclusions then certainly seem reasonable.

When I replaced my harmonic dampers, I was kind of operating under the idea above and concluding as you that new rubber is just better than old, even if the old system might be a tuned mass damper system.
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By Crumpler
#35252
Rhetorical question:

I’m reading post 300-something on Jerry’s dust pan thread today. Why doesn’t he catch heat for not being a vendor on RL?
He used to be, pretty certain, but not for years now and still selling products.
I’m not ripping on Jerry, never met him, it just seems pretty selective who gets persecuted and who does not.
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By Geza-aka-Zombo
#35410
Crumpler wrote: Wed Nov 18, 2020 3:49 pm Rhetorical question:

I’m reading post 300-something on Jerry’s dust pan thread today. Why doesn’t he catch heat for not being a vendor on RL?
He used to be, pretty certain, but not for years now and still selling products.
I’m not ripping on Jerry, never met him, it just seems pretty selective who gets persecuted and who does not.
Like someone PM'd me when they took down my post - from George Carlin - "It's a big club and you ain't in it".

Update to the Ultimate 928 Damper Hub - chips are being made right now and the kit is currently on track for early December availability. I'm hoping folks post their thoughts/impressions on RL, just to stick it in their eye. Thanks for the support everyone!

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