Harmonic Vibration/resonance

[Murd]

My engine has a high pitched harmonic vibration or resonance between 2000-2100 rpm, always has since I got it on the road a year ago. It happens whether driving or parked so unrelated to the rest of the drive train. I occurs both while reving up or coming back down through that rev range, throttle part open or closed. when parked with the hood up you can see and hear the air cleaner top buzz like crazy.

The engine runs smooth at all other RPMs, which leads me to think it's not an engine balance issue? It is a stock '88 5.0 short block/rotating assembly with a new stock replacement harmonic balancer. I am missing a couple teeth on the flywheel ring gear, not sure if that's enough to cause this? My only engine Accessories are the water pump and Alternator, I haven't tried removing the belt but imagine that's a good place to start?

Some searching seems to indicate this could be a resonance from the exhaust system, read someone's old post that they were told it had to do with the side pipes.

I try to not operate in that rev range as I'm afraid it's causing damage, however that makes it really annoying on the hwy in 5th as that range covers about 105-115 km/h that traffic often flows at.

Questions would be:

Anyone else experience this?

What is causing this?

Is there a fairly simple fix? Timing change?

Most important, am I hurting anything? I can live with it, especially at hwy speeds you can't really hear it.


Thanks in advance for your help!

[Gordon Levy]

The balance is off, usually ether the balancer is wrong or the wrong weight is on the flywheel. A stock 5.0 is a 50oz balance. If you are using the original balancer it may have spun. What flywheel do you have?

[Murd]

Gordon,
It has a Powerbond PB1084N 50oz imbalance OEM replacement harmonic balancer, bought it new during my build, now with 3000 miles on it. I first noticed this shortly after I got the car on the road, less than 500 miles in. When you say spun, do you mean the outer weight spun on the rubber damper connecting it to the inner hub or that it's come out of the keyway on the crank.
Flywheel is whatever was on it when I pulled it from the mustang, I assume stock, I do recall seeing Ford cast into it. It does have a newer King Cobra clutch that also came on the donor.

If it was engine balance, would I not feel vibration at other engine speeds as well?

Thanks,

[Gordon Levy]

No, the vibration would fall at those revs, like an out of balance tire. It higher revs you generally don't feel it. Did you use the location pins for the pressure plate?

[Murd]

I recall lining up the pins into the pressure plate, although now of course I would question everything. I can probably get a view through the clutch fork opening to confirm, correct?

[Murd]

would missing teeth on the ring gear/flywheel be enough to cause it?

[Murd]

I just went back to two year old pics I took when mounting the clutch, bare flywheel has 3 equally spaced alignment pins, with the cluctch mounted I can only see one of them, possibly a second due to the angle of the photo, I'm quite sure its lined up with all 3 and not mounted on top of one of them, I pay more attention to detail than to do that.

I only see one broken tooth on the ring gear.

[Gordon Levy]

1 broken tooth won't do that. Now you probably need to dig further into it

[Murd]

110803110804

Gordon,
Thanks for the help, attached are pics of my flywheel and clutch mounted to it. Don't worry about the missing engine plate, it made it on. Opps…..

When you say dig deeper, do you mean internal?

Any chance it's an accessory? worth the time to pull the belt off?

Should there be a line on the balancer to indicate if it has spun?

How damaging is this vibration and am I taking a big risk continuing to drive it?

Thanks again

[Boydster]

I would take the belt off just to verify. Not hard and it eliminates that whole front end of accessories.

[Murd]

Checked it with the belt off. Exactly the same.

[j.miller]

If you feel it in your butt it's in the back of the car. If you feel it in your hands it's in the front of the car. Everything vibrates (that's physics). What you are experiencing is two vibrations waves crossing at a particular RPM range. To get rid of it you only need to eliminate one. RPM range is high for tire imbalance and the fact that it does it driving or not eliminates drive shaft imbalance. Accessories like power steering, alternator, pullies is generally in the 35-5k range. So it sounds like you are back to balancer, flywheel, clutch assy. I would barrow a balancer from someone to start the elimination process. New doesn't mean good. A new short block can have an imbalance in the crank.....if it can happen it does and it will...da bat

[BadAsp427]

So I have the exact harmonic vibration right at 2200 rpm. I only feel it through my side pipe bracket that was up next too and pushing on the body. I put a piece of rubber heater hose on the bracket between the body and the bracket and the issue is 99.5% gone... I think the only reason I feel it now is that I look for it. Once I get my Gas-n Pipes on, I'll adjust the brackets to that it is not tight up against the body.. (it's not suppose to be anyway) and then I should not have any problem with the vibration.

[SJDave]

I had similar vibration, at same RPM range with a King Cobra Clutch mounted to a billet steel flywheel with all the teeth. Turned out the pressure plate assembly was out of balance quite a bit, worst that my local engine builder had ever seen. His balance machine almost ran out of range to test it.
In the attached photo you can see the lump of steel welded to the pressure plate housing to bring it into balance. This amount of steel looks like it's in the same ballpark as the mass of a tooth on the flywheel. If the two missing teeth are in the same on the same side of the flywheel, that is even worse situation.

Unfortunately to fix this you have to pull it apart, take the pressure plate to an engine builder who can check the balance dynamically. If the pressure plate is OK....then you know it's the flywheel...maybe time for new one.

Before I fixed mine, I could move the resonance point by disconnecting the side pipe hangers, that move it down a couple hundred RPM. As I played with it more, I started reading stuff on Mustang forums about big weights added to the differential housing and shock absorbers from the engine flex plate to the frame, to quiet down torsional vibration on Mustangs. I actually bought some shocks to do this but never installed them. It was the worst in 5th gear also, almost unnoticeable in 1st, 2nd and 3rd.

HTH,
Dave

[Railroad]

I do not know what it is about 2200 rpm, but I have 2 rides that do that.
#1 is a 351W in a 66 Mustang, C4, 9" 3.50 gears, stock drive shaft. Engine bored, balanced, new harmonic balancer, Holley projection throttle body. About 2200 rpm in 3rd gear, I get rising and falling high frequency buzz.
#2. Is a 99 Mustang Cobra. Car came from the factory with the buzz. Ford sent a NHV tech to each dealer that reported selling cars with the issue. Tech described the issue as a stacking effect. They clocked the driveshaft on the rear and the front, but could not eliminate the issue. They shipped a new driveline, trans, driveshaft and rear end, IRS. Set up was tested on a car at Detroit, removed and installed on mine. All was good. I eventually changed to an alum flywheel and driveshaft, improvement in throttle response, still no vibs.
I know the OP has narrowed it down to the engine and believe his issue is probably like mine on the 351w. I cannot speak for the quality of the balance job on my engine, so very well could be the cause.
I will be watching to see if anything comes up.

[Murd]

Thanks for the replies all,
Jeff, I agree trying another balancer would be the next step, already contacted a buddy yesterday who uses a lot of 5.0s in hot rods he builds as a side business, he has a couple in crates he said we can pull a balancer off for me to borrow. Try to get it done next week.

Kind of makes me feel better that I'm not the only one, I received a PM from another member who also has a 5.0 in his FFR, says it's been rebuilt twice (don't ask) by a reputable builder, he's tried everything and it won't go away, he mentioned the old post we both read about a Florida speed shop saying it was due to the exhaust setup. He said he's just decided that's how they are and living with it.

I shot a video of it, how do I post a video? Do I need to create a youtube or Vimeo account?

I'm still in touch with the guy I bought the donor from, he had the car a while and was still driving it when I bought it, he doesn't recall any vibration. The only rotating part I replaced was the balancer.

[Murd]

I finally got my buddy’s balancer last night, swapped it out this afternoon, exactly the same vibration at same rpm so harmonic balancer is eliminated.
I did add an eccentric for my mechanical fuel pump, I can’t imagine it’s heavy enough or off centre enough to be the cause?
That’s the only other rotating part I can think of I changed.

[rich grsc]

Just reread this post. You say the engine is a 1988? I thing the early pre-89 were a 28 oz flywheel, didn't they go to 50 oz when they made the HO 5.0? Original '88 crank?
Oops, should have did a quick search first, the 28 oz was pre-81, not 89. Still think you'll have to tear into the engine to solve it, or at the least have the balance checked on the flywheel.

[Murd]

87-93 were basically the same engine. I think they changed from 28 to 50 in the 70s?
Regardless if you look up the correct damper for an 88 on summit or anywhere else it’s 50.

[i.e.427]

What motor mounts? Stock rubber, polyurethane or solid?

[Murd]

The energy suspension poly mounts FFR supplies.

[Railroad]

Pull 1 muffler

[i.e.427]

The energy suspension poly mounts FFR supplies.

The previous owner probably didn't notice anything because the Mustang had factory rubber mounts. I've owned a number of mid 80's 5.0 Mustangs and have seen/had a mid rev vibration before. In one instance going to a ATI Rattler dampener solved the problem. Dealer had installed stock dampener 3 times and still had the vibration (luckily still under warranty). Fluidampr may also be another choice

[Murd]

The previous owner probably didn't notice anything because the Mustang had factory rubber mounts. I've owned a number of mid 80's 5.0 Mustangs and have seen/had a mid rev vibration before. In one instance going to a ATI Rattler dampener solved the problem. Dealer had installed stock dampener 3 times and still had the vibration (luckily still under warranty). Fluidampr may also be another choice
Thanks for the suggestion. Doing some research, looks like ATI super Damper and TCI rattler. Recall which it would have been?
Also looking into fluidampr. Seems all 3 have people love and hate them. All are pretty pricey too!

[Murd]

Pull 1 muffler


Definitely thought of this, those 4 bolts really suck! I will probably try this for kicks at some point.

[i.e.427]

Thanks for the suggestion. Doing some research, looks like ATI super Damper and TCI rattler. Recall which it would have been?
Also looking into fluidampr. Seems all 3 have people love and hate them. All are pretty pricey too!

ATI, TCI, FBI, CIA, it's the Rattler. https://www.summitracing.com/search/product-line/tci-rattler-torsional-vibration-absorbers

[Murd]

Thanks for clearing that up.

[sread]

My 86 - foxbody did the same thing after full rebuild / balance, and installation of Energy Suspension poly mounts. It's the mounts - they are very firm and don't dampen the vibrations like the stock pieces. I swapped balancers too trying to cure it - no change.