It's a rubber dampner on the crank pulley intended to help make engine internals last longer due to vibrations caused by the motor.
You can run a solid aluminum pulley but you will likely find more metal shavings in your oil pan in the future & loads of premature wear on engine internals like valves, pistons etc.
I read a great article about the wear years ago when someone installed an aluminum crank pulley on their Sentra R - the pics of the engine ripped apart after several thousand miles was pretty amazing.
I'm not sure if the page still exists, I'm too lazy & too old to try to search right now.
There's really no real need to run a lightened underdrive pulley on a street car.
As was mentioned above, do things to remove weight from the drivetrain itself + run lighter weight rims.
You've heard of unsprung weight right?
Install a C/F driveshaft which is a ~30lb weight savings over stock, lightened f/w, shimless (shim under buckets)
Make what you have more effecient, then do things like strip the interior if you want to go deaf, tear out the A/C so you can burn up & tear out the power steering so it will be as uncomfortable as possible to drive.
Speaking of Sentras makes me think of the magazine that "made" a kid's Sentra a 10 second car. Good stuff. :twisted:
