Could it be aerodynamics?  Maybe something is a bit loose or vibrating when it's hit with the air moving at that particular speed.  

351w firing order, but it had the same problem with a 302 using 15426378 firing order many years ago. 
The harmonic feels like a bearing that is not happy somewhere, but every bearing on the car has been replaced multiple times. Spindles replaced.  Everything.

Maybe a little higher frequency than that.

Steve I'm getting ready to install leaf springs, new shackles, and my 3.25 3rd member... 
I also noticed I have a vibration at + 100mph ... 
going to look at my T-5 tailshaft bearing as the issue ...
u-joints - wheel bearings - drive shaft was balanced on T-5 install all are good ..

Did you find out what was causing  your issue ??

MS wrote:

I haven’t really driven the car during covid, so it is in the garage.

You need to change this situation Steve... I get withdrawals when I don't drive mine...
Regarding the vibration... Is there a way to get the car off the ground, run it at the speed where the vibration comes in and somehow see what's happening from the underside? Maybe park the car over a pit and lift the rear end off the ground? Obviously the car would need to be secured and safe whilst this was done. Maybe a shop could do something like this?

Last edited by Toploader (1/29/2021 9:04 PM)

MS wrote:

I have considered that. Been too busy building new shop to worry about it.

Not really wild about running my car, jacked up in the air, at 75 mph!

I did that one wheel-at-a-time...open diff...and proved that the vib was in the right side of the 8".  I had tripple checked the wheel, tire, axle, and even balanced the brake drum.  Had to be something to do with the housing, maybe bent.  Put in the 8.8 and no more vib.

I did have the car pointing OUT of the garage at the time.

BB1
 

You could build a thing kinda like a "line lock", Steve.  Something that would drop the hinny at about 6000 rpm.  Then just hang on.  That'd be real hit a the Bash.

BB1

MS wrote:

Not really wild about running my car, jacked up in the air, at 75 mph!

Yeah, I hear you, but I am not sure how else you can do it though? In my opinion, you would need to do it over a pit and have a few buddies around. Someone would need to be in the car the whole time as well. Somehow you would need to secure the car as an extra precaution.
Saying all that... If it were me, I would search around for a shop that could check it out and diagnose. Sometimes it's just easier and safer to pay someone who has all the right equipment for the job.

Last edited by Toploader (2/02/2021 7:05 AM)

Most successful tool I have used to chase a vibration is my little camera that transmits to my phone. I have mounted it all over and under cars so I can watch the recorded (not while driving) video. I was amazed at how much a rear axle can bounce around with a little excitation. the scarriest one was watching the front suspension during hard corners. What looks to be built really solid is rarely that way!!

red351 wrote:

The shaft runout on both ends was acceptable by the book. When I added the GV overdrive, I needed to shorten the driveline. Being cheap as I am I was just going to cut the original and notice it was a bit bent in the middle. I made a new one and the wave was gone.

Simple as that.

When I was installing an aluminum Explorer driveshaft I not only checked the ends, but also the center run out, which was within spec. I still had the same wave vibe you describe, which occurred with both the FMX and AOD. Gonna take the 69 out on the highway today and check for vibes, but I'm not too optimistic.

Bob I found the bent laying a couple 4' levels along it when the shaft was still long enough. They both verified it.
It wasn't much.

Last edited by red351 (2/03/2021 8:38 AM)

Bolted to Floor wrote:

The problem I have with running a car at 75 on jack stands for a vibration issue is it vibrating off the jack stands. I don't need any new doors in the house. How would you secure a car to keep it in place if it come of the stands? I was once told the attachment point for a safety harness should be able to support a truck....that's for the average size person. What does it take for a 3500 pound car in motion? 

Chassis dyno?? 

The good part of that is the car is secured stationary, not in motion like your body is when driving 60 mph.  So there is no moving body to try and stop. The car would have to accelerate from a static position before it needed to be stopped.

Mythbusters did an episode about driving a car along the highway at 70 mph, then driving it up a ramp into a truck also going 70 mph.  The car simply drove up the ramp and was able to easily stop once on the ramps.

With the car in the air, the only dynamics are the flywheel effect of the rotating wheels. Arresting the car’s forward travel won’t take near as much force as if the car were actually moving.  NO, I do not care to test this theory!