[skorpioskorpio]

The Atlas hits it's undesireable harmonics around 65-6700 rpm, it is rev limited to 6300. However there are available harmonic balancers to move that all up 1000 rpms. Bi-metal balancers can make these engines free of destructive harmonics well into the 8000 rpm range. There are guys racing them that routinely and intentionally push them past the harmonics and into the 9000 rpm range and unintentionally past 10K.

Remember that straight sixes have impecable balance and it is the length of the crankshaft and the resonance frequencies that are the problem not the rotating balance. If a 6 never stays at the rpms of these frequencies for any period of time they are not self destructive. The rotating assembly of the Atlas is very capable of very high rpm operation the valve train doesn't float until past 10K, though I don't know how long and sustained I'd want to push a long timing chain.

The Atlas crank is designed without a keyway for the balancer so to add a high rpm capable balancer the crank needs to be removed and notched for a keyway.