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99% of the time, a vibration after a gear/rear swap is caused by the driveshaft.

either get an alum one, or take your current one into a good shop and make sure it's balanced
 

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I read through this post and did not see if you tried just sitting still and revving the car in neutrol? Mine still vibrates when I do this and would seem to eliminate anything from the mid trans back. Mine has an engine vibration somewhere, I changed the balancer, no help. I have a new flex plate but no time to change it. I think the rubbing investigation may be worth my time for sure when i get it. I have not looked into the trans mount either. Good luck, i feel your pain.
 

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Not the engine. I just built it because the person I bought the last engine never balanced the rotating assembly and it actually DID have a bad vibration. This one is smooth. Well, as smooth as a 393w can be with a 7000 rpm cam in it…
It is definitely drive train, either the transmission or driveshaft, obviously not likely the transmission. The shop who made the shaft is recommended by everybody in my area, so taking it in for rebalance is last resort.

Thanks
dave

I read through this post and did not see if you tried just sitting still and revving the car in neutrol? Mine still vibrates when I do this and would seem to eliminate anything from the mid trans back. Mine has an engine vibration somewhere, I changed the balancer, no help. I have a new flex plate but no time to change it. I think the rubbing investigation may be worth my time for sure when i get it. I have not looked into the trans mount either. Good luck, i feel your pain.
 

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One telling sign is whether you feel the vibration in the seat of your pants or in the steering wheel more. Typically that will point to front suspension or everything else. As was said, a LOT of vibration issues are related to transmission mounts.

I see you used pinion wedges to get change the pinion. But I don't see what you measured the pinion angle to be. This is something you can't eyeball. Pinion angle is HUGE in clearing vibration issues. I see a lot of classic mustangs with with a TON of positive pinion angle. With rubber bushings I'd want to see about -5 to -6 degrees pinion and with the good poly bushings about -2 to -3. I've seen classic cars with as much as 5 degrees positive. Under power, the pinion is going to rotate up and if you start with too much positive pinion angle you end up with harmonics transmitted through the driveshaft as the u joint slightly binds in it's travel.
 

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I had not considered the front suspension. I have a rack in there with new wheels and new tires and it was recently aligned and bump steer set. I have run the car in the air and the vibration is still there. It has been narrowed down to the trans/ driveshaft.
I have a digital magnetic angle inclinometer which I intend to use to measure the angles. I have not done it yet because I do not see a clear way to measure the angles accurately, but I have a couple of ideas.
I was guessing before with the wedges, going 2-4 degrees in both directions with no change in the vibration.
It does seem reduced when accelerating and worst when coasting. This could indicate too much negative angle, assuming “pinion up” is positive. More acceleration would rotate the pinion up, making the trans line and pinion line more parallel. However, I rotated the axle a good deal in the positive direction with no change.
Anyways, this is all guessing. Once the angles are set right I can eliminate this variable.
Thanks
Dave



One telling sign is whether you feel the vibration in the seat of your pants or in the steering wheel more. Typically that will point to front suspension or everything else. As was said, a LOT of vibration issues are related to transmission mounts.

I see you used pinion wedges to get change the pinion. But I don't see what you measured the pinion angle to be. This is something you can't eyeball. Pinion angle is HUGE in clearing vibration issues. I see a lot of classic mustangs with with a TON of positive pinion angle. With rubber bushings I'd want to see about -5 to -6 degrees pinion and with the good poly bushings about -2 to -3. I've seen classic cars with as much as 5 degrees positive. Under power, the pinion is going to rotate up and if you start with too much positive pinion angle you end up with harmonics transmitted through the driveshaft as the u joint slightly binds in it's travel.
 
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