[Dano1972]

Please bear with me as this will take awhile to explain. I have a rhythmic vibration that I will describe in more detail after I give you some history on my vehicle. It's a 72' olds cutlass with a transplanted 468 inch BBC. The car has the original B-O-P TH-350 that was built by a very reputable shop about two and a half years ago. A 12-bolt rear end that was recently built by the same shop. A new custom drive shaft that has been balanced. All new front and rear suspension. All new wheel bearings all around. Pretty much everything but the tranny is less than one year old.

I recently bought a more powerful 468 bbc and swapped it in. I bought the motor with about 1000 miles it from a guy out of town. The engine runs great, has a ton of power. The guy I bought it from said it had been balanced with the stock damper and flywheel. I have the damper but the flywheel didn't come with for some reason. The problem is that I have a vibration/rumble that is rhythmic. At 3000 RPM it is one second intervals of vibration on and off. If I drop it to 2750 RPM it's the same vibration but long 2-3 seconds of rumble with one second of silence between. It does not do it when parked in the garage at 3000 rpm. It does not do it if I put it in neutral and coast. It only does it under load going down the street. It is most noticeable in 3rd gear. At 4000+ rpm it seems to fade out. At less than 2000 RPM it is not there.

The thing is, the car was absolutely perfectly smooth with my last BBC. All the suspension and drivetrain hasn't changed, just the motor. I called the guy I bought it from. He said he didn't have the flexplate but it was just a stock GM one. I have now put 3 different flexplates on it. It had the vibration with all of them. I got out the wallet and bought a TCI Rattler damper. It seemed to help , but the vibration is obviously still there. The previous owner swore to me that it was balanced, didn't vibrate, and said I should look elsewhere for the problem. I believe he is telling the truth. When I had the pan off the engine I could see several holes drilled in the crank counterweights. I could also see that the rods have been weight matched. There was no mallory added to the crank.

If the rotating assembly was misbalanced, wouldn't it be a constant vibration that got worse as RPM increased?

I know that it seems unlikely that the tranny could have a problem around the time of the motor swap, but It could be possible.

By the way this is a Mark IV 468 that is externally balanced and has a two piece rear main seal. I have no idea what to do. Everyone I have talked to is baffled.

I don't expect many replies because of the complexity of this problem, but I desperately need help. I can't afford to keep throwing money at it hoping to fix it. Please give me any advice you can on finding the cause of this. Thanks guys.

[454me]

The one time I had a rotating assembly out of balance it vibrated all the time. I would look at a rear transmission bearing being bad. I have seen drive shafts loose in the rear bearing cause a coming and going vibration. What I have done but can be dangerous is jack the car up and run it in gear with stands under it. You can lay beside NOT UNDER the car and sometimes see what is shaking.

[pmeisel]

The "come and go" with speed and load changes indicate a drivetrain issue more likely that the engine...

454me suggests a rear transmission bearing, and that could be. I would also take a look at u-joints -- has anything changed in the driveshaft geometry? Sometimes a change in the angle of the drveshaft between the trans and the axle can cause a problem.

[NorthStar]

Possible that you have wrong flexplate on there?
How about a motor mount? With all that new power maybe its flexing enough to cause driveshaft angle to be excessive?



http://www.sallee-chevrolet.com/frame.html

http://www.sallee-chevrolet.com/Pric...646-0.886.html



FLEXPLATES
00471597 BBC and SBC - A/T Mark IV 396/427 and some small-block Chevrolets, internal balance. Large Converter Patern Only. Heavy duty
14001992 BBC A/T Mark IV 454 external balance
14096987 BBC M/T Mark V 168 tooth
03993827 BBC M/T Mark IV std. weight external balance
03963537 BBC M/T Mark IV light external balance


DAMPERS
03879623 BBC 8” 427 internally balanced
10216339 BBC 8” 454 externally balanced
10051171 BBC 8” 454 external Fluidamper

[jpelli]

I had developed a vibration with my car at 3500 rpm , below it was gone above it had a humming noise with it. Turned out I lost a drive shaft weight. A big one at that 10 oz out of balance. Something you may want to check out.
Joe

[Dano1972]

Thanks for the replys guys. I am starting to suspect the tranny. I'm sure I have the right flexplate. It's a Hughes pt #HP3994. BBC 454, 168 tooth, Ext. Balanced, Mark IV. I took it for a 30 minute test drive today. I found that it doesn't vibrate at al through first and second gear at any RPM. It seems to vibrate the most at 2800 RPM in third gear. I would take it to 4000 RPM in second gear and hold it there and listen...No Vibration, so I would then shift into third and instantly hear the vibration again.

Come to think of it I really beat the hell out of it before I took the old 454 out. It's possible I messed something up then. I'm thinking of buying a hughes turbo 400 unless anyone has something bad to say about them.

454me - Thanks for the info about the rotating assembly.

To pmeisle: Driveshaft geometry has not changed. I spent some time obtaining 2 1/2 degrees negative pinion angle with the opposing and equal 2 1/2 degrees at the tranny yoke. The drive shaft has always had four small weights, two on each end. They are all still there.

[pmeisel]

Only in third, probably a transmission bearing going bad. Happened to me once coming home from a road trip, vibrated a little in third and during 3-4 shifts, eventually burned up. Only good thing was that it only had 13,000 miles, I worked in transmission product group at the time, and the chief engineer had Ford put in a new one for me.