This.

 

I have a turbo and I have run BOTH stock and UDP. From my personal experience I DID NOT feel any difference in power or torque. Now its a matter of engine safety....I chose safety and went with the stock pulley.

The UDP is a bad design from the beginning. It should have been the same diameter as the stock pulley, this way your lights won't dim at idle.

 

its an underdrive pulley..its going to be smaller.

 

inertia is proportional to the mass and radius of an object turning.

I.E. a thin disk, the inertia required to spin it is I=1/2(M*R^2)

so you can free up power by lessening the mass, but much more power is freed by shortening the radius. A combination of both obviously frees the most power.

 

Bad design, dont think soo. My lights dont dim and I run the pulley. But then again i also swapped out my stock head lights for others. Secondly, talk to ROB at RRM if your not getting the info you want from here. Yes the stock pulley will be better for your car with higher boost. Some people run the UDP with there turbo kit and no preblems, but i kept my stocker for when I got turbo. Switching back to the stocker on a turbo system is fine, you will never see the loss that the pulley gives you N/A.