Vibration And Engine Noise At High Rpm

[John Redling]

1941 Plymouth L-head 6 runs good until 45 to 50 mph. At that speed engine gets loud and front vibrates. Can I purchase a overdrive unit or purchase a rear kit to lower ratio. If so where can I get them. Any other ideas. I now have 3speed. Thanks John

[Plymouthy Adams]

you can find lower rear ratio for swap in that are bolt in..you can also upgrade to the newer style rear axles but will require driveshaft work to adapt. $$$  Installing the R10 OD in a retrofit is doable but also a bit expensive finding a R10 unit.$$$$....while these may address your noise due to higher RPM it will not likely help with the vibration...look for bad tires, out of balance or worn components, do also look at the motor mounts and rear shackle bushing as these can cause shift in driveline and alter your phasing and cause vibrations

[Don Coatney]

Are your tires balanced? Is your driveshaft in good shape with no "free" play? What have you looked at so far? The  more information you can give the better your chances for helpful assistance.

[TodFitch]

Front vibrating won't be solved by putting in an overdrive: You'll need to fix your front end.

 

You can probably swap differentials in the original rear axle or you can swap rear axles.

 

But the original drive train should be good for running at 65 MPH. It will be turning over more than a modern car at that speed, but that is not a problem. The original brakes and suspension are probably getting near their safe top speed at that point anyway.

 

So you have a couple of choices. The easiest, simplist thing is to fix the original front end so you don't get the shaking, then see how it runs. It may just be that you aren't used to the sound of a car of that era going 65 and think it is broken. Or maybe it really does have some issues. If it has some issues then decide how you want to go forward. Might be rebuilding to stock or might mean modification. But fix the front end before you throw money at the engine/transmission/rear axle.

[greg g]

What is causing the vibration?  Tire balance comes and goes at different speeds, May be bad a 55 and mostly gone at 60, a drive line shake froma U joint or maybe the front pulley vibration damper usually come on at 45 and stays there.  And again if you haven't checked to see if your E brake might be dragging, now would be a good time.  A change of gear ratio or OD would just move the vibration to a higher road speed if it is in the drive line.

[Don Coatney]

  A change of gear ratio or OD would just move the vibration to a higher road speed if it is in the drive line.

I sort of disagree. The OD will only lower the engine RPM's. Driveline will still be spinning at the same speed unless the differential ratio changes.

[John Redling]

Found vibration is in engine.  Looking for crank shaft and pistons for my 41 Plymouth L-Head 6cyl

[TodFitch]

Found vibration is in engine.  Looking for crank shaft and pistons for my 41 Plymouth L-Head 6cyl

 

Same pistons, crank bearings and rod bearings for '33 through '41. Same crank and rods '34 through '41. Pistons are definitely available new. Crank and, if you need them, rods will most likely need to come from a donor engine.

[Don Coatney]

Found vibration is in engine.  What exactly did you find?

[pflaming]

My truck suddenly had a vibration above 55 mph. I greased the u-jounts and the drive-shaft in/out extension and it smoothed down. I'm going to put the truck up on a hoist soon and do a complete bolt check. I checked the nuts on the drive shave connection to the hand brake drum and they, while not loose, were not at tight as I once had them. . .  I probably have about 2,000 miles since i checked, just too easy to do not to.

[John Redling]

After checking everything and finding out the engine is the cause of the vibration I am going to pull the engine.  We thing the crankshaft is out of balance.  We will check everything and balance everything. I guess that is the way to go.  Thanks everyone for their help, John

[Don Coatney]

Was your engine miss-firing?

[John Redling]

No, engine runs good.  No metal in pan when dropped, no smoke, no burning oil, just a vibration.  Otherwise engine runs good.  Any suggestions?

[TodFitch]

My experience has been that roughness in the engine mechanical parts smooths out, at least as far as I feel them, with increasing speed. I'm sure the bearing loads increase and all that with potentially dramatically reduced engine life (visions rods departing the block). But the vibration should be some harmonic (multiple) of the RPM and when you get up to 3000 or so RPM those vibrations just aren't felt so much as heard. So I'm skeptical of being able to blanket say its been isolated to a crank that you "think" is out of balance.

 

So I guess I'm in the Don Coatney camp of doubting the diagnosis and my curiousity is aroused.

 

Is it an ignition problem? Are valves floating or sticking? Does it run smooth at idle? At what RPM does the problem become noticable? How were you able to isolate it to the engine? How were you able to isolate it to the crank?

[Don Coatney]

Couple more questions. Is this vibration present only when driving or can you make it happen by reving the engine when at a stand still? Is this something that just happened or is it something that appears to be getting worse? When did you last do a compression check and read the spark plugs? How is the oil pressure? What is your ignition timimg set to?

[DJ194950]

Is your tranny removed, clutch plate, pressure plate?

I found that my flywheel was loose at the bolts.

Is the front pulley tight and running straight?

This is an addition to things to look at on top of other items mentioned.

 

Good luck.

 

Best to ya,

 

Doug