Causes of 2100 RPM engine vibration.

[AL in NY]

I have a very loud and noticable engine vibration at 2100 RPM's which I can accelerate through and the engine returns to running smooth a glass. This vibration is not as noticable in 1st or 2nd gear, but increasingly noticible in 3rd and 4th(F150 OD trans).

I have just really started putting some big mileage on "my" new engine recently, that is how I figured out it was engine vibration and not engine knocking. I have been fighting a rear main oil leak since last spring which limited my driving until now. I made the trip to the 2012 New England Meet last weekend and put almost a 1000 miles on my 30 CCPU. Made the entire trip without any major problems, just had the drivers side window freeze up on the trip. But this trip really highlighted the vibration problem. Had noticed it before doing local driving, but didn't figure out it was engine vibration until this long trip.

My engine has a counter-weighted crankshaft drilled for pressurized oil and was balanced by Rich Falluca at Antique Engine Rebuilding. The flywheel was lighted to 34 pounds and balanced a few years ago by Bob Bouldin in Belfast, NY with another crankshaft. The rods are Rich's inserted rods and the pistons are Pete's modern Chevy style pistons made for the Model A.

My questions (finally) are: Is it possible I have some sort for crankshaft harmonics occurring at 2100 RPM's as a result of not having all my bottom-end parts balanced together? Has anyone else seen this type of vibration with a counter-balanced crankshaft?

The main and rod bearing clearances were checked in June and found to be .0015" on all. I have execellent oil pressure, 20 lbs at idle and 35 lbs above 1000 RPM's. The engine performs beautifully with plenty of power and it idles quietly at 400 RPM's. Right now I am trying not to run the engine very long at that RPM, but I am considering pulling the engine at the end of the driving season and having all the components balanced together.

[MikeK]

I vote harmonics. There is an inherent coupled imbalance in an I-4 design that cannot be eliminated by any degree of dynamic or static balancing. It is a rather narrow RPM band and can only be eliminated with a gear driven counter shaft.

The fact that it goes away at higher RPM attests to not having a primary balance issue. If you did, it would get worse with more RPM.

Changing the mass of pistons, rods, crank, etc. will shift the I-4 imbalance up or down. Henry's motor had it, but at a lower RPM. Even in "modern" I-4's with counter balance shafts all is not roses. The CB shaft cancels the harmonic at its peak, but actually ADDS imbalance in other RPM ranges. You end up with an engine that is slightly buzzy at all RPM's instead of smooth, very buzzy, smooth.

"Doctor, Doctor, it hurts when I jump on my left foot." Dr: "Well, then Don't do that!" Just drive through 2100 RPM and call it a day. Hopefully that's not at your favorite cruising speed, but Murphy's law may say otherwise.

Also, that may just be the harmonic point of your frame/body combo.

[AL in NY]

Thanks MikeK!!!!!!

[James Rogers]

Mike has it right. There are harmonics that are inherent in the design of the 180 degree opposed piston engines. If the engine had been designed with the crank and pistons at 90 degrees it would be smoother.

[Kohnke Rebabbitting]

The flywheel was lighted to 34 pounds and balanced a few years ago by Bob Bouldin in Belfast, NY with another crankshaft."END QUOTE"

If the other shaft had the Flywheel bolted on to it when balanced, it would be out of balance now. The crank shaft, and Flywheel should never be bolted together, to be balanced!

If the rods and pistons were not balanced, and also checked for alignment, that is also a source of out of balance!

If your shaft is weighted, and you know it is balanced corectly, there is nothing more you can do for the crankshaft, as long as it was ground on true centers!

The one thing that I didn't hear from you is what did you do about the pressure plate. We balance flywheeel, and pressure plate together, and then mark for assembly. If the flywheel was balanced, and the pressure plate just bolted on, you are way out of balance!

[Bruce Adams]

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Funny, I get a ROAR at 48 MPH on my Antique Engine's job with balanced shaft, lightened wheel, touring cam, bored valves, HC head, but I blow right through it, and its gome by 50MPH.
I do wonder WHY I get a ROAR at that RPM in third only with stock tranny and rear end. (No jump on the speedo)
It seems to happen most times while accelerating through that stage on my way to 55-60 cruise range. I DO NOT hear it when going to 35mph insecond.
Thanks for any explanations.

[AL in NY]

kohnke; I didn't mean to imply that the older crankshaft and flywheel where balanced as a unit, they weren't, but as for the flywheel/pressure plate balance now.....that was never done.

Between your reply and "mikek's" and "bruce's", I'm not so sure that rebalancing everything is going to fix the problem.

Will engine damage result if I leave it alone and just not run the engine very long at that RPM?

[J and M Machine]

Al:
The problem may lie in your flywheel /pressure plate.
it would be wise to have those balanced as a unit.
Since the flywheel was balanced, then you add an unknown balance "the clutch" to it then there is the problem.

When there is unbalance in the engine whether it be 4 cylinder 6 or 8 it's going to multiply and get worse as the engine accelerates.
Since you've never felt it around town it was there you just didn't make it to it's peak of harmonic distortion.

I would recommend you rebalance them together and then see if the vibration goes away.

[AL in NY]

J and M: as I stated originally, I can drive thru the vitration the same way "Bruce's" engine "roar" disappears by 50 MPH, my "roar/vibration" is gone at 2300 RPM's. Wouldn't a mis-balanced flywheel/pressure plate continue to cause vibration as the RPM's continue to increase?

[Chris in CT]

Hi Al, MikeK has nailed it pretty well. You have a number of very good suggestions above, all of which could reduce, but not necessarily eliminate, an harmonic resonance at a certain speed in your engine. Also, as stated above, making modifications to the existing rotating assembly may succeed at shifting, reducing or "smearing" the harmonic resonance, but not eliminating it.
If this thing is bothering you to the point that you feel you have to do something about it, then pull your engine and follow the excellent advice given above. Just keep in mind that what you will probably get is an improvement in engine vibration damping, but you will not completely eliminate it. Happy motoring!

Chris

-----------------------------

www.burlingtoncrankshaft.com

[Dave in MN]

Al,
When we balance an assembly, we complete it in this order:
1) Balance crankshaft (after grinding and facing the flange)
2) Balance the flywheel (after lightening and facing)
3) Add the pressure plate to the flywheel and balance again adding only to the pressure plate (as the flywheel is already balanced)
4) Assemble the crankshaft, flywheel and pressure plate and check the entire assembly for balance
5) Index all mating of parts with paint to make sure it is assembled as balanced

My touring engine has a Burlington crank, lightened flywheel, V-8 clutch cover, weight matched pistons and rods all balanced as above and I have a narrow band of vibration that shows up in the same range as you indicate...only noticeable in third gear. I know my assembly is right on...I agree with those above that say it is normal. Even knowing I did all I could to achieve good balance, it is always an anxious moment pushing through that RPM. I have a Mitchell overdrive so I can avoid running at my problem RPM.
Even though we are saying it is normal... I would still take it apart to eliminate the unknowns in your own system.
Good Day!
Dave in MN
www.durableperformance.net

[J and M Machine]

Al: depending on what the inbalance is the flywheel/clutch combination could attribute to that vibration and make it worse.
Typically the dynamics of an unbalanced engine get worse as the engine speeds up.
However the unbalance of the clutch and flywheel could be making your situation worse.
You'll never get rid of the 4 cylinder shake but by properly balancing all of the parts you'll move the resonance to a higher rpm level.
Still;Where the engine portion is balanced I'd recommend balancing clutch/flywheel as a unit as I have shown in the prior post.

[Jim Brierley]

MikeK most likely has it right. There are actually 3 vibration points in the A (or B) engine but the most notable is usually at 2300, not 2100 that you have. I built a balanced BB engine some time back thinking it would be the smoothest one I ever built, wrong! It had that vibe at 2300 so I just didn't drive at that RPM and put many miles on it with no problems. If you have a tach and run the engine at that RPM with no load, ie sitting in your driveway, you probably won't hear the vibe. Not sure if a front damper would help or not???

[steve s]

Mine has a vibration point at 1500 rpm, noticeable in all gears but mostly in 3rd. Also a Falluca rebuild with inserts and large intake valves; balanced crank, "secret" cam grind, and V8 clutch provided by Dick McKee many years ago; Cling's serpentine fan belt system.

FWIW, Dennis Cling claimed that the stock fan belt can be significant source of perceived vibration, with its own harmonics.

Regarding Jim's damper question, I had been running a damper for many years before the Antique Auto rebuild, and, although I can't be positive, I don't recall noticing that 1500 rpm vibration with it on. However, I also recall thinking when I first installed it that I couldn't tell any difference. My understanding is that it dampens torsional vibrations along the length of the crank, which the crank feels but are not so much transmitted to the rest of the car. The damper was an alleged repro of a Winfield design sold by, as I recall, a CA company run by a Mr. Scott.

I have always been impressed by how the car runs smooth as silk once I get up over 50 mph, with or without new rebuilt engine or F150 transmission.

Steve

[RichPA]

I think you can precision balance and machine until you have no money left and you will still have a vibration.The way Henry attached the engine,trans and exhaust to the frame is practical but has no consideration for the driver or passengers.The inherent vibration will be felt unless there is better insulation installed using aftermarket parts to absorb it.

[AL in NY]

RichPA, my vibration is at one RPM only and quite noticable, more like a low "roar" as Bruce stated about his engine. The engine mounting to the frame
magnifies it, but it's not the cause of it. I'm leaning towards pulling the engine and checking the balance of the flywheel/pressure plate combination. Also, will probably try again to stop my rear main seal leak by trying the chevy rear seal.

[RichPA]

Al,all engines vibrate,thats why all cars have rubber mounts.You will never get the vibration out of your engine.The mounting is not the cause,it is just not able to dampen and absorb.The front mount is steel spring and the rear is a flat rubber pad that has no give.The tailpipe is bolted right to the frame.This all transmits the vibration directly to you.I live with the vibration in my car.Some folks install the Float A Motor kits with good results.You may also try lowering the oil level a little,your leak might stop.

[MikeK]

Originally Posted by RichPA

Al, . . .You may also try lowering the oil level a little,your leak might stop.

The crank center, and rear bearing, are already WAY lower than the oil level. Several inches. Even lowering the oil level three quarts won't help! As long as the pump intake tastes oil, the valve chamber will be full, feeding the same amount of gravity feed oil to the rear bearing. You will, however, get hotter oil because it has less cooling dwell time in the pan.

If you get to the point where the rear cap oil drain tube is not submerged, the leak will get worse, not better.

[AL in NY]

Already found out I loose oil pressure ( 0 PSI) on right hand sweeping turns at 45 MPH if the oil level is down one quart. The oil pump becomes uncovered when the oil in the pan moves to the left side on a right hand turn. So lowering the oil level is not an option to lessen my rear main seal oil leak. The pressure recovers when you exit the turn. I have been thinking of adding a front-to-back baffle to the bottom of the dipper tray to prevent this. Not a good feeling to see the oil pressure drop from 35 PSI to 0. I did use the "rear main janitor" on my trip to Maine and I lost/collected about 6 ozs. of oil in 10 hours of driving. The leak wasn't as bad as I first thought. Myself and my driving partner split a quart of oil after two days of driving. I'm still thinking of adding the chevy rear seal to eliminate the leak!!!!

[bettlesr]

Al in NY:
I have the same right turn problem in my Sport Coupe. While I don't have a leak, I do have a roaring noise a little under 2000 rpm and a drop to 0 psi oil pressure on hard right turns. The engine is built very similar to yours and the F150 trans is just waiting for the cold weather to tie the car up. I am currently running 1 quart over full which helps but doesn't eliminate the problem (with no leak luckly) and thinking of making a baffled and trap door pan similar to what I had to have made for my Lotus Elan. My new oil pump has the pressure relief built into it.