In Search Of - Vibration

DorianL

solid fixture here in the forum
Staff member
Anyone remember that old TV show "In Search Of" ? :D

This will be one of those longer running threads (cuz it is too :mad: cold to work on the car consistently). So it's more about picking your brains.

I took my car out twice this weekend trying to gain some insight into where the vibration may be coming from.

What we have is a 1969 corvette with a 400 SBC and a T5 transmission from an 1991 Camaro.
The nearly-new 10:1 performance SBC 400 came out of a lux-o-barge. It was mated to a TH350 and had no obvious vibration. It is externally balanced. Accordingly, the flywheel I threw on there is also externally balanced. I reused my clutch kit.

Back to my test drive.
If it makes any sense, the vibration feels like it is synchronized with the exhaust pulses. It feels like a rumble and maybe hearing the exhaust is reinforcing the feeling. It is also more noticeable on acceleration and not so much cruising at higher RPMs.

I am suspecting that this may relate to contact between the shifter base and the transmission console. I am very sure they are touching.

I also noticed that when I put my foot in it, the shift lever comes over quite a bit. I can't quantify it, but it feels like a lot; it yanks around in my hand. The motor mounts and the trans mount are new.

I am speculating that I mounted the trans too high when fabricating the removable crossmember.

I suspect that, in addition to the touching shifter base, when I accelerate, the trans itself might be coming over and touching the fiberglass transmission tunnel. I seem to recall there isn't much clearance under there.

My plan to test this theory is to replace the trans mount with a lower profile one giving me more clearance.

Here comes the weird question. I don't think lower profile trans mounts exist... can one trim one 1/2 an inch, and... would polyurethane be a better idea ? (All current mounts are rubber.)

What are your thoughts ?

:cool:

D
 
What is the mount made of(tranny mount), regular rubber, polyurethane, solid? Is the mount centered in the crossmember? Had a friend that made his own crossmember, the mounting plate was off center about 3/8 of an inch, it jerked when he revved it up, also the driveshaft rubbed when he made a sharp turn. I would confirm that the mounting holes are exactly center, or are in a location that holds the tranny centered. I have seen a few cars that the holes had to be a little off center, in order to mount the tranny centered.
P.S. Just saw in your post, that the mount is rubber, I would use urethane!
 
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Does it have the single rear T 5 trans. mount on it still ? I had a vib. like that in my 55 chevy truck until I put a bar with 2 rear trans. mounts under it with steel and a 1/4 in. hard rubber under trans. single bolt hole?
 
Part ordered :D

Does it have the single rear T 5 trans. mount on it still ? I had a vib. like that in my 55 chevy truck until I put a bar with 2 rear trans. mounts under it with steel and a 1/4 in. hard rubber under trans. single bolt hole?

I’m using a trans mount with two bolt holes kinda like this one.

F0F667F6-B354-4726-BE22-7FCD44C4C4C3.jpeg

I think the torque on that 400 is really twisting that tall rubber mount as well. Hm curious what difference this will make.

I spent a lot of time trying to make sure that trans is as centered as possible. I have no doubts there.
 
I had one of those single bolt or stud tranny mounts once. I didn’t like it! I just brought up the centering thing to put a bug in your ear. Sometimes we forget, I know I do. I think losing that 3/4 of an inch will help and stiffen it up some too! Good luck Dorian!
 
I think you realize what it is thats half the battle now to find the solution but when you said about the shifter tilting a smaller thickness mount should really help that with less to flex
 
I think you realize what it is thats half the battle now to find the solution but when you said about the shifter tilting a smaller thickness mount should really help that with less to flex
It will flex less, and will move away from the tranny tunnel by 3/4 of an inch!
 
exactly what I was thinking I don't know the thickness of the mounts but in theory a 1/2 inch mount should flex 50% less than a 1 inch mount but I'm also wondering if his mount is broken because it still shouldn't have that much flex and yes I know its new that only means that its shinny not defective
 
Stock mounts are 1.75, the replacement Dorian ordered is 1 inch! So his tranny will move away from the tranny tunnel by 3/4 of an inch! It will more rigid, plus will give the added 3/4 inch of clearance! So it should be 75% more rigid by your estimate!
 
I'm also wondering if his mount is broken because it still shouldn't have that much flex and yes I know its new that only means that its shinny not defective

Agreed. I had the same thought. The shifter came over so far that it also set me wondering if the rubber mount was torn. I probably will put the Corvette up on stands to check this out.

I ordered the part. There is no direct shipping to Belgium, so it has to go o my freight forwarder in Florida. Luckily shipping to Florida is free. It may be a few weeks before we test this out.
 
I was thinking the same thing, either it’s defective, broken, or maybe the rubber is not stiff enough for the torque of a 400. I agree, stock mounts are kinda cheaply made at best! But they are designed for lesser duty also, not for performance related applications.
 
Check These.
 

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Thanks T. I do think the vibration is not related to speed, but rather RPM.

That being said, I have: replaced all the U-joints, balanced the driveshaft, replaced the transmission, replaced all the rubber mounts, made sure the driveline geometry was spot on...
 
Thanks T. I do think the vibration is not related to speed, but rather RPM.

That being said, I have: replaced all the U-joints, balanced the driveshaft, replaced the transmission, replaced all the rubber mounts, made sure the driveline geometry was spot on...

Well that only leaves the engine balance--flywheel--clutch--only way I know how is one at a time. Repair or replace--and I know how hard it will be to do and to get replacement parts or s machine shop.

If the shifter moves side to side when accelerating a lot then I suspect engine mounts/trans mount. try chaining the left side of engine.

Aint working outside fun ??
 
just listening to the conversation, it sounds like either the damper/balancer on the front of the crank
and/or the flywheel/clutch assembly are not properly balanced
 
I was thinking the same thing, either it’s defective, broken, or maybe the rubber is not stiff enough for the torque of a 400. I agree, stock mounts are kinda cheaply made at best! But they are designed for lesser duty also, not for performance related applications.
I don't know about the lesser engines 1969 had some pretty good muscle under the hood we ar
 
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