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Old 02-23-2001, 09:08 AM   #1
donjuan
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Post Power Pulley Article

I was asked to post this here for reference. I just thought it was interesting.

Some food for thought about UD pulleys by Steve Dinan of Dinan BMW...


The Danger of Power Pulleys &
Understanding the Harmonic Damper

By Steve Dinan


I have been threatening for a long time to write a series of technical articles to educate consumers and to dispel misconceptions that exist about automotive after-market technology. Motivated by problems with customer's cars resulting from the installation of power pulleys, I wish to explain the potential dangers of these products and address the damage they cause to engines.

The theory behind the power pulley is that a reduction in the speed of the accessory drive will minimize the parasitic losses that rob power from the engine. Parasitic power losses are a result of the energy that the engine uses to turn accessory components such as the alternator and water pump, instead of producing power for acceleration. In an attempt to minimize this energy loss, many companies claim to produce additional power by removing the harmonic damper and replacing it with a lightweight assembly. While a small power gain can be realized, there are a significant number of potential problems associated with this modification, some that are small and one which is particularly large and damaging!

The popular method for making power pulleys on E36 engines is by removing the harmonic damper and replacing it with a lightweight alloy assembly. This is a very dangerous product because this damper is essential to the longevity of an engine. The substitution of this part often results in severe engine damage.

It is also important to understand that while the engine in a BMW is designed by a team of qualified engineers, these power pulleys are created and installed by people who do not understand some very important principles of physics. I would first like to give a brief explanation of these principles which are critical to the proper operation of an engine.

1) Elastic Deformation

Though it is common belief that large steel parts such as crankshafts are rigid and inflexible, this is not true. When a force acts on a crank it bends, flexes and twists just as a rubber band would. While this movement is often very small, it can have a significant impact on how an engine functions.

2) Natural Frequency

All objects have a natural frequency that they resonate (vibrate) at when struck with a hammer. An everyday example of this is a tuning fork. The sound that a particular fork makes is directly related to the frequency that it is vibrating at. This is its "natural frequency," that is dictated by the size, shape and material of the instrument. Just like a tuning fork, a crankshaft has a natural frequency that it vibrates at when struck. An important aspect of this principle is that when an object is exposed to a heavily amplified order of its own natural frequency, it will begin to resonate with increasing vigor until it vibrates itself to pieces (fatigue failure).

3) Fatigue Failure

Fatigue failure is when a material, metal in this case, breaks from repeated twisting or bending. A paper clip makes a great example. Take a paper clip and flex it back and forth 90° or so. After about 10 oscillations the paper clip will break of fatigue failure.

The explanation of the destructive nature of power pulleys begins with the two basic balance and vibration modes in an internal combustion engine. It is of great importance that these modes are understood as being separate and distinct.

1) The vibration of the engine and its rigid components caused by the imbalance of the rotating and reciprocating parts. This is why we have counterweights on the crankshaft to offset the mass of the piston and rod as well as the reason for balancing the components in the engine.

2) The vibration of the engine components due to their individual elastic deformations. These deformations are a result of the periodic combustion impulses that create torsional forces on the crankshaft and camshaft. These torques excite the shafts into sequential orders of vibration, and lateral oscillation. Engine vibration of this sort is counteracted by the harmonic damper and is the primary subject of this paper.

Torsional Vibration (Natural Frequency)

Every time a cylinder fires, the force twists the crankshaft. When the cylinder stops firing the force ceases to act and the crankshaft starts to return to the untwisted position. However, the crankshaft will overshoot and begin to twist in the opposite direction, and then back again. Though this back-and-forth twisting motion decays over a number of repetitions due to internal friction, the frequency of vibration remains unique to the particular crankshaft.

This motion is complicated in the case of a crankshaft because the amplitude of the vibration varies along the shaft. The crankshaft will experience torsional vibrations of the greatest amplitude at the point furthest from the flywheel or load.

<IMG SRC="http://dinanbmw.com/html/graphics/image2.jpg" border=0>

Harmonic (sine wave) Torque Curves

Each time a cylinder fires, force is translated through the piston and the connecting rod to the crankshaft pin. This force is then applied tangentially to, and causes the rotation of the crankshaft.

The sequence of forces that the crankshaft is subjected to is commonly organized into variable tangential torque curves that in turn can be resolved into either a constant mean torque curve or an infinite number of sine wave torque curves. These curves, known as harmonics, follow orders that depend on the number of complete vibrations (cylinder pulses) per revolution. Accordingly, the tangential crankshaft torque is comprised of many harmonics of varying amplitudes and frequencies. This is where the name "harmonic damper" originates.

<IMG SRC="http://dinanbmw.com/html/graphics/image3.jpg" border=0>

Critical RPM's

When the crankshaft is revolving at an RPM such that the torque frequency, or one of the harmonic sine wave frequencies coincides with the natural frequency of the shaft, resonance occurs. Thus, the crankshaft RPM at which this resonance occurs is known a critical speed. A modern automobile engine will commonly pass through multiple critical speeds over the range of its possible RPM's. These speeds are categorized into either major or minor critical RPM's.

Major and Minor Critical RPM’s

Major and minor critical RPM's are different due to the fact that some harmonics assist one another in producing large vibrations, whereas other harmonics cancel each other out. Hence, the important critical RPM’s have harmonics that build on one another to amplify the torsional motion of the crankshaft. These critical RPM’s are know as the "major criticals". Conversely, the "minor criticals" exist at RPM's that tend to cancel and damp the oscillations of the crankshaft.

If the RPM remains at or near one of the major criticals for any length of time, fatigue failure of the crankshaft is probable. Major critical RPM’s are dangerous, and either must be avoided or properly damped. Additionally, smaller but still serious problems can result from an undamped crankshaft. The oscillation of the crankshaft at a major critical speed will commonly sheer the front crank pulley and the flywheel from the crankshaft. I have witnessed front pulley hub keys being sheered, flywheels coming loose, and clutch covers coming apart. These failures have often required crankshaft and/or gearbox replacement.

Harmonic Dampers

Crankshaft failure can be prevented by mounting some form of vibration damper at the front end of the crankshaft that is capable of absorbing and dissipating the majority of the vibratory energy. Once absorbed by the damper the energy is released in the form of heat, making adequate cooling a necessity. This heat dissipation was visibly essential in Tom Milner's PTG racing M3 which channeled air from the brake ducts to the harmonic damper, in order to keep the damper at optimal operating temperatures. While there are various types of torsional vibration dampers, BMW engines are primarily designed with "tuned rubber" dampers.

It is also important to note that while the large springs of a dual mass flywheel absorb some of the torsional impulses conveyed to the crankshaft, they are not harmonic dampers, and are only responsible for a small reduction in vibration.

<IMG SRC="http://dinanbmw.com/html/graphics/pully_wp.jpg" border=0>


In addition to the crankshaft issue, other problems can result from slowing down the accessories below their designed speeds, particularly at idle. Slowing the alternator down can result in reduced charging of the battery, dimming of the lights, and computer malfunctions. Slowing of the water pump and fan can result in warm running, while slowing of the power steering can cause stiff steering at idle and groaning noises. It is possible to implement design corrections and avoid these scenarios, but this would require additional components and/or software.

Our motto at Dinan is "Performance without sacrifice" We feel our customers expect ultra high performance along with the legendary comfort and reliability of a standard BMW.

While it is common that a Dinan BMW is the fastest BMW you can buy, performance is not our only goal. Dinan isn't just trying to make the fastest car. Instead a host of considerations go into the development of our products. Dinan puts much more effort into these other areas than does our competition.

These considerations are Performance, Reliability (Warranty), Driveability, Emissions, Value, Fit and Finish. We feel that the power pulley is a bad way to get extra power from and engine and the potential for serious engine damage is too great.

This is a simplified explanation meant to be comprehensible by those who are not automotive engineers. In trying to simplify an extremely complex topic some precision was sacrificed although we believe this explanation to be as accurate as possible. We encourage our customers to educate themselves and understand the automotive after-market because we believe that our products are the best researched, engineered, and fabricated products available.

For those interested in a more in depth and technical explanation of this topic, the reference book is Advanced Engine Technology, written by Heinz Heisler MSc,BSc,FIMI,MIRTE,MCIT. Heinz Heisler is the Head of Transportation Studies at The College of North West London. His book is distributed in this country by the SAE (Society of Automotive Engineers).

EDIT--fix pix.




[This message has been edited by donjuan (edited February 23, 2001).]
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Old 02-23-2001, 09:38 AM   #2
RidinLow
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That's a pretty interesting read. I kind of thought that the I6's wouldn't need a harmonic dampener because they're balanced, but I guess that their crankshaft is much longer than the H4's?
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Old 02-23-2001, 09:42 AM   #3
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Yeah that's what I came up with too, the extra length. I don't know how that info would differ between I-6, H-4, V-8, W-8, or whatever, but it's interesting info...
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Old 02-23-2001, 09:51 AM   #4
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I wonder what is the longest duration (in actual mileage) that an Impreza 2.5RS owner has had the UOR pulley on their engine... And if they have had any problems with their engines...
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Old 02-23-2001, 10:22 AM   #5
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Hmmm I'll have to check my friend's M5, I remember when we were rebuilding it, it didn't have the harmonic balancer on it (It's an E28 chassis, 3.5L DOHC I6). The I6 is really really balanced, I don't think it even requires the harmonic balancers. For those who have a Porsche, check to see if you have a balancer. I know that RUF and a few other tuners in Germany use UD/light weight crank pulleys. But with today's tight tolerences, I there's no "real" need for the harmonic balancers on the H4 or I6's, I do know that the V6/8's will need it.
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Old 02-23-2001, 12:57 PM   #6
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hmmm I thought that the H4's aren't required to have the dampening... might be different on a i6 though
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Old 02-23-2001, 03:28 PM   #7
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it would be nice to have Trey's input on this subject. i know he sells them (unorthodox pulleys)
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Old 02-23-2001, 03:32 PM   #8
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I beleive H-4's dont have a harmonic damper/balancer to start with.....do they? If they dont replace away!
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Old 02-23-2001, 03:42 PM   #9
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When I had my crankshaft pully off for my cam install, I noticed that there is a large rubber disk attached to the pully. I would assume this is part of the harmonic balancer of the pully. It fits the description of the harmonic damper perfectly.

Shiv refuses to adapt his Tec-II to use an underdrive pully, so he is aware of possible "issues" without harmonic dampers. I have also heard that the H-4 is internally balanced, since everything is opposed and symmetrical. I didn't have any plans of trying an underdrive pully because of the hopes that I may, one day, own the Tec-II.

-Jon
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Old 02-23-2001, 10:57 PM   #10
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Well, I am also under the impression that the H4 doesn't need a harmonic damper and UD pulleys are okay.

Larry
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Old 02-24-2001, 01:19 AM   #11
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just as I bought a UD pulley. This really makes me doubt about installing it. Pretty interesting article. You can find pro and cons about using a pulley in this forum, however this article beats all of them. I guess I will have to stick that pulley in a dark place (storage room)


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Old 02-24-2001, 10:13 AM   #12
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I'm going to be doing an auto-x this weekend, and I posted a question to members on the mailing list concerning my mods and what class I'd fall into.
Some said my UD pulley would move me out of stock class, but the official answer I got was that it was ok, because the stock pulley is not a harmonic dampener, and therefore it is legal to change the pulley to an UD one. I was told this is because the H-4 is a balanced engine. That said, I've had the UD pulley on for about 5K, no problems yet!
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Old 02-24-2001, 10:17 AM   #13
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Quote:
the stock pulley is not a harmonic dampener
The stock pulley has a hard rubber sandwich. You make the call-- what purpose could that rubber serve? Why not make it out of solid metal if it wasn't meant to damp vibration?
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Old 02-24-2001, 10:23 AM   #14
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The stock pulley is indeed a harmonic dampener. Compare it to the rendering shown in the first post.

The UD pulley used to put you into a SP class, but now it just moves you to ST. There's no way will you be running stock class with a pulley.
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Old 02-24-2001, 10:40 AM   #15
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donjuan --

great post! thanks for the info and the diagrams.

-- brian
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Old 02-24-2001, 11:44 AM   #16
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Let me clear something up for you guys talking about naturally balanced engines. Torsional vibration has got nothing to do with whether you have a naturally balanced engine or not. All crankshafts, regardless of what engine it is from, will experience some degree of torsional vibration, some more than others depending on how long the crankshaft is and some other factors.
Your crankshaft is most likely to break/fail when the torsional vibration frequency matches up with the natural frequency of the crankshaft.
Now, a crankshaft damper pulley's job is to mess around with the torsional vibration frequency. It shifts this frequency up or down or spilt it in two, thus reducing it overall amplitude. Bottom line is, it makes sure that the torsional vibration frequency is nowhere near the natural frequency of the crankshaft, where bad things happen. Our H4 engines have very short crankshafts, that does help a little.
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Old 02-25-2001, 12:02 AM   #17
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good info!
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Old 02-25-2001, 12:15 PM   #18
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Gee! I sent back my UD pulley because so many people said my A/C would be compromised. Now I really feel OK about sending it back!!
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Old 02-25-2001, 12:27 PM   #19
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My take on UD pulleys has always been simple: Is the increased (but still small, I'm sure) probability of catastrophic engine failure worth a small performance gain? Not to me

Also, there have been numerous Miata oil pump failures believed to be caused by UD pullies. Of course, all these failures (AFAIK) have occured on turbocharged Miata engines suggesting the likelihood of harmonic-induced engine failure rises with engine output and engine redline. You can guess why I don't use them with my TEC-II kits since I expect most customers to eventually run turbos and scs at higher-than-stock redlines.

Cheers,
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Old 02-25-2001, 07:49 PM   #20
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This is perfect timing!!! I was just about to do a search un UD pulleys!! I was planning on getting the Cobb Tuning stage I kit this summer, and was going to try to find info on whether UDs were really worth the trouble. This answers my question for sure. Don't think I'll bother. Too much trouble to go to for not enough increase and so much potential for disaster. Great info!!
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Old 02-25-2001, 10:43 PM   #21
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Cool. Man, by reading this article I just learned a grip. Well thanks to this I know not to go with a power pulley, doesn't really matter because I plan on getting a UD pulley anyway. But its always good to learn something new!
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Old 02-25-2001, 10:55 PM   #22
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Umm, power pulley=UD pulley.
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Old 02-26-2001, 01:16 AM   #23
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Great article. Thanks for the post. I believe I have had my pulley on my car sometime either late 1997 or early 1998. I have to find my receipt. At any rate, I've had it on my car for around 70k miles. No problems here that I am aware of. That's not to say it won't cause a problem on another car or that I won't have problems soon. You just never know!

If you want to prevent problems from aftermarket parts, keep your car stock. That's the best advice I can give you.
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Old 02-27-2001, 01:07 AM   #24
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ive had the underdrive pulley on MY01 for 10,883 miles so far and have nothing bad to say about it.
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Old 02-27-2001, 01:23 AM   #25
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Serious technical question:

Has anyone that's had an underdrive pulley on for many miles had their crankshaft endplay checked? Just curious, because if not then you really may not have "no problems".
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