Hello!

So, this is yet another post in a seemingly endless saga (if you're curious, look up some of my past posts...).

After another 9 months, I finally have my car back. It's almost all new (2003 C5Z). Freshly built stroker (makes 450 at the wheels BOTH hp and torque), new drive line (except transmission), new clutch, new axle, quibos, propeller shaft input shaft, new shocks, all new poly bushings, new wheel bearings, Accumsump, Morroso catch can with dual lines form both valve covers and on and on and on...

The mounts (engine and transmission) were also change to Hinson Motorsports mounts.

So yesterday, I finally get to drive the car. It's powerful, it handles better than it ever has... and it has an odd vibration...

In the 2,000 - 4,000 rpm range, there is a noticeable (physically and audibly) rumble. I've narrowed it down to the engine (at 60 mph, go into neutral, engine at idle, not vibration. Rev the engine while still in neutral, vibration).

I'm hoping that it's just the engine mounts being stiffer than stock... or maybe it's not in which case, I may have a problem.

So my brothers (and sisters), what are your thoughts/experiences in this matter?

Thank you for your time!

Jeff

 

Make sure the exhaust is not touching anywhere.

Was the crank balanced with the clutch assembly?

 

I'll check the exhaust...

No, the crank and clutch were not balanced together...

 

On LS1/6 engines with a manual transmission, you need to be concerned about noise/vibration/harshness due to the clutch. ALL MN6/MN12 C5s had an ADDITIONAL "HOT BALANCE" procedure done to the flywheel and the damper. The additional critical balance eliminated that vibration that you have.

So,,, Now you have to do some experimenting to see IF, you can reduce or eliminate the issue.

The easiest way that I know is to remove the lower bell housing cover. Mark each one of the pressure plate bolts with a number. remove ONE BOLT and add a weight (washer) to that bolt. Spin up the engine and see if the vibration gets BETTER or WORSE.

Continue to add test and remove the washer until you find a quadrant that makes it better. Once you find that quadrant add additional washers until the vibration is better or eliminated.

NOTE! I have NO idea how far you are out of critical balance. One washer could be too much to start with. Its all a big experiment.

You might have to use 1/2 a washer to start or one that is thinner than a regular washer. Give it a try.

When all is said and done, use a precision scale and weigh the mass that it takes to make the problem better. Depending on the thickness of the washer, you MAY NEED a LONGER pressure plate bolt to have full thread protrusion! That longer bolt will need to be taken into consideration weight wise.

Let us know about you thoughts.

I helped a forum member from Canada solve the same issue successfully using the washer method.



Bill

 

usually with an aftermarket full rebuild, the specs on the balance of the rotating assembly are held to much tighter tolerances than from the factory negating the need for the hot balance procedure.

I would ask the builder what kind of tolerances they kept it to. It should have a zero balanced flywheel on it.

 

Assuming the engine is blueprinted and zero balanced, the dampener isn't delaminating, and the clutch assembley is zero balanced. You won't have engine rpm related balance issues.

I would start by trying to narrow it down. Find someone with a more extreme car then yours, and take them for a ride and see what they think. They might not even notice it cause its just part of having a modified car and normal nvh.

It is easy enough to get the clutch and flywheel checked once you get it out.

Assuming you have a new dampener that is some reasonable brand that you don't need to worry about the balance of it. That just leaves the engine being out of balance.