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First of all, speaking from experience, I have installed plenty of Z1 1pc shafts. From the very early version (smaller diameter, steel and painted black). It was well-balanced (no vib at low speed) but once at high speed >100mph, the vibration is crazy. This is due to the low first bending frequency of the shaft, nothing much you can do about it. Completely destroy the trans and differential. The later version of Z1 shafts I installed were much better. Bigger diameter makes a big difference. Not sure it will survive Vmax though. But kudos to the improvements. Factory Nissan design the 2 pc shaft not because it's harder to build a "consistently smooth" 1 pc shaft, in fact, it's easier and much cheaper to build. 2 pc shaft has much shorter center to center length, thus 1st bending frequency is very high. That means shaft critical speed will be very high (could be over 20k rpm) which the Z will never see this speed even at vmax. This chart give a better idea of shaft lengh, diameter, material that will effect driveshaft critical speed (rpm). I didn't measure the length/diameter of the Z1 latest version shaft but it should fall into one of this. Note: this speed has nothing to do with balancing. A perfect balance shaft will be the same. 
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No Russell, that's not true - 
