I have this horrible vibration coming from the motor/ tranny. the whole motor and tranny shake. It is ok at idle but anything over 1500 rpms is pretty bad. it feels like it is coming from under the car(the bellhousing). i darn not try to drive the car in fear of doing damage. even when i crank the car i can feel something thumping or vibrating under there. its not the starter, i made sure that it fully disengages and pulls away from the flexplate, the flexplate had no cracks and spins perfectly true when the car is running. could it be the converter? it is lined up properly and everything. i cant seem to figure out what is causing this. would a bad harmonic balancer cause it to feel like a horrible vibration coming from the bellhousing area. Does anyone have any idea? could it be a bad converter?
Matt

 

Did you just put the tranny on the engine. Sounds like an improper weight flywheel.

 

It could be caused by anything that is spinning (crank, cam, balancer, flywheel, converter, driveshaft, etc.).More information is need for us to help you. Didthe vibration just started all of a sudden? Did it start after you changed something? Does it do it while in park?What rpm does it start? Does it get better at high rpms? Is there a noise associated or just a vibration? Does it do it in park? Does it do it in gear? Is it RPM or speed related?

Flywheels and harmonic balancers have to match the rotating assembly balance (i.e. 50oz, 28oz, or internal 0oz). You would know if it were that because the engine would have started vibrating when you changed that part.

A worn out converter can also cause some bumping or vibrating when it startsto failbut it will not do it while in park.

 

Is it a mechanical vibration, or something like the fan nicking the shroud?

CP

 

welcome to the club ... i'm a vibration sufferer as well
what di dyou do before it happened?
as well:
does it happen as well when standing, revving in neutral
ig yes, does it happen with clutch pressed as well?

 

well i can start by saying that it is an internally balanced motor. its a pretty wild 383 stroker that is in my other car(72 Nova). I picked up the whole assembly(motor and tranny) to put in my nova. i pulled the motor apart to inspect everything and freshen it up because it was sitting for a few years. Everything looked good and clean so i put it back together and got it running. the vibration starts at about 1500 and gets better after 3500 rpms. this is all done in park and not drive. i dont have plates yet so i cant take it on the street to test it. The vibration is bad enough to make the cheeks on my face shake. It feels like its coming from the flexplate but i inspected it and couldn't find any cracks. if it were a bad damper could you still feel it from the bellhousing area? and when i am cranking it, it feels like something is thumping under the car.
any advice would be great
Matt

 

You got me. My old Cleveland has an unidentifiable vibration that came in at idle and left around 4500. I ended up pulling it because of low compression issues. I suspect that the vibration was coming from uneven forces on the crank from radically different compression values in each cylinder. It basically had one cylinder way down on compression and the other varied quite a bit. What are your cylinder pressure readings? To answer you question, a jacked up balancer or the incorrect balancer/flexplate can cause crazy vibration. I know because of putting the wrong flexplate on a 351w in my Bronco. It was a Plain Janer that felt like a 300 duration cam race motor. After getting the right flexplate it drives smooth as silk.

 

If it is a Chevy engine, (which I assume it is since its in a Nova) you know you have to run a 400 flywheel on 383s correct? Unless it is a forged aftermarket crank, chances are that it truely is not internally balanced. Now, the old 350s and other Chevy V8s were internally balanced, but not the 400. You said that you pulled it out to freshen it up... did you install the same flywheel that you took off when you put the engine back in?

 

Yea it is a chevy motor, its a 383 stroker with 13:1 compression, .645 lift cam canfield aluminum racing heads that have been flowed and alot of carb. the motor is built pretty good and it has 7 passes on it and he ran a best of 10.80 in the 1/4 mile in a 72 nova just like mine. It has a fluidamper balancer and i have no idea as to what flexplate it has, all i know is that is is an SFI certified plate. it is the same one that i took off the motor and just re installed it the same way it came off. The only other thing i can think of is that it could use a real good tune. currently i am runnig stock plugs and 100 octane fuel. i picked up some 112 octane and autolite racing plugs and will see if that makes a difference. The motor def had a miss and its because of the plugs. maybe the miss and the high compression is just throwing it for a loop.
any other ideas?
Thanks alot
Matt

 

That could be part of the problem, especially with that high of compression. I would continue on the path you are headed down and see where you end up.