×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Strange Vibration at 1200 - 1400 RPM in V8

Strange Vibration at 1200 - 1400 RPM in V8

Strange Vibration at 1200 - 1400 RPM in V8

(OP)
I'd appreciate some engineering perspective on a problem my engine has had for a while (ever since I've owned it).  I'm not looking for normal mechanic-type troubleshooting advice, people intimately familiar with these engines have weighed in and come up with nothing.  It is very unusual.

My '52 Ford truck's flathead V8 has a vibration at the specific RPMs listed, and only there.  The vibration seems like a couple fore-aft, rather than side-to-side, as it wags the alternator that way (which is mounted rather high in front and on a 3/8"-thick plate) visibly -- think "pendulum".  

I've had the engine nearly completely apart; have not removed the crank.  All bearing clearances are in spec. The flywheel has been balanced with the clutch mounted. Compression is uniform within 5 - 10 psi across all cylinders. All plugs are burning clean. Oil pressure is great, about 50 psi at any speed above 1,000 RPM, cold or hot.  I have taken off all drive belts and it has no impact on the vibration, so it is clearly something inside the block.

The only things I've found are:
1.  one of the connecting rods is not original.  There are two of them stamped as #2, with the odd one at the #3 position.  All are the correct piece for the engine.  Since the engine was rebuilt in the '70's, I'd suppose the #3 was replaced for some reason with another stock piece.  No indications on the bearings of a twisted or bent rod, all pop up to the same deck height.

2.  cylinder wear is at the limits, tapered from .005 - .009".  These use 4-ring, long-skirt pistons with fairly high tension by modern standards.
 
These engines are a 3-main-bearing design, 6.8:1 compression ratio, side-valve arrangement. I did not know this problem existed when I had the engine apart, so I didn't weigh the piston assemblies, it was a clean-up operation.

My question really is, is vibration at this range indicative of a specific type of problem, e.g., a once per rev harmonic?  A bent crank? Could it be pistons rattling in the bores? I'd love to know what to look for before resorting to a complete teardown and rebuild.  These are not cheap to rebuild.  Other than this vibration, the engine runs great.

Any ideas would be welcome!

RE: Strange Vibration at 1200 - 1400 RPM in V8

I didn't notice that my LT1 had a vibration at 2000 rpm until my friend Ted pointed out that his did.  It never got worse, or better.  It may not be possible to counter _every_ imbalance that exists in an engine, especially with a three-bearing crank.

Drive the bitch till she breaks.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
Maybe I should clarify, this is bad enough to rattle the entire exhaust, all the connecting linkages, even the pedals.  It hasn't gotten worse or better in 3 yrs, but is not typical of these engines.  With their low compression, huge flywheels, they are usually silky smooth.

RE: Strange Vibration at 1200 - 1400 RPM in V8

Ross, what you describe is decidedly not common.  My first engine was my '49 Merc.  With the lack of skills as a kid of 18 it came out smooth as silk, even with all the mods I made. The only problem I had with vibration was a bent (?) 1st motion shaft (input shaft) in the trans.  It was enough to shake the whole car but stopped when the clutch was depressed.

The stock rods are often swapped and most modern rebuilders use French rods or custom. My '53 59AB was finished Thursday the 20th.  No vibrations on the test stand, but it will be fitted with a tq converter to C4, so no flywheel/clutch to bother with.  Your correct about the cost, the flathead has become an "exotic"...$7909 with my stock crank and rods.

Before a complete teardown, maybe try another flywheel or ck runout on yours.  You said it was balanced?  Stock flywheels are a dime a dozen around here, no one uses stock any longer. Other than that, it sounds like it's time to take the crank out!!!

Common problems in a flathead were blown head gaskets...Worst case, a cylinder would hydraulic and bend a rod.  However, you said all pistons came up equally. ???  Besides, I don't think that would cause the vibration you speak of.

I will pick up my engine at H&H next week, I'll ask Mike about your vibration.  He's the expert these days.

When your tear it down, let us know what you find. Good luck.

Rod
 

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
" Your correct about the cost, the flathead has become an "exotic"...$7909 with my stock crank and rods.."  -- YIKES!  Did you pop for a blower, or lots of eye candy?!?! No question H&H is top drawer.  

Joe Abbin of Roadrunner Engrg is here in town and I had him listen to it, he was puzzled, too.  What puzzles me is the RPM it occurs at, it seems low for most kinds of balance problems, which I think would include a bent crank nor a bent crank flange.  I'll check the runout, although it was surfaced correctly, referenced off the flange area.  The balancer removed material 180 deg. from where the factory did... always seemed suspicious to me.



 

RE: Strange Vibration at 1200 - 1400 RPM in V8

No blower it would not fit the image I want in my 23. Exotic is right.  I build modern (well, sorta modern) engines here at my shop, but it was actually cheaper for H&H to rebuild it than it would have cost me just for the parts...Never mind all the hassle of running all over chasing parts.

I had it done to look like the 4th and 5th engine on the H&H site.  All black.  Offy heads with just the tops of the fins polished and chrome acorn nuts.  All the bells and whistles on the inside, reground cam, adjustable lifters, Manley valves, etc. My core was a '53 out of a wagon that only had ~60k miles on it.  Actually, I could have used it as is, but I wanted something a bit better.  I sold my Metropolitan for $8k so, that was the budget.

Give Mike a call, he's a nice kid. He is really into flathead V8's.  One on the bench w/supercharger and bling was $28 large!!!

Rod

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
Very nice TrackT -- and is that a Dominator I see behind it??

RE: Strange Vibration at 1200 - 1400 RPM in V8

1948 Norton Inter/ES-2, Ex Bill Young, four time TT champ with the Norton Manx team.  Hopefully I'll have it up on Ebay soon.

Rod

RE: Strange Vibration at 1200 - 1400 RPM in V8

>>>  The balancer removed material 180 deg. from where the factory did... always seemed suspicious to me.
<<<

... and to me.

 

Mike Halloran
Pembroke Pines, FL, USA

RE: Strange Vibration at 1200 - 1400 RPM in V8

Hmmmm.  Yes, especially if the "clutch and flywheel were balanced together"...AND then the balance marks were not aligned.

Rod
 

RE: Strange Vibration at 1200 - 1400 RPM in V8

I agree with the strange weight removal.  Its been too many years to remember if the front pulley/balancer is rubber sleeved, and thats where I would put my two cents worth.

Franz

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

RE: Strange Vibration at 1200 - 1400 RPM in V8

From the description is sounds like it is strictly rpm related, not load or road speed?  How about when accelerating a bit in high gear at that speed uphill?

Is this your setup?
https://www.nostalgicairparts.com/ac/ford-flathead-alternator-bracket-124.php
I'd run it for a minute with the alternator belt removed, then with the alternator heavily braced, and then with the alternator removed.

Points triggered ignition?  I'd watch the ignition timing and dwell to see if something goes unsteady in the 1200-1400 rpm range.  Also manifold vacuum.  I might cup my hand over the carb throat or pull the choke slightly when it goes unsteady to richen the mixture, just to see what happens.

My preference is to ID the frequency of the vibration.(s)
If mechanical unbalance is behind it, the vibration will be 1X. The opposite is not necessarily true.
A real vibration analyzer is best.
A reed tach like this might be enlightening.
http://www.shop.com/Genuine+Briggs+Stratton+19200+Tachometer-398088266-p+.xhtml

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
Thanks for the responses.

No harmonic balancer, plain old cast iron sheave.

The clutch and flywheel were balanced individually, then together, with clear markings for reassembly.  He added two pieces of 3/8"-dia. rod, a little over an inch long, to the clutch cover to get it balanced.  I also just realized that this vibration occured with a completely different clutch assembly (running a diaphragm clutch now, previously had a Long-type stock clutch).

Yes, that slingshot is what mounts the alternator.  I used a shorter belt in an attempt to get it lower (less pendulum), no difference.  I have run it with no belts at all, no difference in RPM that it occurs at or in amplitude.  I have wondered if the alternator is picking up a vibration and amplifying it?  But this type of mount is very common and no one else is complaining about vibrations.

Tmoose, I am not clear on what the reed tach would show?  

 

RE: Strange Vibration at 1200 - 1400 RPM in V8

the reed tach would show the frequency of the vibration, and to some extent amplitude.
If the big vibration is at crank rotating frequency, unbalance is a suspect. If the predominant vibration is at some other rpm/frequency, then the diagnosis changes
 

RE: Strange Vibration at 1200 - 1400 RPM in V8

We use vibration analysis on our equipment all the time (Paper Mill). So there may be some sources for vibration testing in your area other than automotive. Once you know if the vibration is one times crank revolution etc. you can do the math to figure out what part of the motor might be the actual vibration source and not just some multiple of the frequency like the alternator shaking.

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
I'll check the flywheel runout and run it with the alternator off the engine this weekend (my daily driver is down).  Thanks for the ideas.  I'll see if I can scrounge up some vibration measuring equipment.

RE: Strange Vibration at 1200 - 1400 RPM in V8

Do you know anyone at the around your local airport in the maintenance business? There may be someone there who can hook up a vibration survey on the car. Aircraft Mechanics are usually gearheads, and might enjoy seeing a running L head Ford.

Here's a spectrum analyzer, used to be state of the art for balancing Helicopter rotors & propellers. Now coming on the market fairly cheap, I paid $1000.00 for a full kit. Hook a couple of velometers up, and run the engine. Little plotter shows the peak IPS vibration and rpm where it's occurring.

http://www.used-helicopter.net/wp-content/uploads/2010/12/-19241206534520460.jpg

no financial interest
 

RE: Strange Vibration at 1200 - 1400 RPM in V8

It sounds like some kind of resonance to me, possibly something to do with the sprung centre clutch plate or engine mounts.

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
It does this in neutral, clutch in or clutch out.  Motor mounts have less than 2,000 miles on them.

RE: Strange Vibration at 1200 - 1400 RPM in V8

Have you watched the timing and dwell yet?   

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
It has an MSD electronic dizzy and timing is very steady with advance working properly.

I am anxious to completely remove the alternator and run it.  The slingshot-type mount these use is 3/8"-thick plate.  The stock generator mount is a casting that is deeply ribbed to stiffen it against exactly the type of wagging I'm seeing.  I've previously only removed the belt off it.  It is possible it is picking up a natural fore-aft force and resonating.  If that is the case some stiffening ribs can be welded on.

RE: Strange Vibration at 1200 - 1400 RPM in V8


  Even if the alternator (and its mount) is not the problem (and I suspect it is) - surely that mount is very poor engineering practice - I am surprised it works at all.   

 

RE: Strange Vibration at 1200 - 1400 RPM in V8

I suspect the alternator is responding, not resonating.

I.e., the source is elsewhere.
 

Mike Halloran
Pembroke Pines, FL, USA

RE: Strange Vibration at 1200 - 1400 RPM in V8

Rod, that is a nice ride! I  love the "Indy Roadster" look of the front end. What were your influences?

RE: Strange Vibration at 1200 - 1400 RPM in V8

I got interested in sprint cars, midgets in the 50's with my cousins. Always wanted to do a street roadster in the style of a sprint car. Life has a habit of giving you what you need instead of what you want. I'm involved in vintage racing these days and so many of the old sprint, midget, Indy roadsters are being returned to a useful live again...Temptation?

To make a long story a bit shorter... I found an old 50's/60's Kellison bodied dirt car from Oregon. The "real deal"... Too much to resist.  First engine was a Esslinger Pro4 which I promptly broke.  New engine will be my '53 flathead V8.
My son's an automotive artist and did the paint and body work. I did the grill from a raw sand casting. Frank's Hot Rod Upholstery of Temecula did the camel leather. H&H Flatheads did the new engine.
I should have it running again late summer.

http://www.youtube.com/watch?v=1uspjugZUKU

Rod

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
Well, I have the answer:  I removed the alternator completely and there is almost no discernable vibration. I had a trouble light hanging from the hood while I ran it and it picked up some vibration at around the same RPM, but I was running with the choke on and it wasn't running very cleanly (and since the alternator belt also runs one water pump, I couldn't let it warm up).

As soon as I hefted the alternator out of the bracket I figured that was the case -- it isn't light (GM internally regulated 100-amp unit), at least 20 lbs.  The bracket is simply not able to resist fore-aft vibration despite being 7/16"-thick.  So I will weld a flat bar 1" deep along the edges of the slingshot to give it more stiffness fore-aft, and if necessary add a brace back to the intake manifold.

Thanks for all your help!

RE: Strange Vibration at 1200 - 1400 RPM in V8

I'd still like to know if the frequency of vibration is engine rpm, or "something else".

Lots of folks have had several bouts of expensive work done on driveshafts AND wheels-n-tires trying to kill a frustrating vibration.  But 10 minutes work with a vibration analyzer usually clearly picks up wheel speed ( around 800 rpm) or driveshaft speed (2000-3000 rpm) and pretty much eliminates one group from the lineup.   

RE: Strange Vibration at 1200 - 1400 RPM in V8

(OP)
I'd like to know too, and when the weather clears, I'll pursue analysis at the local airport or with some folks at a local plant.  Right now, it's 8 deg.F. which is 10 deg. below the ASHRAE 99% Low.  I'm curious but not THAT motivated.

RE: Strange Vibration at 1200 - 1400 RPM in V8

Where I am (in Oz) it hasn't been under 30 deg.C for a week - in some areas it hasn't been under 40 deg.C for a week.   

RE: Strange Vibration at 1200 - 1400 RPM in V8

A brace from the back of the alternator to the engine would likely help a lot. Running on an angle approximating something around 45 degrees and going from the alternator backwards to the engine would likely make it even more effective.

Still, like others, I doubt that the alternator itself is the actual problem. But, you can stop it from amplifying the vibration which could make the vibration acceptable.

RE: Strange Vibration at 1200 - 1400 RPM in V8

Something to consider with belt driven equipment is belt fat spots and pulley/sheave eccentricity creating once per rev "tugs" of extra belt tension that can excite all kinds of things. Triggered strobe lights used to be standard equipment with vibration analyzers, and made locating guilty components "stand still." One time we selected a few sets of "good" v-belts (one just for today and a few years worth of spares for the future) for one of those fancy bug killing HVAC units by measuring the vibration on the hospital director's desk.

Running without the belt pretty much eliminated all those items.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources