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I've been having a rather unpleasant high speed vibration in the 70 for the last couple years. I finally rid the car of it once and for all. I thought it was the driveshaft so I put a Denny's in it, still there. I ruled out the rear and transmission since it was only happening under a load. It turns out it was the transmission mount I had in there. It was one of those polyurethane Energy Suspension mounts. After comparing the Poly one to a stock piece, the Energy Suspension piece was 1/2" if not more thicker than the stock mount which in turn threw the pinion angle way off causing the car to vibrate over 65 MPH. I put the stock mount in, vibration gone.
 

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Have to wonder about the engineering (lack of?) that goes into many of these aftermarket "performance" parts...
 

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I avoided that happening to me by removing my energy suspension tranny mount right after installing it, since it raised the rear of the tranny more than I liked. Waste of money on that mount. But the 2 ES engine mounts are fine.
 

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x2 on not using their trans mount-better fit with stock one, and i prefer the trans to be able to move a little if needed, i feel it takes some stress out of it
 

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And it's not a good idea to have a stiffer (as in polyurethane) trans mount than engine mounts. Many broken trans cases have been caused by that. The other way around is OK though.
 

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Would have never thunk it! I was just about to go out get one for my car too. I'll stick to stock.

When I was taking apart my car a few weeks ago, the cross member had two home-made spacers made from 1/4 plate steel. I had never seen this done before in a Chevelle and still don't know why they were there? Pinion angle I guess? The opposite from what you're doing. Guess I'll find out more when I get mine back on the road.
 
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Would have never thunk it! I was just about to go out get one for my car too. I'll stick to stock.

When I was taking apart my car a few weeks ago, the cross member had two home-made spacers made from 1/4 plate steel. I had never seen this done before in a Chevelle and still don't know why they were there? Pinion angle I guess? The opposite from what you're doing. Guess I'll find out more when I get mine back on the road.
It could have had one of those thin mounts, I've a couple. I have a poly mount in the 67 which I'm going to change before it hits the highway.
 

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I've been having a rather unpleasant high speed vibration in the 70 for the last couple years. I finally rid the car of it once and for all. I thought it was the driveshaft so I put a Denny's in it, still there. I ruled out the rear and transmission since it was only happening under a load. It turns out it was the transmission mount I had in there. It was one of those polyurethane Energy Suspension mounts. After comparing the Poly one to a stock piece, the Energy Suspension piece was 1/2" if not more thicker than the stock mount which in turn threw the pinion angle way off causing the car to vibrate over 65 MPH. I put the stock mount in, vibration gone.

This is great to know since i'm having the same problem with my 64 el camino that was my fathers. Right now it has a 400/400 out of his (got smashed) 68 gto. I was wondering if any of you could help me with this since i have no idea what mount was used and he's not around to answer these questions any more. I would really appreciate some input on this if you all don't mind. thanks:)
 

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Ya know, when I did my TH400 to TKO conversion, I measured the distance from the transmission mounting faces to the shaft centerlines, and they differred by half an inch. I did the trigonometry and could see that it was going to change the angles noticeably. That's why I bent my cross member down half an inch, so the TKO would come out exactly like the TH was. Some here thought that was a hill-billy thing to do....

When I got it back together (same angles before/after), I bought a digital angle gauge, and found that the pinion angle was way off anyway! I bought some Edelbrock adjustable upper control arms, set the angles both exactly the same before I even drove it, and no vibes. So, in my opinion, get one of those little digital angle gauges off of eBay for $40, and check the angles for the as-is condition, and correct if needed. It's easy.

So, would I like a rubber mount to replace the ES Poly I have now? Is the drivetrain any less "buzzy"? I have new ES Poly's on the motor, too.
 

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I have similar vibration at higher speeds. I also have the ES trans. mount in. I'll be swapping that out soon and repost if there's a change. Thanks for bringing this to the community's attention. :)
 
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