Nope, they aren't good for ANY engine, period.. They suck, plain as that.With a turboed engine an Underdrive pulley will not benefit you. In fact it could ruin you
Pulleys are normally good for NA engines
From WikipediaI read that aluminum lightweight pulleys, underdrive or not, adversely affect the motor because the do not have a harmonic dampener. Not really sure what that is other than a fancy name for a big heavy chunk of steel but didn't want to take a chance after reading it. I did have one on a 2000 Legacy GT and did notice quite a difference in acceleration. I think there are much better (safer) options for gains. My .02
Wow, its amazing how many people come on here and say WOW!Once your car reaches about 1500 rpm, your alternator is at maximium output.
Its also incorrect to say they don't work on turbo cars. They offer the same improvements on any car, regardless of turbo or not.