Cummins harmonic balancer
Cummins harmonic balancer
(OP)
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!
Starspangled
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!
Starspangled





RE: Cummins harmonic balancer
Pat Daly, where are you?
Rod
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
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!
Starspangled
RE: Cummins harmonic balancer
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
RE: Cummins harmonic balancer
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....
Starspangled
RE: Cummins harmonic balancer
Good luck. ISZ
RE: Cummins harmonic balancer
4.5L QSB engines
http://ww
4.5l 4BT engines (doesn't specify mechanical or electronic fuel control)
http://w
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~~~~~
ISZ
RE: Cummins harmonic balancer
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!
Starspangled
RE: Cummins harmonic balancer
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.
Rod
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
Why not look into a viscous damper? Many small to medium Diesels use them.
RE: Cummins harmonic balancer
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....
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
To me it sounds as though it is worth gettting your engine balanced before worrying about TVs.
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Cummins harmonic balancer
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!
Starspangled
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer
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
RE: Cummins harmonic balancer
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.
Starspangled
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
RE: Cummins harmonic balancer
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.
RE: Cummins harmonic balancer