Wiles 4” unit, not as smooth as a stock 2 piece but smoothest of 3 brands I tried and I’ve had all up to 200 mph. M6 drive shaft spins much faster then a A8 drive shaft at the same speed so that’s a factor that comes into play as well with drive shaft vibrations, I’m M6 so with the 3.90 rear I run at 200 mph the drive shaft was spinning about 11,000 rpm’s verse probably around 7000 for a A8 drive shaft with a 3.09 rear. That’s flying for such a long unsupported in the middle piece.