[Cloudsurfer]

If you have a dual-mass flywheel on your car, changing from an un-damped "normal" size pulley to an un-damped underdrive pulley is fine, as you haven't made any changes to how things work (now, whether these motors should have come from the factory with a harmonic balancer is a whole different discussion).

If you have a single-mass flywheel, then balancing the rotating assembly and running a damped pulley is a requirement if you expect the motor to last.

Porsche did go to a dampened pulley on the M97 3.8, and I have to assume that they had their reasons, as those cars still came from the factory with a dual-mass flywheel.

Road Sport Supply sells a damped underdrive pulley, really designed for the M97 3.8 but as long as you use the longer 997 bolt it works just fine on any of the M96 family.

If I were going to bother going to a UPD, I would use the RSS unit and add some damping factor to the motor.