Crankshaft Vibes

[6TIME]

Hey guys, I've been running a 4CW crank for the last year in my 4.6 build and I'm getting a vibration around 3300-4200 rpm. I'm just curious if anyone out there has encountered this? The motor is a .030 over 4.0 block, 7120 head with LS1 valves and springs, Short snout 4cw crank, Lunati Cam, .043 quench height, 26cc pistons all mounted to a 5speed. The rotating assembly has been completely balanced by a very reputable shop. It runs great as a daily driver but I'm looking to find the source of this vibration. I do have a heavy 12cw crank that I could drop in there, but not quite sure yet if it's the solution. I also noticed that The new Scat cranks are 4cw but heavier than Oem 4cw cranks at 56lbs. Anybody getting the same vibrations at those RPM's? It looks like it may be some type of harmonic. I wonder if Scat had a reason for making their stroker cranks heavier? Thanks

[SilverXJ]

Being 4 or 12 weight is not the source of the vibration. There is something else unbalanced here. Did the machine shop spin balance the crank, flexplate/flywheel, and harmonic balancer? Were the pistons and rods weight matched?

[6TIME]

Everything that attached to the crank was spin balanced, and the Rods and pistons were all weight matched.

[Cheromaniac]

Better check the outer ring on the HB in that case.

[6TIME]

Damper was new. Already swapped it out, no change. Tried swapping pressure plates also, no change. Just curious if anyone has any sort of vibes with the 4wt crankshaft before I go tearing it down to have them recheck the balance job.

[SilverXJ]

I have the 4wt. No vibes what so ever. And that is with stiffer engine mounts. Perhaps the balance job wasn't that great.

[6TIME]

Silver, Thanks that's what I need to hear. Obviously somethings not right because I have factory engine and tranny mounts. Looks like i'll be calling the machine shop and tearing my motor down. Did you have your motor balanced?

[SilverXJ]

Yes, I had the engine internals balanced. Crank was spun, flexplate was spun and harmonic balance was spun. Pistons and rods were wieght matched.

[lafrad]

you checked plugs and re-indexed the cam and crank positioning sensors, right? I'd also pop the valve cover and make sure all is well there, and a spring hasn't broken or something.

there are PLENTY of issues with tuning and sensors and etc that would only make themselves apparent at specific RPM.

[6TIME]

All of the plugs look great. Distributor is indexed correctly. Compression within 5psi in all holes. Besides the vibes, the motor runs great with plenty of power. Thanks for the replies so far.

[6TIME]

Tore it down today. All rotating parts will be checked by the machine shop. Found a new problem though....Destroyed cam bearings. Seems as though this is becoming more common. Motor has 26k on it.

[SilverXJ]

Did you notice anything that would have caused the bearing failure? Missing front oil galley plug perhaps? What was your oil pressure? Any other bearings show excessive wear?