Hi guys. Thanks for the help, in advance.

1971 Olds 442, 350, Holley Sniper EFI, 200-4r, 3.73 Eaton Posi.
Horrible vibration starting at 54 mph.

Back-story:
First off, this is a daily driver. My son and I built the car over the past 2 years and now it's his DD. He'll be leaving for college in just a few days. During his second outing with the car, the TH350 burnt up the forward clutch pack. We replaced the TH350 with a 200-4r. This transmission with the peg-leg 2.56 Cutlass rearend meant shifts to OD at 75 mph. Anyway, not much driving going on yet. Earlier this week, we upgraded the rear to a 3.73 Eaton Posi. I say we, Bruce Ponti @bponti442 was the master (excellent work and a clean-freak...definite recommend).

While doing the first "break-in" of the gears and posi, the car has a horrible vibration at 54 mph, and it doesn't come out of it. Not engine related and it's not wheels/tires. Yes, I have searched posts and read threads. They've definitely helped, but we haven't totally found the culprit....Yet!

Been a long-time observer and peruser, but haven't spent much time actually posting. Hopefully, I can add to the knowledge base from now on.

 

Did you use the same driveshaft with the OD, from the original 350? If so, I’m guessing the vibration wasn’t there before the gear swap. Try flipping the driveshaft 180 degrees in the rear end yoke. If that does t help, try reclocking the converter on the flywheel.

I assume moving the crossmember back for the new trans didn’t cause clearance issues with the exhaust? Transmission mount?

 

Matt, thanks for the input.

Since that original post was supposed to post this morning, let me fill in some more data and detail.

Loosened the torque converter and slid it back to ensure the vibration is not in the engine.
Vibration did not happen with the old rearend at less than 70 mph, so it isn't wheels/tires. (vibration happens sooner now, since the new 3.73 rearend increases the driveline (trans/driveshaft) RPM)
Same driveshaft as with the TH350, and same crossmember. No clearance issues.
Clocked the driveshaft 180, and tried again. Vibration was a little better (not much better), but still very bad.
Checked the yoke to tailshaft clearance, and there is almost 2-inches of yoke showing behind the 200-4r tail-shaft. Best I can tell, 3/4" to 1" is what is recommended.
Removed the driveshaft to have it balanced and noticed a weight is missing on the shaft.
Also, talked with the driveshaft engineer (owner) and he's confident that my problem is the yoke not deep enough in the trans. ...and the missing weight.
Instead of lengthening the driveshaft, he said that we can "cheat" and just use a 4L60E yoke which is more than an inch longer. This will put more of the yoke in the transmission.
Really hoping this cures it, or at least helps.

 

I did the same conversion with my Th 350 to 200-4r at the same time I did my motor rebuilt. I ended up with a bad vibration as well around 65-70 in OD... I had replaced the U-joints front and back using the same driveshaft and yoke when I installed the rebuilt motor.. No vibration with the old set up. Come to find out after I pulled the driveshaft again, that body under coating tar had dripped off the body onto the top of the driveshaft during the cam break-in.
Did the weights fall off during the replacement, or were they off before you changed tranny's? I installed new u-joints again just to make sure. I had about 1" clearance approx.

 

Wow, that's pretty crazy that the tar would put it out of balance. Glad you got it figured out.

I'm not sure if the weight was gone before or not. They're rebuilding the driveshaft with the new 4L60E yoke and u-joints, so it should definitely eliminate the driveshaft as the culprit.

 

FYI, I wanted to finish this thread, since so many threads never seem to have a solution. It ended up that the driveshaft shop called me and told me the driveshaft was bent beyond repair. The made me a new driveshaft tube and put my original ends back on it. This means that I now still have the T350 yoke on it (running a 200-4r) and I have 3/4" to 1" of stick out from the tail of the transmission.

Vibration is gone.

 

Thanks for following up.

 

x2

 

X10 on closing the loop. Makes searching with a solution that much easier.

 

Excellent!