weird vibration

the car whent on the dyno and still a vibration . which was felt thru the dynopack also but he says all the 944 suffer like this on a dynopak..

On my LM7 build there were originally some strange vibes in those rpms that went away after grounding both heads to chassis.

what did the head grounds do to it ? i could try did urs do it parked and driving was worst ?

just had a look back through the post trying to remember everything discussed. so you rebuilt the engine yourself, did you change anything in the rotating assembly? rods, pistons, pins, crank, etc? did you have the rotating assembly balanced? im wondering if something is mismatched weight wise in the rotating assembly, like say you put some aftermarket pistons in and they were heavier or lighter, the bob weight of the crankshaft is now out and you will get a vibration. could even have been something that happened before you owned the engine, if the engine was ever rebuilt at some point.

if the clutch assembly is balanced, there isnt much left other than the engine itself. the only other test you can do would be to remove the clutch assembly, install an automatic flex plate and run the engine, either back in the car or on a test stand.

matty89 wrote:
So i pull the flywheel out and pressure plate and got them balanced...
Its not worst than Ever since its zero balanced....

It "is" or "isn't" worse than ever?

its worst now that it is balanced. before i got her balanced i decided to add weight to the flywheel by added washers on the pressure plate bolts . which helped the vibrations go down but they were still there but endurable/hardly noticable . Now that she is balanced to zero its like i was on the worst gravel road i have ever driven on only at 3000-3200 then stops and at 4000 it does it again and stops after that....

sharkey wrote:just had a look back through the post trying to remember everything discussed. so you rebuilt the engine yourself, did you change anything in the rotating assembly? rods, pistons, pins, crank, etc? did you have the rotating assembly balanced? im wondering if something is mismatched weight wise in the rotating assembly, like say you put some aftermarket pistons in and they were heavier or lighter, the bob weight of the crankshaft is now out and you will get a vibration. could even have been something that happened before you owned the engine, if the engine was ever rebuilt at some point.

if the clutch assembly is balanced, there isnt much left other than the engine itself. the only other test you can do would be to remove the clutch assembly, install an automatic flex plate and run the engine, either back in the car or on a test stand.

i will have to find a flexplate but what would that tell me ? it would say i need my clutch assembly to weight the same ? if it doesnt vibrate

really its just one of those things you could try to go "yup its 100% for sure the engine", however with what you have tried thus far im 99% sure your dealing with a rotating assembly balance problem.

i remember years ago my dad built a 454 for a friend of his and it had a bad vibration at idle, then it went away and came back to mid range rpm. after lots of frustration trying to find the source the owner got tired of it and put a 502 ramjet in instead. after years of sitting in the corner the engine came back apart, and it turned out that the machine shop removed the dowel pin in the crank flange during machining. the balancing shop didnt catch that it wasnt there and balanced the crank without it. when the flex plate got bolted on a new dowel pin got put in, and that was the source of the out of balance condition. funny to think something the size of the dowel would cause that much vibration.

matty89 wrote:its worst now that it is balanced. before i got her balanced i decided to add weight to the flywheel by added washers on the pressure plate bolts . which helped the vibrations go down but they were still there but endurable/hardly noticable . Now that she is balanced to zero its like i was on the worst gravel road i have ever driven on only at 3000-3200 then stops and at 4000 it does it again and stops after that....

I had a similar problem when I changed engines... I used my old 350 flywheel on a 383 which I quickly found out was zero balanced. The vibration was awful as soon as I started it. I knew right away it must have been zero balanced. So I ripped the 383 out and bought a zero balanced flywheel and everything was good.... sounds like your situation is gonna take a little more to figure out though... goodluck!

Take off the belts and run just long enough to see if the vibration is still there and you've eliminated everything except the rotating assembly and the valve train. It's always a good idea to balance the assembly unless you're freshening up a running engine. It's likely a mismatch between the balancer/crank/flywheel because of the specific RPM and you affected it when you balanced the flywheel.

sharkey wrote:really its just one of those things you could try to go "yup its 100% for sure the engine", however with what you have tried thus far im 99% sure your dealing with a rotating assembly balance problem.

i remember years ago my dad built a 454 for a friend of his and it had a bad vibration at idle, then it went away and came back to mid range rpm. after lots of frustration trying to find the source the owner got tired of it and put a 502 ramjet in instead. after years of sitting in the corner the engine came back apart, and it turned out that the machine shop removed the dowel pin in the crank flange during machining. the balancing shop didnt catch that it wasnt there and balanced the crank without it. when the flex plate got bolted on a new dowel pin got put in, and that was the source of the out of balance condition. funny to think something the size of the dowel would cause that much vibration.

on my crank to flywheel there wasnt a dowel pin .... but i placed the dowel hole toward the dowel of the flywheel.......

ls engines dont use a dowel pin in the crank flange. you have the flywheel oriented correctly as well. i was just mentioned that to show how something small that can get missed can cause massive vibration issues.

Matty89, did you ever end up solving this one? I have the exact same problem and have been going through the same process as you and I'm stumped.