I hear guys talking about using a 292 Chevy harmonic damper on a 261 Chevy.I know it will press on but.......My 261 project has a stock damper that's 50 years old and needs to be "rebuilt" Before spending 150 bucks on the old one,a new 292 damper is 75 bucks or so depending on style.It heavier if that's a good thing,comes in a variety of with belt grooves in different locations.
The trick is,will it fit without machine work which cost more than the damper.Cutting off a little from the snout is easy of course.Thanks.

The engines aren't that close.

Here's a quote from a tech article here:

"Harmonic Balancer: A 3 groove 292 Chevy 6 balancer, part #10141202 makes an excellent, and improved, replacement for the not so good 261 unit. It should have the rear groove turned off and it’s snout shortened slightly. There was an article on this in an earlier 12 Port News. Or for a Stamped, legal size envelope, I can send you a photocopy."

Here's the whole article:

http://www.inliners.org/tech/261_build.htm

You might find a high school machine shop willing to do it.

We disagree.
They used it because they didn't know any better, it was available, it fits, and it appears logical ("A, therefore B").
The design factors differ quite a bit between the 2 engines, especially as to length, stroke, counterweight placement, number of mains, and journal sizes. Other than the brand and firing order they're not very close.
This means that the dangerous RPM points ("orders") in the engine where destructive crank harmonics occur will not be at the points the damper is tuned to "see", they'll be above or below. Prolonged cruising at a critical RPM causes breakage if not damped.
It's not going to blow up, and it's better than none, or a know-to-be-bad original.
It's just not as good as the correct damper.

Tht's why I asked the question.I'm aware of the harmonics and the twisting and untwisting of a long crankshaft.Any stock damper is at least 40 plus years old and the rubber "dampening" is likely gone away.I have sent dampers to be rebuilt,what are the odds of the correct rubber being put back in so the damper is tuned correctly?
I have a nice looking 270 GMC damper sitting here,that might be fine on a 261 Chevy,or not.

Panic,

I've seen the small block V-8 dampners suggested for the 250-292. Using your explanation for the difference between the 261 and 292, why do the small block dampners work on the 250-292? They would not seem to be a good choice.

Thanks,

Dave

very interesting subject! thanks for sharing the knowledge.

So the bottom line may be that the old but correct damper is worn out so to speak,a damper "rebuilt" by specialists might not be tuned correctly ,or damper from another engine is probably all wrong.
So...how come we don't see more people running over their crankshafts? Pure luck of not running at a critical rpm?

You Know gents, if that 12 Port tech article was written by some Yahoo back yard mechanic, I would lean toward Panics assessment, but it was not. Pat Smith was technician of the year for many years and the crew chief for Dyson Racing. He is not just saying what might work, he is telling you what will work. Pay close attention to the type of 292 harmonic dampener you find, they are not all equal. The good one is hot bonded. A very good source of info on this is Tom Langdon @ Stovebolt. These are people with hands on experience, BTW, Tom was deeply involved with GM's advanced powerplant division. These are serious people with excellent advice to follow.

There is an engine builder that goes to my church that will wax eloquent for hours on topics like this - when asked for his thoughts on this, he basically said that for the most part, automotive "Crankshaft Torsional Absorbers" are tuned to a specific frequency predicated on the engine design as produced at the factory. . . and it is affected by flywheel / clutch inertia, transmission type, piston weight, crankshaft counterweights, bla bla bla . . . his thought is that with any non-stock motor and drivetrain that it is highly unlikely that the "CTA" that is on the motor is optimized for the new resonant frequencies generated by the modified rotating assembly. Or something like that

In other words - his thoughts are to get something that damps out frequencies that are reasonably close to what the motor had when stock - if you have a lightened things up - find one with a higher natural frequency - if you have gone with a heavier clutch setup going into a truck from a car - look for a unit with a lower natural frequency.

If anyone has interest, he has taken the time to put his thoughts on the web related to this . . . http://www.epi-eng.com/piston_engine_technology/crankshaft_torsional_absorbers.htm


Also - if anyone ever wants to know the natural frequency of a damper, I can easily set up a test bench and conduct a sine sweep and find the resonant frequencies pretty quickly of the damper itself.

That rather ends my interest in the subject. Thanks for playing our game.

Am I missing something here? Why did you delete everything you wrote on this topic? (and why do you make a habit of doing that so often on this site?)

If this is a game - why are you the only one playing it?

I didn't bookmark the url panic posted earlier, can someone repost it for me?

Thanks,
Stephen

There's always this one.

http://www.bhjdynamics.com/index.php?main_page=product_info&cPath=1_2_35&products_id=39

Kinda pricey, makes the $150 rebuild of the stock damper look cheap.

FWIW, I have no affiliation with BHJ or any of their dealer network.

Ken,thanks for posting that link,I've read some of his other stuff too. Well,many of us have taken long rides in modified inline engines,mostly 50 year old 4 main bearing inline 6's with millions of cycles on the crankshaft.Droning along for hours at 2000-2800 rpm.The harmonic dampers are just old ones with fresh paint or "rebuilt" by one of the several places that specialize in it.These rebuilders appear to use an injected silicone rubber and don't have have a room full of engineers to figure out all this "dampening" logic.Some engine have lighter pistons,some are carefully balanced,but overall I'm not hearing about crankshaft failure.
So...I suppose you might say it's not that big an issue on a street engine not spending more a few seconds at high rpm.

This seems to be the BHJ technical article that panic had posted. Seems to be in agreement with the EPI article.

In the BHJ technical article the last few paragraphs says it all.More or less ,when in doubt,run a stock type harmonic damper on a modified street engine.
Panic likely deleted his posts because they were in contrast to this final statement,an article he posted?.Personally,contrast is good,makes you think about options to common wisdom.And while BHJ is a harmonic after market company,their engineer recomends using a stock stype damper on most street engines.

i have been following this enlightening conversation with interest and, as always, i am getting an education.

i am interested not least because -- on the recommendation of Tom Langdon -- we mounted a Fluidamper on our GMC 302. That engine has run extensively on the dyno but only a very few miles on the road as yet. i have not detected any vibration, but now i'll be feeling and listening for it. i'm sure that Fluidamper was intended for a V8, and i hope that it will not prove to be an expensive mistake.

God's Peace to you.

d
Inliner #1450

There was an article in Inliners about 302 used for circle track racing.The engine builder claimed the Fluid Damper caused bearing wear issues and maybe crank damage.He switched to a ATI elastomer type damper and solved the problem.This was a racing engine,not a street engine,might make a big difference.But I wouldn't run anything on an engine recomended by anyone unless they have actual experience on their own engines.

FWIW - I had a stock balancer rebuilt by a guy named Dale in Oregon. From appearances, he does good work. I have put just over 12,000 miles on that rebuild over the last 2-1/2 years. Some of it buzzing along at 2000 rpm in OD running 65 mph - easy duty. Some of it doing stoplight drags and the occasional speed run. The motor has seen 4500 rpm hundreds of times and 5000 rpm more than a dozen times. So far no signs of problems with the damper or the motor.

The crank resonant frequencies that are served up by an old inline six where weight is high and stiffness is low are very low in comparison to modern engines. . . you need to keep that in mind when looking at options.

Thank you Hoyt, I wanted to compare them to other articles I find.

I too, based upon a recommendation posted here a couple of years ago, put a FluidDamper on my engine. (Which has yet to run, so no experience points.)

Even though the FluidDamper is not necessarily tuned to the engine (although it is marketed for the 292) I was comfortable with that sugggestion as it seemed to me that the FluidDamper would be more universally applicable to a modified engine.

My reasoning:
The conventional damper is a tuned mass/spring, and as such will resonate to particular frequencies. The period of the crank resonant frequency absolutely cannot match the damper resonant frequency or they would couple and uncontrolably add. The conventional damper elastic medium doesn't consume (generally) energy; it adsorbs and returns vibrational energy from and to the crank, but at a different time and rate than the cranks natural period. In other words, it would seem that the damper isn't really a 'damper' but a vibrational reflector that scatters the energy pulses over time to prevent build up at certain frequencies.

As mentioned previously; any changes to the engines design rotating assembly is inevitably going to change the resonant periods of the crank, and it would seem to be pretty much a crap shoot whether that change is going to move the resonant frequency of the rotating assembly away from or closer to the resonant frequencies of the damper..

The FluidDamper on the other hand, is not a 'tuned' damper as I understand it. It is simply a fairly large mass buffered by a viscous fluid. When vibration excites movement of the mass relative to the housing, the fluid shears, and actually consumes energy; it doesn't return energy to the crank. Consequently, it seemed to me that it should act as a non-frequency-specific broadband damper that should continue to work regardless of changes to the resonant frequencies of the rotation assembly.

Since buying and installing the FluidDamper, I've heard of some failures that now have me wondering, although TLowe's crank snout wallowing failure might actually be an indication that the FluidDamper was so successfully consuming energy that the wallowing might have been a side effect of saving the crank... Difficult to know.

I've read through the recorded processes needed to flight-qualify the Wright and P&W twin row radials prior to WWII. Crank dyanmics are increadibly convoluted, and that was on short two-throw cranks. Our relatively limber and long cranks are no doubt spectacularly difficult.

Yes and just the same for the aircraft "Inlines" (Allison and Merlin V-12's) All kinds of harmonics mostly controlled by friction dampers and asessories driven by torsional shafts.
I'm rebuilding a F head Willys (Jeep) engine at the moment.This 4 cylinder engine with a redline of about 4000 rpm has no damper.

I am going to shoot my mouth off here a bit at the risk of getting run into the ground by profesional mechanics and engineers. But here goes....

I am a dentist. I have no formal training in engineering or mechanics. i have had a 235 since high school and I like them. That is my only qualification: I like the 235 series engines.

I helped develop that BHJ balancer. It was for the Webrodder series I was helping Frohmader do on 235's. Then Frohmader turned into a dick causing the series to go nowhere. But that is another discussion for the future....

When the time of the build up came to do the balancing, one complaint I had was that there was no GOOD balancer for a 235 much less one that had 2 grooves (Brian called them "shivs") for this series. Well, together we came up with what BHJ produces today. there were some arguments along the way, but suffice it to say it is produced today for us. Yes, it is expensive but IMHO it is the best thing out there for harmonics for us

A separate argument, and the one I have more trouble with than the discussion about the science of balancers is about the credibility of parts for 235's (and others) if they are or are not published in the 12 port news. This will probably get censored but I dont care because its the truth. I submitted several articles on parts such as this for publication and they got nowhere with the 12 port news. I was never told why, they were just thrown in file 13. I dont get one nickel from BHJ producing and selling this balancer. I needed a 2 groove balancer and they asked me to help, so I did. But I still feel that this is one piece that we really need for our engines rather than alter a 292 balancer. Bur it seems if it is not approved by the "team" there is no ink. and that is too bad, - this thing flat works (my 56 is proof). I had nothing to gain by selling it - only to help inliners. It got nowhere because the powers that be decided so. And yet the reference that gets continually cited is a reference to an old article done by a 12 port advertiser for profit who alters a production balancer and says its great. And he has no science behind it!! Everyone cites this article as "proof" because its in the 12 port news. I dont get it.


Argue all you want - this piece has the science behind it. Call Brian Clarke at BHJ and ask him.

I don't have a dog in this fight. I'm pretty much a Ford man. But I have run damper development tests on a dynamometer. If the 12PN advertiser modified a 292 damper it could be so overkill in the RPM range that 235s normally operate in that it will work fine. If the cost is less than an engine-specific more pricey damper for the 235 I call that "value engineering".