I've owned my 76 for over 7 years and made alot of upgrades over the years. The car originally had the L82 350 T10 4 speed, the car has always had some vibration at anything over 3500 rpm. It also has a type of roaring I thought was maybe coming from my trans, it grows loud to soft and does so consistently over and over but you can only hear it (also feel it) at a sustained high rpm. I insalled GM 330hp crate engine about 4 years ago with no change. I just installed and have been running a T56 for month or so. I still have the vibration and the roaring, so I read some threads and thought you know I've never changed my differential fluid and I thought maybe its empty?? So last week I checked it and it was completely full so I did my best to get out what was in there and I added fluid and the additive. The roaring has slowly decreased, I plan on adding another bottle of additive and keep an ear on it. So now I have a vibration that I'm trying to figure out, it starts at about 3300rpm and gets worse until I shift although it isn't scary vibration it's just the shift plate and console tend to vibrate and I have headers and flowmasters so my car is already loud and then you throw in the vibration. I also have some steering wheel shake or vibration almost like it goes through the entire car. So with the new 6 speed I can head out on the interstate and as I'm going through the gears, It's the same thing I was experiencing with my 4 speed vibrations at the higher RPM range the difference now I can shift into 5th and sixth gear and as long as I'm running around 2500 rpm the car drives like a dream at whatever speeds. I'm just trying to figure what could be tightened up or balanced and give a smoother accelleration at the high rpm range??
Should I expect a certain amount of vibration? Any input would be appreciated

 

Not sure what you have check already previously, but several things come to mind.

1. Check the motor mounts for condition and tightness. Should be torqued to 35 ft lbs. What could be happening is as the engine is torqued either some part of the header is making metal contact or the exhaust pipe where it goes through the frame holes is making direct contact. It happened to me on both accounts.

2. Check the transmission mount for condition and tightness.

If the engine (through the exhaust) is making any contact with the frame you will feel it in the wheel. In my case I had a header tube that was laying on the frame just above the motor mount. On the other side I had a tube that was touching the bolt on the idler arm under engine torque. If the motor mounts are original, they could well be shot or sagging.

Also, I would suggets you have the tires "road force" balanced. This balancing will compensate for out of round wheels and tire. Made an absolutely huge difference in the smoothness road manners of my 69 vert.

You may also want to check the u joints in the drive shaft and the balance of the drive shaft. Since it is RPM related this is not likely the cause but I have seen some weird things come of ujoint/driveshaft problems.

 

you know a bad fan clutch can also cause a vibration look closely around the shaft and see if the fan blade wobbles at all

 

Are you using the same balancer off your old engine and put it on the new one?
I've seen were they have slipped and cause your engine go out of balance and do something like that.
But then your timing marks would be off a little.......
Or maybe someone put a 400 flywheel on it and cause the same thing.
But ain't the 330 hp crate engines externally balanced already???

 

I purchased a new balancer from Car quest 7" Chevy Small block harmonic Balancer part number PIO DA-3071.

 

Went out and checked it just now, no wobble fan is tight and looks and feels fine, but thanks for the input.

 

I did have new mounts installed with the new engine but I will check them. Also I will take a close look at the headers and see if they are touching the frame.

I'm about to purchase new tires soon and have alignment so we will see if that helps.

Just had new trans mounts installed with the 6 speed. Also had new u joints and drive shaft shortened and balanced. Being that it was doing the same thing before the 6 speed, I would assume its' not in the driveshaft unless it's in the half shafts??

Thanks

 

Based on what you've said so far, I would think the probable cause is an out-of-balance pressure plate.
I've had trouble with this a couple of times myself. Does it vibrate in neutral at 3500 rpm? If yes, then it substantiates this.

Roger

 

I did too. I forgot about this. It was way out of balance and required a substantial weight welded to it to correct. Good advice.