Building revs with balance is why drivers love Subaru underdrive pulleys. If you have gone through the trouble of customizing your turbo setup then likely, there is a missing piece to the puzzle. For most WRX and STI owners, that is a Subaru underdrive pulley. The balancing act of downshifting and trying to maintain boost can grow tiring. It can also lead to laggy performance. Subaru underdrive pulleys lighten the load on the belt, resulting in quicker response. You can also count on a more even feel when on the road. It is all done with science. GFB engine pulleys combine lighter weight with less overall mass that makes cruising smoother than before.
One of the chief complaints Impreza owners have had is about the hitching going from low RPMs in high gears and shifting down. Go Fast Bits fixes this with their lightweight underdrive pulley kit. It is simple physics. The pulleys are smaller, thus mass is reduced. Less rotational inertia is what GFB engine pulleys are based upon. Removing weight/mass from the car results in better balance, especially in the lower gears. GFB engine pulleys do not make more horsepower. However, they do allow more power to be available due to the decrease in rotational mass. Going from 1st to 2nd to 3rd or down from 4th to 3rd becomes a lot quicker. That is because the GFB engine pulley helps catch revs before a steep drop.
This is not a single piece. Each underdrive pulley kit includes crank, alternator, and power steering pulleys along with their belt(s). The material used is key in the lower density. Go Fast Bits uses 6061-T6 aluminum in their underdrive pulley kits not just because it is light, but also due to its durability. There is no chatter during rotation. Plus, the aluminum helps each component in the underdrive pulley kit resist corrosion thanks to an anodized finish.
You do not need a special tool for installation. Unlike the factory parts, the belt included with the Go Fast Bits pulley kit fits over just one set of ribs. The only tedious part to the procedure (in some models) is removing the OEM alternator pulley, which is usually made easier with air tools. All GFB engine underdrive pulley kits come with instructions.