BTW I have searched for a week.
I have had the poly trans mount in for some time and I have always had vibrations at high speed since. When I installed it I left the preload plate out. I also could not get the the mount to line up right so I drilled another hole just off to the side of the slot. I now got a new x-member and am going to enlarge the slot and see if my vibe is caused from the trans being off to the one side. I remember trying everything to get the mount to line up so we'll see what happens.

So my question is Preload plate in or out?

 

I have a Prothane mount, which is the one everyone recommends (Direct replacement, no messing with a preload plate) and after installing it I get a vibration at certain RPMs.

After doing some reading, I think it's because of the poly mount itself (As it transmits more vibrations).

 

I get a pretty bad vibration like something is out of balance.

 

If you could not get the mount to linw up, and redrilled, you have probably ofset the tranny, making the driveshaft run at a different angle than it should, which will cause harmonics, which you are feeling.


Reinstall it, the right way, and use the preload plate as per instructions.

 

If it isn't severe, the trans being off to one side won't affect anything. Cars actually have a slight amount of side offset from the factory, if they didn't the u-joints wouldn't work properly. I doubt if that is causing your vibration. Some vehicles have the rear drvieshaft installed diagonally from the yoke af the rear to the transfer case (not entirely uncommon on 4x4s).

The vibration is most likely the result of the poly mount being stiffer than the stock rubber, which absorbs a lot more vibration, which is why they use the rubber one in the first place. Anytime you replace rubber with poly you will pick up some NVH (Noise/Vibration/Harshness), which is an OEM term for one of the design criteria standards that a car must meet. This enures that the car will be pleasing to drivers of all sorts from teenagers to old ladies. If the car rattled and vibrated at speed it would be a hard sell to all but the most hardcore enthusiasts, and sadly that market share is just too small. Consequently the OEMs try to build a vehicle that is totally quiet, does not have any vibrations at any speed or RPM, and rides like it was on a cloud.

Most of what gets a car to that level is what enthusiasts seek to change when they modify the car for more performance. The car is too quiet, so we go with lower restriction mufflers and/or cats and bigger pipes, which also increase power. Vibrations are a cost of stiffening up the suspension and chassis, we just live with them. If the car bucks a little over rough pavement and the shocks/springs are a bit stiff, it's the cost we pay for the car carving a corner when called upon. We aren't the rule, we are the exception.

I've noticed vibrations with poly trans mounts on a variety of cars. The preload plate on an Energy mount adds about 1/4" to the mount height, which changes the driveshaft angle, and is most likely responsible for the vibrations at critical shaft speed. I usually leave these plates out and have not had a problem from doing so. Most often I just run a rubber mount at the trans though. I will run poly motor mounts instead, allowing for the trans mount to dampen at least some of the vibration. A lot of guys I know are doing the same thing with good results. One benefit of the poly mount is the built in interlock though, so if the mount breaks the trans can't move all that much. It's a toss up I guess.

 

I agree, it is a price that we pay. I just think it is annoying and it makes the car feel like a heap. I hate going triple digits and the dash cover starts shaking. It just feels scary. I am waiting for something to fly off at those speeds. I would like my car to just glide along at 100+ and not shake itself to death. I was considering going back to the rubber mount but I am sure I will destroy it when I bolt on the drag radials and take it to the strip.
I am going to try a few more things to see if I can get rid of the vibrations.

 

I would be more concerned about the engine mounts with drag radials. I'd try a torque strap and poly mounts with a rubber trans mount.

If the car is shakign that voilently then you've got a driveshaft angle issue. My friend's Cobra did the same thing at around 100MPH. He had lowered the car and used a poly mount. Once we got the angles right the problem went away.

 

Maybe i should look into an adj. troque arm.

what is your goal when setting up the drive line angle?
Is there supposed to be no more than a degree between front and rear?

 

An adjustable torque arm isn't a bad idea. Ideally go with a divorced setup that moves the arm off the side of the trans. That's why a lot of hard launchign 3rd gens break the trans mount. A setup like Spohn's is a lot better. The crossmember absorbs all the load instead of the trans mount.

When setting the driveline angles you want no more than a degree variance from front to back. However, on a race car you may need a different static angle at the rear to account for the change in angle under hard acceleration as the pinion end moves up. This way the angles are the same when the car is loaded up going down the track, even though they may be 2-3 degrees different sitting still.

 

I installed it with mine. I also lowered the crossmember about 3/8" using spacers between the crossmember and 'frame' rails.

 

Well tonight I tackled the mount. I swapped in the new x-member and a poly torque arm bushing.
I had to do a bit of pulling but I got it to line up. I took a few blasts down the interstate and it seems that the vibes are gone. I was going 110 and it was smooth.
I do notice some slight vibes at low speeds and anything over about 1/2 throttle. Say for instance taking off from a stop light. I don't know if it is actually caused from the new torque arm mount or the new position of the x-member and trans. I will keep an eye on it as the bushings work their way into place.

 

BTW
I left the preload plate out

 

Well the slight vibrations are from the passenger side header hitting the chassis! It looks like I have to get under the car again and move some stuff around.

 

Went back to the way I originally had it. Couldn't move it far enough to the drivers side to get the headers away from the frame. I will just deal with it.