For those that have not followed my thread, I have a 68 Imperial with some engine work on the 440, and a freshly rebuilt transmission. I finally got the chance to take the car on a freeway drive with limited traffic, so that I can try some a bit higher cruising speeds. I did not try full throttle accelerations, not enough room for that (plus, I have decided to rejet my Ederblock AFB on the secondaries, so I know the performance is not all there yet). However ... As I was exceeding the 2800-3000 rpm range (85-90 mph, speedo reads about 8 or 9 mph fast in this range), a high speed vibration was felt. When I went a bit faster, the vibration seemed to disappear. At 100 (3200-3300 rpm), this vibration was no longer there. The frequency is too high for it to be tires. I do not recall having this vibration prior to the recent work. My guess is that going faster yet, the vibration will not be there.


Question: Could the people that rebuilt the transmission have assembled the driveshaft wrong? Short of removing the driveshaft and having it rebalanced, is there something that I may be able to do that is easier? I have never had to deal with this before, and I am not sure where to start.

 

I have no suggestion. But some of the big C bodies had a weight bolted to the tail shaft

 

With the speedo being a little incorrect, have you change the tire size lately? The speedo will be off if this has happened, and the speedo gear wasn't changed accordingly.


Can you tell where the vibration was coming from?
With this recent engine work, was any part of the exhaust removed? You can check to see if it is near or touching any part of the car.


Did the shop alter the drive shaft in anyway?
Quick test, go under the car and grab the drive shaft. There should be little to no play in it. This test will check its joints.

 

Was the convertor changed or had any work done to it?
If you can find what rpm it defiantly does it, bring the rpm up to that in park or neutral just to see if its in the engine, trans. You will feel it coming through the steering wheel.

 

Guys, thanks for the suggestions.


The tire size has always been 235/75R15. My complain is not so much the speedometer error. A 7-10% speedometer error is generally typical in old cars, and I am used to it (8% fast is the standard error in modern European cars). I know the speed to rpm ratio anyway, so I have a good idea how fast I am really going.


I do not think the exhaust was touched on the back of the car. Obviously, the manifold/pipes connections was undone, and I have an exhaust leak on the passenger side, but I cannot see how that can be related to the vibrations. I don't think the exhaust is touching the body, and I do not hear any bangs.


Last weekend I found a tall curb to park the car, put the car in neutral, and went under the car to check the sloppiness of the driveshaft. Very little, and all of it was in the differential gears. No relative movement on the U-joint connections. I did not see any dents on the shaft, nor any missing weights, but I could not really reach well enough for a thorough inspection. Ideally, the car should be raised on a lift.


We did not replace the torque converter. The shop wanted to use a 70's 400 converter that they found locally, I asked them to use the old converter instead, despite of the protests of the mechanic. On 1st gear, I have revved the car many times to 4500-5000, I do not feel much vibration. On 2nd gear you do feel it when you rev it up, but that's because you bring the speed to the 70's and 80's (5000 rpm on 2nd gear is a true 100 mph, speedo says almost 110!).


Drove the car again this weekend, but not as fast as last weekend. May be the vibration is lower frequency than I thought, and may be its one of the new rear tires (improperly balanced ? ). Will see. For now, until I can check on the tires, I will try to stay outside of the vibration range (~80-90), and at any rate, the vibration is not that severe.

 

As to the drive shaft ? Some of them had a round weight as part of the drive shaft. I have seen it installed backwards ! with the weight at the rear..

 

A simple way to check tire balance, besides having someone follow you and looking for any wobble, is to jack up the rear of the car, and check yourself. Make sure the car is properly supported of course.

 

Interestingly, I think I did see some weights on the rear! But I doubt they removed the driveshaft completely from the car. They only undid the front, the connection to the transmission. I hope ...


This car has limited slip, so I would have to raise both tires to check for wobble. I think I will just take it back to the discount tires, I noticed that the rear left tire has no balancing weight on the outside, so may be the weight they used fell off....

 

It may have big washers tack welded to the drive shaft That is normal. I am talking about a big doughnut, part of the drive shaft

 

Would the donut be on it with a standard trans too?