I purchased my 69 Cutlass conv. a year ago. It spent half that time in a body shop being stripped and painted. Since then I have been trying to get it running and driving in a manner I am happy with. A long time ago a PO put in a 455, a TH400 trans, and a 3.23 posi rear. It has a vibration on the highway at 50-70. The speedo is off due to the rear change so thats a guess. When I put it in neutral at that speed it goes away. To me that meant drivetrain. First I had the wheels balanced just to eliminate any possible issue there. Though there were new tires on the car they were all out of balance. Then I pulled the driveshaft and took it to get balanced. Factory says it can't be out by more than .015. It had been shortened and not rebalanced and was out .065. Both those improvements helped but did not eliminate the issue. I have also gone through the front and rear suspension replacing all bushings and center link as well as shocks. OK, to the point, somewhere in the CSM I found a section on vibration that goes over 3 possibilities. Unfortunately I haven't been able to find it again. But what it suggested was rotating the torque converter 110 degrees. Has anyone done this? Can it be done with the trans in?
If I need a new torque converter I have read that HP etc. determine the best selection. Until now I thought they were basically all the same. What would I need to know if I needed to purchase one?
Another question is my RPM seems higher than it should be on the highway. From what I've read the 3.23 rear is a reasonable rear for everyday use. I have checked several sites using tire size, rear, and a given speed and my RPM seems about 400 higher than it should be. All 3 speeds work but it feels like it should shift into another gear. Can that be torque converter related?
thanks,
Steve

 

Yes you can rotate the torque converter with the trans in.

You didn't mention condition of u joints. I would consider them suspect if you haven't swapped them.

You should look into changing the driven speedometer gear in the trans to the proper tooth count. Sometimes you have to replace the drive gear as well, but depends in the what housing is currently installed.

https://www.tciauto.com/speedometer-gear-calculator

 

The U joints are good and I tried changing the driven gear but it needs the drive gear as well. I'll tackle that over the winter. Thanks on rotating the converter. Thats good news. I just wonder if anyone has had success with that. I can see in there since there is no cover on the trans at the moment. I assume there is suppose to be one.

thanks,
Steve

 

Torque converters have some amount of slippage unless it's a design that utilizes a lockup converter, and the amount of slippage can vary a lot depending upon the specifics of the converter. On my car I measured the converter slippage at around 300 RPM, so your 400 RPM number is in the right range.

 

I had just watched a youtube video by Summit on torque converters and they were talking about the stall aspect, when they are designed to stop slipping. I will dig into it and try and get some accurate rpm info. Thanks for the response, at least I feel like there is a possible reason.

Steve

 

Try reclocking the converter, you may also try flipping the driveshaft 180 degrees in the rearend yoke.

 

Did you inspect the tires and rims for hop or run out? Start there before digging as deep as a torque converter. Start simple...
A good tire tech can observe the rims/tires on the balance machine for abnormalities. A tire hop or bent rim will still balance and still vib at high speed. Can you grab another set of tires/rims from someone to slap on and road test?

A dragging brake shoe or caliper can induce a vibe at speed. So can a bad or loose wheel bearing.

Next, pull the shocks and inspect them. Believe it or not, a bad shock can transfer road irregularities into the chassis and feel like tire or driveline balance.

Get another opinion on the driveshaft. Ive had bad balance jobs before. (Dennys Driveshafts dot com if you need a new shaft).

If the above fails to eliminate the problem then its time to isolate the driveline vibe.
While in Park bring the engine up to the RPM that produces the vibe. Do you feel it? If yes go to the next step.

Next, disconnect the converter from the flexplate and push it back into the trans. Start the engine and bring it up to that same RPM. Vibe still there? No then its something from the converter back to the rear end. If yes, the flywheel is now suspect. Its on the engine side, not the trans.

If you decide you need a converter, (which is doubtful, as they are balanced when assembled and don't become unbalanced unless they come apart internally in which case you will induce different problems from what you are experiencing)....look at TCI, Hughes, B&M. Jegs or Summit dot com. You might like the Saturday Night Special for some added off the line umph or Street Rodder for Stock style lock up.

Heres the converter selection chart at Jegs
https://www.jegs.com/p/TCI/TCI-Break...47572/10002/-1