Balancing Idiot 383 lt1

I have a scat crank with 6" eagle forged rods. I had the motor balanced and blue printed, but can't find information any more from it. The motor runs and sounds awesome, but not as smooth as i would expect. Seams to have a vibration through the entire RPM band. Is there an easy way to tell if the flywheel should be balanced or if it should be neutral? I'm totally lost and disappointed/concerned after a lot of money and time put into this engine.

Did your machine shop check your balancer and flywheel after the internal balance.

I don't know i bought it as an aborted project from someone else. I know pistons, rods and crank, so would there be a good guess if it was internal or external? The vibration is bad, once i get off idle seems to stay just as bad through out.

Most 3.75" stroke cranks will be external balance, as will nearly all one piece seal cranks.

Ok how sure are you about your last post? From what i've beed reading most of the aftermarket cranks for 6" rods are getting internally balanced. I'm starting to lean towards this, but guess i would like to know from someone else that had this problem how bad their vibration was? I've read a few different posts where people had this same problem.

for a correct balance job the machine shop needs the entire rotating assy, including the balancer and flex-plate or flywheel, and most 1 piece seal stroker kits are balanced internally and the balancer and flex-plate are balanced neutral,

check with your machine shop, you probly need to take the flex-plate off and have the weight taken off and have it neutral balanced

that is what i was thinking also. Neutral balance the flywheel.

Mine with a scat cast crank and 6.0 rods was internal both ends.

There is absolutely no way to know for sure without pulling the rotating assembly and having it balanced again. Like others have mentioned, the machine shop that balanced it should know which flexplate and balancer to use. If the engine is already in the car, you could mix-n-match different flexplates and balancers to see what gets rid of the vibration. Just know that, even though you may not feel the vibration, there still could be an im-balance problem.