Crank Dampers - How They Work

Engine building advice and Q&A with Jeff!

Crank Dampers - How They Work

Postby FRP » Thu Jul 18, 2013 11:14 pm

Hello to all, sorry to be so late posting again, just been buried at the shop and tired once I get home.

The crank damper on a reciprocating engine is a very often overlooked part of the engine's power and durability equation. They seem innocuous enough at first blush, but are in fact an extremely critical element in both power production and engine longevity. The damper has but one main function, which is to control torsional (twisting) forces within the crank in such a way as to reduce the amplitude of the "rebound" forces that occur after power pulse has deflected the crank, as it twists against the load and then tries to return to it normalized position / shape.

You can imagine, especially on hi boost big power engines, how these forces would congregate within the crank structure and magnify themselves if something did not serve to stabilize the crankshaft. That is the job of the damper - to provide stability to the crank by adding mass at the end of the crank and by acting as a "phase cancellation" device to effectively appose the rebound action that takes place after a cylinder firing has applied twist to the crank structure. If not for the damper the amplitude of the rebound forces would increase at varying rates depending on the material strength of the crank vs. the forces and frequencies (rpm) being applied, and, wreak havoc with the stability of the engine's cam and ignition timing as well as drastically reducing bearing life, breaking gears in oil pumps, and eventually failing the crank itself.

Crank dampers on production engines are generally 2 piece units with a hub, and an outer ring separated by an elastomeric ring that bonds the outer ring to the hub. The durometer rating (stiffness)of the elastomeric ring and the weight (mass) of the outer hub are calculated by the engine designer to address the most critical rebound frequencies anticipated, based on the design power output and rpm range. The damper functions, in the most simple terms, by allowing the inner hub to momentarily "out run" the outer ring (by the inherent flex of the elastomer material) and for the "catching up" motion of the outer ring to meet and appose the rebound forces as the crank tries to snap back to its normal shape. In other words by phase cancellation. You can now see why you would not want an old worn damper with a hardened, and even cracking elastomer ring, on your high performance engine!

The real problems for crank dampers begin when people like us, start drastically increasing the power output (and therefore crankshaft torsional deflection amplitude) of our engines. Not only is the tuning of the damper inadequate, but the actual physical mounting of the crank damper to the crank nose becomes a potential point of failure. This is a BIG problem on our V.A.G. engines that try to retain the damper with a big bolt that squeezes the face of the damper hub against the nose of the crank. With increased power output there is simply not enough surface area available to pass the forces between the damper and the crank! That is why they have their special "diamond encrusted washer" between the damper hub and the nose of the crank. They are trying to add "tooth" to this junction beyond what is available with steel against steel! And, this is why just trying to bolt an improved aftermarket type balancer (be it a fluid type or tunable elastomeric type) through a stock type mounting, fails to address all of the issues on big power engines.

I can absolutely guarantee you that the flywheel bolt loosening problem on some of our V.A.G. engines with turbo / power upgrades is being caused by the factory crank damper being totally over-matched by the cranks torsional deflections at higher power levels.

At FRP, I had to engineer a solution for the AAN engine in the Bonneville car - if it was ever going to survive the abuse of extended operation at 9,700 rpm and 1000 WHP pushing against that big wind load.
I worked extensively with the engineers at ATi (makers of the ATi Super Damper which is fitted to every Sprint Cup Car and every NHRA Pro Stock car) to design a real race damper solution for the Audi I 5. The units inertia weight and the stiffness of its elastomer rings are tuned to support the engine's extreme output and rpm.

The FRP / ATi unit is designed for use with a dry sump system and requires the removal of the four lobe oil pump drive lever that sits against the front of the number 1 main journal. The damper's drive hub is designed for a .001 interference fit, presses over the complete length of the crank nose, and bolts solidly against the front bearing journal, retained by the stock damper bolt. The crank nose is broached for a 3/16 ths. Woodruff key to provide correct clocking of the damper.

An additional feature of the drive hub is an integral cam belt drive machined into the hub - no more separate cam dive gear. I only mention this because it provides an example of crank damping which is robust enough in design and mounting for the job at hand. It can be seen on the web site at fourringperformance.com .

I would be completely open to working with Hank or Marc or whomever to try and develop this style of damper and mounting for other V.A.G. engines if the interest arises.

Again, sorry this took so long and I hope it sheds some useful light on the subject. Happy Motoring!
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Re: Crank Dampers - How They Work

Postby speeding-g60 » Thu Jul 18, 2013 11:38 pm

20v 4 cylinder. 9500 rpm and more, solid liftered, 45 psi, >800 AWHP. interested. will be dry sumped this winter.

TIA :)
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Re: Crank Dampers - How They Work

Postby PRY4SNO » Sat Jul 20, 2013 6:32 pm

Great read! Thanks for taking the time to develop and share about this new product :)

One question, with your research, what range (rpm/hp) did you determine the OEM damper to be insufficient? Or, at what point would someone who is building their 20v i5 consider an upgrade?
Find me on Instagram @pry4sno

1992 80 quattro 20v /// Eventual AAN'd Winter Sled

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Re: Crank Dampers - How They Work

Postby Mcstiff » Sat Jul 20, 2013 7:29 pm

Very cool! The worst part is the cost of the dry sump lol.
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Re: Crank Dampers - How They Work

Postby FRP » Sun Jul 21, 2013 6:08 pm

Dear PRY4SNO
Good question. My impression is that a stock AAN damper is good to about 550 to 600 crank hp - if it is good shape without excessive miles or dry rotting of the elastomer ring. (with longevity) Above this level the crank deflections begin to really work on the bonding of the inner hub and outer ring to the elastomer ring. I know when we first took the race car to Bonneville we had a GT40 and about 700 WHP. After the dyno runs and about 8 runs on the salt (progressively faster for acquiring an "A" license - with the last 2 runs at 222mph) when we got the car back the elastomer ring was badly cracked. I put the damper in the press at the shop and the center hub just popped right out. Now let me say that this kind of abuse won't occur on a street car or DE car etc. but it does indicate that the 700 WHP level will tear up a used damper - even though it was a good looking sound appearing used damper. More importantly there was a lot of "cold welding" occurring in the joint between the damper and the crank nose. The crank bolt was still fully torqued with the locktite very intact but the crank nose and damper surfaces along with the diamond washer looked shredded with lots of micro welding and tearing on the mating surfaces. For whatever reason - maybe more internal surface area on the elastomer ring - it seem to me that the earlier v belt style damper from the 3B was tougher and more able to handle high horsepower. Maybe some of our other readers here will have an opinion / experience to back up my impression about the older design damper - I would be interested to hear anyone's thoughts. I would certainly suggest that anyone doing a big HP build on one of these motors try to start out with a new damper. With fresh elastomer and zero work cycles.
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Re: Crank Dampers - How They Work

Postby PRA4WX » Sat Jul 27, 2013 12:03 am

Fascinating reading. Thanks! 8-)
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Re: Crank Dampers - How They Work

Postby yodasfro » Sat Aug 03, 2013 9:03 pm

Jeff, what are your thoughts on using the thinner diamond coated washer from the diesel 5 cyl.(pn#074105193) on the gas audi 5 cyl. (early AAN/7A/3B/MC) motors that originally did not come with one from the factory?
Austin 90' 90q 20v 91' 200 20v wagon 92' urs4
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Re: Crank Dampers - How They Work

Postby speeding-g60 » Sun Aug 04, 2013 12:07 am

EKA-Grip...... yeah....
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Re: Crank Dampers - How They Work

Postby loxxrider » Sun Aug 04, 2013 8:33 am

Austin, I think if you are really after getting the most insurance out of the stock damper, you really need to pin it. Even then, you are putting a pin in shear which is never sound design practice.
-Chris

'91 Audi 200 20v - Revver/BAT project
'91 Audi 200 20v Avant
'01 Anthracite M5
'90 M3
'85 Euro 635csi
'12 X3
E34 530i (maybe rear-mount soon)
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Re: Crank Dampers - How They Work

Postby yodasfro » Sun Aug 04, 2013 11:45 am

Chris, that wasn't really the question. My question isn't related to a high rev/ high HP application just a stock motor.
Austin 90' 90q 20v 91' 200 20v wagon 92' urs4
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Re: Crank Dampers - How They Work

Postby loxxrider » Mon Aug 05, 2013 9:38 pm

I don't understand how what I'm saying doesn't apply? On a stock motor, get a hardened cog, torque it properly (correct torque and without an impact), and be done with it. If that still bothers you, you can pin it with a dowel pin for a hell of a lot cheaper than one of those $90 washers from Audi.
-Chris

'91 Audi 200 20v - Revver/BAT project
'91 Audi 200 20v Avant
'01 Anthracite M5
'90 M3
'85 Euro 635csi
'12 X3
E34 530i (maybe rear-mount soon)
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Re: Crank Dampers - How They Work

Postby yodasfro » Tue Aug 06, 2013 1:12 am

The washer I'm talking about isn't $90. I can 't see how pining and a hardened cog would be cost effective for an otherwise stock motor? My question is about adding a diamond coated washer for more tooth between the damper and the end of the crank snout to an application that didn't have one from the factory.
Austin 90' 90q 20v 91' 200 20v wagon 92' urs4
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Re: Crank Dampers - How They Work

Postby audifreakjim » Tue Aug 06, 2013 11:55 am

So they are actually diamond coated? I thought you were just saying that because they were so expensive, lol. It sounds like a good solution to help with the problems Jeff mentioned above.
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Re: Crank Dampers - How They Work

Postby loxxrider » Tue Aug 06, 2013 10:04 pm

yodasfro wrote:The washer I'm talking about isn't $90. I can 't see how pining and a hardened cog would be cost effective for an otherwise stock motor? My question is about adding a diamond coated washer for more tooth between the damper and the end of the crank snout to an application that didn't have one from the factory.


How is it not cost effective in the long term? It is basically obligatory to replace the cog with a new one every time you pull it off... A hardened cog will pay for itself in as little as two timing belt changes. The pin costs almost nothing, so you are in approximately $200 vs $95 for a stock sintered P.O.S. cog plus a $90 washer (if it costs the same as an AAN diamond washer). That's cheap insurance to me. Noone is having vibration issues with stock 3b setups on stock engines to my knowledge, so I just don't understand why you'd want to go through the effort to do this. Sorry for trying to help :P
-Chris

'91 Audi 200 20v - Revver/BAT project
'91 Audi 200 20v Avant
'01 Anthracite M5
'90 M3
'85 Euro 635csi
'12 X3
E34 530i (maybe rear-mount soon)
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Re: Crank Dampers - How They Work

Postby my2000apb » Fri Aug 09, 2013 7:30 pm

audifreakjim wrote:So they are actually diamond coated? I thought you were just saying that because they were so expensive, lol. It sounds like a good solution to help with the problems Jeff mentioned above.

something like this
http://www.intengineering.com/integrated-engineering-ekagrip-6-bolt-flywheel-friction-disk
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