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Driveline, Engine & DME Tuning
Talk about driveline improvements, NA tuning and DME tuning your E46 BMW here. This includes diffs, intakes, exhausts, chips, software and OBD tuning.

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Old 06-07-2014, 10:58 PM   #1
Rob43
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Harmonic Vibrations & Stuff

I pose this question mostly to you guys with Engineering Degrees. I figure most of you have been reading about the M54B30 & its inherent Harmonic Vibration problem that kills the oil pump at higher RPM's. I have a thought on a simple way to help "Eat-Up" those bad vibrations, so I look to your extensive knowledge in engineering to help answer this seemingly simple question.

My thought is to mount an appropriate size hydraulic shock between the block & the chassis, thinking it would help eat-up some of the harmonics coming off the block. It certainly would be cheap enough, and my thinking says that if the hydraulic shock had a raise in temperature, it's working.

Thoughts ?



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Old 06-08-2014, 02:40 AM   #2
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The E30/M40 has a hydraulic shock absorber on the left side of the engine, next to the engine mount. Are you talking about a similar thing?
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Old 06-08-2014, 12:27 PM   #3
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The E30/M40 has a hydraulic shock absorber on the left side of the engine, next to the engine mount. Are you talking about a similar thing?

Similar, kind of.

I would want this theoretical shock mounted high enough up so that it can get the Maximum rocking movement. My thought is that it would be exposed
to the most vibration mounted this way.


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Old 06-09-2014, 03:50 AM   #4
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As in some honda engines?
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Old 06-09-2014, 05:15 AM   #5
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That's a "bobble strut" and they're used on cars with soft engine mounts to minimize the movement of the engine. Typically FWD stuff with squishy rubber mounts to isolate the unibody from first and second order vibes. It won't work to solve our problem because 1) our vibrations are torsional, running the length of the crank, so the net rocking effect on the engine is zero and 2) they're way too high of a frequency to be absorbed by something like that.
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Old 06-09-2014, 06:11 AM   #6
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Yeah, I don't see that being able to absorb anything useful. I'm sure if you put an LVDT or linear pot in the same position you're talking about, you'd see the amplitude as measured in that direction would be barely distinguishable from noise.
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Old 06-09-2014, 12:42 PM   #7
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I am not talking about a tiny shock like the one posted in post #4, but a larger unit that would really dampen something like an atv or car. So a large diameter unit (30 to 50mm) that holds plenty of hydraulic fluid, but short enough to actually fit in place.

Like most of us I too run upgraded motor mounts, but clearly there is still some motor movement (rocking), just not as much as stock. So lets say for the sake of saying that this theoretical "hydraulic shock" experiences a 1/4 of an inch or about 6mm of piston travel. And in this 1/4 of an inch of travel the damper experiences a relatively high frequency rate, specifically above 5000 RPM.

Wouldn't the shock/damper be "eating-up" harmonic vibrations in its 6mm of piston travel ?



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Old 06-09-2014, 01:04 PM   #8
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So you're saying that shock would oscillate back and forth with a 1/4" stroke? I mean, I'm sure it could under certain load/unload conditions, but certainly not anything that would dampen the harmful vibrations. It sounds like what you're proposing is a linear analog to a axial harmonic damper, which is what we've been talking about. I don't think it's oriented correctly to do any good, though.
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Old 06-09-2014, 01:14 PM   #9
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So you're saying that shock would oscillate back and forth with a 1/4" stroke? I mean, I'm sure it could under certain load/unload conditions, but certainly not anything that would dampen the harmful vibrations. It sounds like what you're proposing is a linear analog to a axial harmonic damper, which is what we've been talking about. I don't think it's oriented correctly to do any good, though.

Yep, that's pretty much what I'm saying.

Keep in mind that I'm one of the ten (10) guys that owns an ATI Damper from the original group buy. I thought up this idea because I was looking for a way to build a "Poor Mans Damper" for everyone else, assuming it works; even a little.


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Old 06-09-2014, 01:46 PM   #10
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I'd be curious to see how well it would work. I just don't see how it could help.
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Old 06-09-2014, 04:52 PM   #11
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I think you're confused about the nature of the harmonics that are occurring in the crankshaft. They are not causing a rocking moment on the entire engine.
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Old 06-09-2014, 04:57 PM   #12
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I think you're confused about the nature of the harmonics that are occurring in the crankshaft. They are not causing a rocking moment on the entire engine.

Certainly not confused.

It's just the easiest way to describe it for the masses. Would you be more comfortable with the word "Buzzzzzz".


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Old 06-09-2014, 05:18 PM   #13
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But it's important to understand how to effectively mitigate it. There's a reason the ATI damper is a press-fit at the end of the crankshaft.
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Old 06-09-2014, 05:19 PM   #14
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You are confused though...

Harmonics/resonance in the crankshaft won't be affected by an engine brace.

Just because the side effect of the harmonics is a "vibrating" engine doesn't mean dampening the vibrations will solve it....

Need a harmonic damper directly attached(press-fit/welded/etc) to the crankshaft...

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Old 06-09-2014, 06:17 PM   #15
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You are confused though...

Harmonics/resonance in the crankshaft won't be affected by an engine brace.

Just because the side effect of the harmonics is a "vibrating" engine doesn't mean dampening the vibrations will solve it....

Need a harmonic damper directly attached(press-fit/welded/etc) to the crankshaft...

This is a reasonable answer, and makes sense.


Any other thoughts /anyone else ?


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Old 06-10-2014, 01:34 PM   #16
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I put in a phone call to ATI Dampers today & had a pleasant conversation with John who is one of ATI's technical guys. After giving him the all the details on our motors & my "Out Of The Box" thinking, John said that the only way to remove harmonics was to do it at the crank. No matter what I could attach to the block, it wouldn't have any real value as to suppressing unwanted harmonics. Oh well, so much for trying to build a "Poor Mans" damper.

I did learn that the rubber o-ring rebuild kit costs about $50 dollars and it is easy for the end user to rebuild his own ATI Super Damper when needed.



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Nitrous is a little trickier than boost, but it's not the spray that kills motors, it's STUPIDITY!!
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Old 06-10-2014, 02:07 PM   #17
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For what it does, the ATI damper really isn't that expensive. And yes, it's the only solution to the problem.
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Old 06-11-2014, 06:57 AM   #18
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a damper like that could dampen the global response of the engine like rigid mode of vibration (lateral, longitudinal, bounce, roll, yaw, pitch on its mounts) but it wont stop the flexible modes of the block and crank

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