Strange Vibration at 1200 - 1400 RPM in V8

[RossABQ]

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!

 

[Tmoose]

Have you watched the timing and dwell yet?

 

[RossABQ]

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.

 

[HRHtex]

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.

 

[MikeHalloran]

I suspect the alternator is responding, not resonating.

I.e., the source is elsewhere.


Mike Halloran
Pembroke Pines, FL, USA

 

[thruthefence]

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

 

[evelrod]

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

 

[RossABQ]

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!

 

[Tmoose]

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.

 

[RossABQ]

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.

 

[HRHtex]

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.

 

[LionelHutz]

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.

 

[Tmoose]

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.