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starspangled (Automotive) (OP)
11 Feb 07 10:24
Hi guys,

I read of a fellow having success using the Cummins 6BT harmonic balancer with an adapter on a Cummins 4BT to solve vibration & balance problems. I'm no engineer so I though I'd check here whether that is a good idea, and if not, why not? Other alternative suggestions appreciated....thanks!

evelrod (Automotive)
11 Feb 07 14:47
Can't say for sure, probably as both engines are cut from the same pie...I'm not sure I would bother as I have a passing aquaintence with a early 60's Cheby pickup with a 4BT...I first used it in Colorado, the summer of 64 ---  last saw it (at over 400,000 miles on the clock) in Los Angeles in the late 70's...engine still ran fine, but the truck was traaaaaaaash!!!

Pat Daly, where are you?

starspangled (Automotive) (OP)
11 Feb 07 16:46
Ha haha, brother if I can hit 400 thousand I'll be one happy guy, and probably quite ancient at the time. The vibration is only an irritation and I am getting used to it, but....
patdaly (Mechanical)
12 Feb 07 12:41

I have a 95 6BT balancer bolted to my 4BT, and I cant tell much of a difference, but then I dont think the 4BT is objectionable except for the shutdown shake.

I would have to question the effectiveness of a balancer engineered for a inline 6 on a 4 though.

Sorry I couldnt be more help.
starspangled (Automotive) (OP)
12 Feb 07 13:14
No, that's just what I'm after, Pat! Due to strange events I didn't make it in the field of mathematics so never realized my dream of engineering. Because of that I don't understand the exact mechanical forces and harmonics and metalurgy involved. I do know that these balancers dampen certain wave forms that develop in a crankshaft, but don't know if that damping or absorption or conversion of forces works the same with any inline crank, or even with same balancer used on a V configuration crank.

There's considerable vibration throughout the powerband in my engine, but you and the other gentleman I spoke of say you haven't noticed that with yours, WITH the 6B damper. Hmmmm....that's 2 for 2.
hemi (Automotive)
12 Feb 07 16:10
If you are expecting a larger crankshaft harmonic damper to help attenuate inline 4 cyl vertical shake, you will be disappointed.  The damper's purpose is to absorb crankshaft torsional vibration.
Unlike an inline 4, any conventional inline 6 has no inherent vertical shake, so the 6 cyl configuration is vastly superior in terms of NVH characteristics.
We are talking about 4-strokes here; the standard configurations for 2-stroke engines are inherently different and have their own distinct balancing characteristics.
starspangled (Automotive) (OP)
12 Feb 07 16:43
Oh, I was referring to the score: 2 for 2, not to a 2-stroke damper. 2 people out of 2 are getting good results, and another from a different forum has now posted the same, so it's up to '3 for 3'. But i was wondering about the aplied science of torsional versus vertical, and have heard something about 'vibrations of 6th order' or somesuch. I'm assuming that is something to do with sineusoidal wave harmonics, as sympathetic waveforms developing another waveform, but don't know for sure.
starspangled (Automotive) (OP)
13 Feb 07 22:59
I should have made it more clear: the Cummins 4BT has no harmonic balancer at all, and no balance shafts. I did a comparatively rigid installation as far as engine isolators go. With mine there's very little chance of falling asleep driving...
Helpful Member!  Tmoose (Mechanical)
14 Feb 07 13:06
It takes two "balance shafts" spinning at 2X engine speed to quell a standard inline 4's secondary vibration.  I would not expect a crank mounted damper/balancer pulley would not help much at all.

Soft engine mounts (more than .25 inc static deflection) would help isolate the engine vibration from the frame.  They might also help de-tune resonant frequencies involving the frame and engine and ?? .  I've had a few 4 cylinder cars whose exhaust systems re-amplified the engine vibration and fed it into the body frame thru the exhaust hangers.
starspangled (Automotive) (OP)
14 Feb 07 15:04
Man, you hit the nail on the head RE: exhaust! Couldn't figure it out at first and then found the rigid mount exhaust section, even with quality flex on each size, was doing its harmonics thing throughout the truck. Big difference when changed.

I believe I used too hard of engine isolators just as you describe, being new to diesel. I will be upgrading to a softer compound and thicker mounts. It's just that I don't fully grasp the science of harmonics per engine type so didn't know if the 6B damper was even physically capable of damping the 4B Cummins.

I'm now going to wait on that and install proper isolators first, per advice here. May then try the 6B damper just as an experiment to see if there's further improvement.

Thanks for the assist!

Helpful Member!  IceStationZebra (Mechanical)
14 Feb 07 21:52
Star, basically the individual cyliders are inputting their force at various points along the crank. This force imparts a torsion (or twist) into the crank. Whichever end of the crack presents less inertia will speed-up & slow down to match this cyclical loading. It is a very complex torsional spring. The main issue is dampening the resonant frequencies of the crank.

On the tranny end you generally have a nice heavy flywheel to help with this, and on some higher end cars they even go to dual mass flywheels. My diesel Olsmobile even had a rubber damper built into the driveshaft. (I found that out after putting in a 455!) On the front end you have the accessories and belt drive but not much else, and there is not much room for a big 'ol flywheel. So the next best thing is a tuned mass damper - which acts like a heavy flywheel at certain frequencies but is basically invisible at all others. There are also other technologies like viscous fluids, etc.

- Do you absolutely need a damper? No, it just makes the crank and other bits live longer. Many drag racers do without, but they don't put on many miles a year.
- Can you use a 6 cylinder damper on a 4 cylinder? Maybe (depends if the 4BT & 6BT are both internally balanced), but it is not tuned to the correct resonant frequency. Probably better than none, but not as good as the right one. I would hazard a guess that there is also a difference between the 3.9 and 4.5 4BT dampers.
-On balance shafts: they are an available opion on the 2-valve 4BT engines. A company I previously worked for tried to get one to compare against the non-balanced version and it was something like 1000 usd and a 9 month wait straight from Cummins. Needless to say we dropped it. Also, most automotive 4 cylinders above 2 liters get balance shafts. Here you have a heavy 3.9 (or 4.5) liter without any.
-softer engine mounts: will help with vibration but watch the exhaust system. We had constant problems on our off-road application. On one machine we fatigued the flange right off of the turbine! On another we fatiqued off the motor mount. On a third we had our exhaust supplier take some vibration data and give us thier recommendation. Within 10 minutes the flex joint had completely failed!

The long and short of it is - the 4BT is a durable but shaky beast! Its going to move and you can't stop it. I would recommend that you stick with the 4 cylinder damper and use a heavy duty flex joint in the exhaust. Don't use convoluted flex pipe - it corrodes and soon becomes a solid pipe.

Have fun. ISZ
starspangled (Automotive) (OP)
14 Feb 07 22:14
Well thanks for some very good information. I haven't heard of the 4.5L 4BT but I'm now going to do some searching for info. I'm currently using the 6BT trans output yoke/damper behind transmission, with 2000 6BT flywheel, no dual mass. It's possible that there was a harmonic damper available for some series of 4BT, seems hard to believe there wasn't.

There are complexities with the balance shaft setup that I don't care to deal with. As for the exhaust I do have a section of heavy duty flex pipe immediately after turbo down pipe and another behind muffler so there's now some flex and float in the system.

It seems that this engine hits hard on one cylinder and others I've heard do the same. 6BT do that too, but not sure why. I will research to see if either engine is balanced either externally or internally. I hope I didn't error in using a 6BT flywheel if there is a difference in balancing on engines.

So, the vibration is tolerable and I can get used to that, but part of my concern was in the possibility of metals fatigue or fastener failure with all that shaking going on. I will look into a 4BT damper as you mention, that certainly must be the best answer! Thanks for taking the time....

IceStationZebra (Mechanical)
15 Feb 07 21:14
Cummins came out with the 4.5L 4BT to try and meet tier3 off-road emissions without resorting to computer control for price sensitive OEM's. I'm not sure how easily you will be able to find one though.

Good luck. ISZ
IceStationZebra (Mechanical)
15 Feb 07 21:49
I did a little digging on the Cummins site. It mentions that the 4.5 just has a longer stroke than the 3.9, so with a little backwards engineering you might be able to fit it into a 3.9 block - but I don't know any of the details, so don't blame me :)

4.5L QSB engines

4.5l 4BT engines (doesn't specify mechanical or electronic fuel control)

Also in case anyone wants to make a mechanical fueled QSB, there is a 4.5L bridge engine being put into Hyster/Yale forklifts. Don't ask about the details though - I don't know what it took. It just showed up from Indiana! And boy did it SHAKE~~~~~

starspangled (Automotive) (OP)
15 Feb 07 23:53
A 4BT STROKER!??!! Oh, man, that's got to be one stout little engine. Now I'll have to start filling the piggy again while I search out a crank!

Thanks much for the links, I'll study up on that engine. I'm beginning to think that part of my vibration problem is from use of a year 2000 6BT flywheel, designed for an engine with different balance characteristics. Vibration is throughout the rpm range indicating it may be external of engine? Whether the 6BT damper would help, due to 6 BT flywheel, I don't know. I believe it would be best to try the 4BT damper first and if that doesn't help look further into the flywheel question and the 6B damper.

Thanks again!

evelrod (Automotive)
16 Feb 07 14:09
Since you have already stated the obvious.....Have you had any of the "stuff" on your engine balanced?  I have a few years with a Cummins and I haven't seen any balance problems!
I also build and balance my own race engines and, over the last 50 years, have found many OEM parts that are 'not exactly' perfect.

starspangled (Automotive) (OP)
16 Feb 07 21:42
Good question, evelrod, bad answer: NOPE!

I hope to adjust the valves tomorrow to eliminate that as a possibility but doubt that's it. Injectors seem to be okay from my limited knowledge. Loosening any one causes roughness, smooths out when tightened.

It has the shake, but there is a distinct vibration that changes slightly with rpm which continues throughout the entire rpm range. There is a sweet spot, probably around 1,700 to 1,900 [no tach yet] where there is least vibration. Vibration can be felt through chassis, floorboards, seat, steering wheel [no P/S] etc. and is very noticeable.
IceStationZebra (Mechanical)
16 Feb 07 22:53
In regaurds to the mechanically fuel injected QSB motors - I just saw a sales flyer for IVECO which showed a 4 and 6 cylinder QSB with rotary fuel pumps. Could be a neat project down the road - take one early Dodge NA diesel, add 4 valves and 1 big turbo! ISZ
Fabrico (Automotive)
16 Feb 07 23:36

Why not look into a viscous damper? Many small to medium Diesels use them.

starspangled (Automotive) (OP)
16 Feb 07 23:43
That's a ridiculous idea, ISZ! I like it of course. Something similar crossed my mind when I went to those links you posted.

I am rather surprised that there aren't more folks pressing the limits on the little 4BT to discover its true power potential. Seeing 6BT running at 800 HP and about 1,500 ft lbs gives one pause for thought. That same state of tune would put the 3.9L in the neighborhood of 475-500 HP and 1,000 ft lbs torgue!

Far exceeds my meager income to even think about going there....
starspangled (Automotive) (OP)
16 Feb 07 23:51
Thanks, Fabrico, I've been considering just that. There is finally some competition in the aftermarket visous units so some are quite reasonably priced. It shouldn't take rocket science to adapt one, and if it does I can always ask here on this forum ha haha! Could solve all the problems.

I wish I could afford to have the rotating assembly balanced PLUS a fluid type damper. If I ever do rebuild it will be getting a bit of Mallory metal here and there, or maybe some QSB parts.
GregLocock (Automotive)
17 Feb 07 4:29
Well, before you waste your money, the TV damper does an entirely different thing to first order balance.

To me it sounds as though it is worth gettting your engine balanced before worrying about TVs.


Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

starspangled (Automotive) (OP)
17 Feb 07 17:15
Thanks,Greg, and you may be right. Problem is I just can't afford that at this point. The conversion to 4BT included swapping out and upgrading the entire drivetrain, running gear/suspension, steering, brakes and rewiring from bumper to bumper etc. and that ate up the whole pie.

I located a new 4BT damper at a very reasonable price so I guess I'll try that first. Future plans include a balance job but for now I'll just have to hope this works. Some new rubber isolators may help also. I'll try to let you guys know how well the damper works for your future reference.

Thanks for all your helpful replies fellas!

Tmoose (Mechanical)
19 Feb 07 19:07
I'd get a vibration analyst to measure the vibration at objectionable locations.  If the frequency is 1X engine rpm then engine balance has something to offer.

If the worst frequency is 2X rpm (very likely with big 4s) then engine balancing (which only effects primary or 1X)won't do much.

Might be able to rig up a reed vibrometer to judge the problem frequency, or search other threads for using sound cards and free signal processing hardware to help with indentification.
starspangled (Automotive) (OP)
19 Feb 07 19:43
Thanks, Tmoose. I live up in the sticks so about the most hi-tech stuff around is the town's 2 traffic lights. I might be able to find those services down in the valley so I'll check next time I go. I'll look into the sound card/signal processing hardware and see what comes up. Thanks again.
Tmoose (Mechanical)
22 Feb 07 12:49
One of the root causes of the secondary unbalance the rod length/stroke ratio.  The other is reciprocating mass. If an engine had infinitely long connecting rods (or weightless pistons, etc) it would have no secondary vibration.  A stroker motor is likely to have rods shorter or the same length as its little brother, with the result a "worse" RL ratio, and even more severe secondary vibration.  

starspangled (Automotive) (OP)
22 Feb 07 13:02
Well thanks! I really like this 'little' engine at present displacement and have no complaints. For what all it can do as-is I can't see it being worth the resources to bother stroking it. Continually impressed with the power curve and MILEAGE! Hard to beat in many respects. Thanks!
r2800 (Automotive)
26 Feb 07 8:38
An inline six cylinder engine is inherently balanced in both primary and secondary orders, as is a V12.Whereas an inline 4 cylinder engine is only balanced in its primaries, Hence a torsional vibration damper from one is not suitable for the other.
Careful attention to dynamic crankshaft balancing and mass matching of the pistons and connecting rods would certainly help.
If soft engine mounts are to be used then they should have sufficient hysterisis to prevent build up and radius arms should be used to reduce torque reaction.
starspangled (Automotive) (OP)
26 Feb 07 9:25
Thanks, r2800, for your reply. I believe I may have discovered the proper damper but need to check with Cummins for part number cross reference. There is an elastometric damper that is described as correct for both 4B and 6B, but of course that doesn't mean the seller fully understands the dynamics, which I'll leave to Cummins I guess.

At this point dynamic balancing is out of the question as it is a 56K engine and won't get torn apart for a long time. It may be that I just have to bite the bullet and get the pricey little fluid type damper, and also address isolator issues as you describe. I'd like to find a gel-rubber type isolator but there are some fitment problems there regarding frame and engine width interference.

Some have said that it may be an entirely different problem, that I just need to adjust the valves and possibly replace injectors or have them serviced. Didn't adjust valves yet but it's on the plate for this week. After that new injectors, and then more research on damper and mounts.

This water is deeper than I thought! Thanks again for your instruction and assistance. Bit by bit I'm learning more about these curious 4 cylinder diesels. In passing: how interesting to ponder a V12 diesel? Thanks!

starspangled (Automotive) (OP)
26 Feb 07 9:40
Hi all!

I started to go through the replies to click the button to thank people for their replies. I realize now that every single reply has valuable information and has contributed to my understanding. So, I stopped adding stars and just want to thank you ALL for your help! Every contribution is appreciated so thanks everyone and I appreciate the help.

r2800 (Automotive)
26 Feb 07 10:00
A V12 diesel has a heavenly sound all of its own and is remarkably smooth a Rolls Royce Condor is a good example as is the Maybach.
SomptingGuy (Automotive)
26 Feb 07 10:12
And if you want to know what Armaggedon sounds like, stand on the platform of a provincial station as an Intercity 125 screams past.
starspangled (Automotive) (OP)
26 Feb 07 10:24
Oh, the fascinations in these mechanical things! I hope in my travels to hear at least one of each. I love the sound of a big aircraft radial, too. Funny how some engines are so impressive for the remarkable quiet of their subdued power while another thrills at it's peak power curve. I suppose it doesn't much matter which one when you're a hopeless gearhead and I confess, I am!
r2800 (Automotive)
26 Feb 07 11:14
You are spot on there is nothing quite like all the seemingly disorganised clatter,rumble and racket of big capcity engines, aero engines have a romance all of their own Pratt & Whitney,Wright and Rolls Royce and all the others too.
The shrill howl of a Merlin at speed is without doubt one of the most fantastic sounds, I'm getting goose bumps as I write this.
starspangled (Automotive) (OP)
26 Feb 07 11:27
Yes! I once attended a race for Unlimited Hydroplanes and I don't think I've recovered yet 30 years later hah! That noise echoing off the waters is beyond description. Merlin's are truly a work of mechanical art with few others that even come close.

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