I ran across an article from Engine Builder that specifically mentions the Buick/GM 3800. The article says it's supposed to be 36.6% balanced, but I assume that's with the balance shaft. When disabling or removing the balance shaft due to running a double roller timing chain, the rotating assembly should be 50% balanced, right? Does balancing the rotating assembly include the harmonic balancer and flex plate? Does 50% balancing the engine benefit high rpm?

 

The balance shaft doesn't impact bottom end balancing technique, its only a NVH reduction item for helping counteract non primary vibration. Actually in the static condition, it's neutral balance down the center shaft axis (same amount of weight on both sides of the axis).

Because of this, the engine does not need to be rebalanced in order to remove it (as evidenced by the tons of people running a double roller timing chain / aftermarket cam on an otherwise stock bottom end).

However the 3800 is an externally balanced engine so the balancer and flex plate are required to balance the bottom end. Youll notice they both have an offset weight which means they're intentionally out of static balance.

Having the engine balanced will lessen the imbalance stress at higher rpm. The stock engine didn't spin that high, so the tolerances didn't have to be super precise. When building/ balancing, you can get much more consistentent throughout.