My first instinct for such a vibration is the crankshaft & the diff pinion relative angles. Ideally, they should be parallel with around 0.5 to a max of 6.0 degrees of angle at both uni joints from memory. The critical point is that each angle must be the same. I.e. if one measures 2.5 degrees, so should the other. Google this info & you will find some very interesting articles & diagrams.
You mention that this alignment has been set correctly? Many EH's that have been modified over the years have the box hanging far lower than they should. The simple reason is the small size of the tunnel, particularly manual bodied cars, and lazy installers. Even auto bodied EH's need to have the tunnel modified slightly to get a Trimatic up far enough to achieve the correct crankshaft inclination.
With regard to EJ/EH's, to have the crankshaft angle, as from factory, the center of the output shaft of the auto should be in the range of 85 to 95mm from the floor directly above it.
Now add to this lowering the rear end, which I assume would have been done.
As leaf springs are reset to be lowered, they flatten out. As this happens, the front of the diff will start to point toward the ground. Wedges can be utilised to compensate for this change of the angle or relocating the spring saddles on the diff to create the correct angle for your particular car.
Lowering a car with lowering blocks does keep the factory setting of diff pinion angle, but brings other issues into the geometry.
Another mod that can change these angles is the old HR front end. Again, if not done correctly, to keep the motor in exactly the same orientation, this can alter these critical angles.
Each item in isolation may not vary the angles much. But if each variation is at the far end, problems may surface.
So, if the box is lower than it should be & the rear springs have been lowered, then I would look far more closely at the crankshaft & diff pinion alignment.
Experiment: Take the car for a drive with a couple of mates in the back seat. A couple of fat bastards preferably. No mates on hand, fill the boot with heavy stuff to lower the body of the car. Try to determine if the vibration changes when compared to just you in it. If so, the crankshaft & diff pinion alignment might be worth looking at.
I had this alignment issue in the car below. Once sorted, I was surprised at the difference it has made.
While under the car, check the diff pinion for sideways movement. A buggered pinion bearing will allow the tailshaft to wobble at high revs sending a vibe through the car.
Just noticed your earlier "unis in phase" question. This is referring to the unis having correct positioning on the driveshaft relative to each other. If you were to slide a broom handle into each of the empty weld yokes at each end of the driveshaft, they should be parallel.
The links below may help with better explanations.
http://www.4crawler.com/...icks/Driveline-101.shtmlhttp://www.hotrodhotline...rive_shaft_harmonics.phpUtility8
Edited by user Monday, 18 October 2010 1:21:11 AM(UTC)
| Reason: Not specified
