Vintage Cars: Chevrolet Drivetrain question, iroc camaro, chev 350


Question
i dont know if they quite call these vintage yet, but since you mentioned chevy knowledge, i figured id try, this could happen with any car i guess.....

ive got an 86 Iroc camaro, has a 350 of some sort its a throw together as it doesnt match parts from their accustomed 5.7 tpi, for one, i just replaced the flywheel, the one from the iroc 350 wouldnt fit, too small for the crank, the one that fit was from an unknown Chev 350 Diesel engine....never heard of before but have heard of an 83 caprice coming with one, so now it starts great, runs VERY strong, but in a mid to high rpm situation i get a NASTY vibration, whether in gear or not, doesnt seem to affect its running as in power and such, sounds and idles healthy, doesnt stall, but once some rpm winds up she vibrates i get a feeling the flywheel is out of balance, the whole car vibrates pretty much...too much to try to pinpoint.....but i dont know these irocs too well, and never run into this issue before, just wondering if not out of balance, what else may cause this, and how bad it can be hurting things, i wont start it again if its causing damage, but even if it didnt hurt anything, id rather lose the vibration, never did that before so im guessing something had to have changed with the new flywheel.....any info helps, if you need any more info, ill gladly provide it, also if it helps at all, theres NO check engine light on, oil pressure is right on, and battery is charging properly,

thanks in advance,
john

Answer
I ran into this myself with a 66 L-79 Nova I owned for a few months.  Bought it without really giving it a hard run, and the first time through the gears at speed, same thing you mention.  Real bad vibration.  Not enough to tear things apart right away, but you just know it can't hold together forever that way and that something is really wrong.

I balanced the driveshaft and the entire clutch/flywheel assembly.  Double checked the crank pully/balancer.

I came to the opinion that it was something seriously mismatched inside the engine as the vibration was matched with the rpm's of the engine like yours it seems.

(Just had a thought, is there a bearing in the crank for the nose of the tranny to fit into?  sometimes these are missing or bad, not even 100% sure newer small blocks have them, but I would think they do, well, back to my other train of thought:)

Could be mismatched rods, a crank out of balance, or could be the mismatch of the flywheel and balancer etc.  

Since you already know you have an odd combination, I recommend that you start by checking the date codes and other codes on the block.  Just google number codes, vin number etc for Camaro and you will find all sorts of resources.

My guess is you may wind up taking everything apart and seeing exactly what you have.  Then make sure it matches and balance the entire thing before assembly.  Some people don't think balancing is necessary for a street car, but the only car I did it for was a 4 cyl MGB.  Yea, balancing a straight four....I got a few chuckles from my friends on that one, but that engine was the smoothest MGB engine I had ever experienced.   I would do it again in a heartbeat.

Don