Well I finally resolved my drive line vibration that I had at 70MPH. It was the driveshaft after all.

Now I appear to have an engine vibration.

It is a small but noticeable vibration mainly between 4500 RPM and 5500 RPM. It is most noticeable when driving it and accelerating hard from 4500-5500 the RPMs in 2nd and 3rd gear. It feels like the vibe comes in a 4500 and falls off at 5500 RPM. I dont feel any vibe at leading up to 6.5K RPM redline. Also the engine is running very well with no other ill effects. No knock counts either.

I can also feel this vibe with the car parked, clutch in and free revving it to the mentioned RPMs. Much less noticeable with the car still than while driving it.

I currently have 10K on the motor and this absolutely was not present from the time it was brand new up until recently. I just noticed this in the last week or two.

The only engine work I have recently done is replace the water pump drive seal in the timing cover, re-lash the valves which are set at 1/2 TPZL, replace the driveshaft, transmission mount. I am confident this vibe is NOT driveline related. It responds to RPM.

I replaced the transmission mount. No change.
I checked the headers and exhaust pipes. Not banging anything.
I check the motor mounts. Look fine.
I check the plug wires. They appear to be on solid.

Any ideas?

 

I wonder if the flywheel is unbalanced just slightly?

 

Check the harmonic damper for a broken rubber ring that joins the two halves.

 

Update:

I am going to stop one tick shy of calling it fixed.

I installed the stock balancer. No change.

I then installed a new set of NGK TR6's (OMG I fuggin hate putting LT1 plugs in) and did not feel the vibe when free revving it in the garage.

I then drove it and humped on it pretty good and the vibe is no where near the intensity of what it was before. Before the change in the ash tray jingled pretty good and of course you could really feel it.

Now the change jingling is not near as intense and the vibe is far less intense as well.

Plus the car feels even more powerful especially up top.

And it idles smoother too.

I think there is a very good chance the #2 spark plug boot was loose and that was the heart of the problem. The plugs all looked excellent and were consistent one to another in terms of burn pattern.

I will drive it some more tomorrow to ensure its good to go. What remaining minor vibes I feel are what I would consider very normal for a h/c loud exhaust 400HP motor.

Thanks guys for the help.

 

If you have a poly trans mount and non-poly motor mounts, or vice-versa, that will also cause a vibration.

 

I am using all rubeer mounts.