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Discussion Starter #1
So I have descovered that I have a pulsing vibration in 5th gear when I am at 70+mph and 2,300+ rpms, but only in 5th gear. I don't think it is the tires, as the vibration goes away the second I let off the gas or push in the clutch. I feel it in the floor and pedles, and it seems to be loader and felt more under harder throttle.

When I put the engine at the same rps in 3rd or 4th, I don't hear it or feel it. Before I start ripping into things, I wanted to hear what you guys thought. I don't think it is the engine, or it would do it in 3rd and 4th gear, so I think it may be the transmission or the drive shaft, or the rear end. It doesn't seem to have any effect on the power of the engine, and I don't hear any difference in the exhaust note.

I have recently welded in sub frame connectors, put in lower control arms, put in wide band gauge, and some exhaust work. What do you think?

Thanks,
 

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only in 5th gear under load? how old is your clutch? it could be early signs of a slipping clutch.
 

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Discussion Starter #3
The clutch is stock, along with the trans...
 

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Discussion Starter #4
So after some research online, I have found that it can be a number of things. From motor mounts, to u-joints, to transmission, or the rear end.

I don't feel the vibration in the wheel at all, I sort of feel it it in the shifter, but I really feel it in the floor and pedals, only under load. I also have an odd vibration that seems to be coming from the intake right behind the throttle body at about 2,200 rpms while in nutrual or any gear. That confuses me, because the only moving part is the throttle body, but that is the place I har it the most when I have the stethoscope on. I don't think they are related, but the vibration under load really concerns me.:headscratch:
 

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Discussion Starter #5
I did weld in the sub frame connectors before I noticed it. Do you think that could have anything to do with it?

Thakns,
 

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I had a similar experience long ago, when I put a set of delrin bushings (plus a full race rear suspension) in my 1977 280Z Datsun.

The handling improvement was apparent, but the sudden buzzing and transmission of noise into the cockpit was extreme. Turns out I had eliminated some critical items that were isolating me from the mechanical world out back, and I finally just swapped them back out of the car and put new OE bushings back in.

Nowadays they have urethane, which is a better stopgap between solid mounts and rubber, but Dendro this might just be that you are now experiencing more noise and vibration from it being transmitted via the new parts you installed.

Another possibility is that it is partly the result of harmonics from the exhaust, again being carried directly into the floorboards. This would be most likely to occur if at any point the exhaust is making contact with the body or those new parts (subframe connectors, for instance).

BUT the fact that it is most noticeable in top gear and "under load" (which would indicate you are moving at a rapid clip) makes me want to check out the wheels and tires first.

If you run the same size tires front and back, try rotating them. If the buzz goes away or decreases (or moves to the front end), you have a clue. If you are running larger on the back than the front (I do), you will need to get the rear wheels re-balanced, and see if that helps.

Finally, this COULD be a pre-existing problem with your aftermarket gears that was subtle and being masked by your car's standard bushings and isolators, but your new additions now allow that high speed buzz to vibrate your unibody. This could indicate that you need to have your gear install revisited.

Best of luck, and I hope this helps.
 

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Discussion Starter #7
Thanks trippleblack! I will start with the wheels. I did mount the side exhaust hangers to the seat bolts this past week. Perhaps I will take the mounts off and see if I hear it then. At least I have a few things to go on.

Again, thanks...
 

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Mounts dead on the seat bolts...

COULD be the culprit right there.

You might try to isolate them with some old shock absorber bushings (if you are a packrat like me and have some sitting around).
 

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Discussion Starter #9
Well it's not the mounts for the exhaust, removed them and it still does it. I noticed it after I welded in the sub frames, so I will take her in to get the rear tires pulled and balanced again. I don't think I felt it after I put the LCA's in, which I put in a week before the SFC's. Hopefully a good balance will take care of it.

Thanks,
 
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