Anyone here with a ZF-6 that has made the mistake of putting in a standard flywheel in place of the discontinued dual mass flywheel knows what let down that rattle it makes is to the street driving experience. I didn't learn until after replacing the flywheel that the C4's flywheel is externally balanced to the engine. If I had looked at the back of my dual mass when I had it replaced a decade ago, I would have seen that it had weighting holes in the back and been able to transfer that info to my then new flywheel, but I have a race that upcoming weekend and was running for a championship. 

Now that my formal competition days are over, and I am looking to cruise the car more, the flywheel is annoying. I don't have any reason to tear down the engine, pull the crank and balance the flywheel to it. Is there another way with the engine in the car to figure out the balancing for the flywheel. 

1990 C4 Z51 ZF-6sp. 

Have you tried a lightweight aluminum flywheel? In the past I’ve balanced the flywheel, clutch and pressure plates together. But I’m sure you’ve done that in the past for competition. Good luck, hope this helps.