Competition Clutch Ultra Lightweight Steel Flywheels (STU suffix) are recommended for forced induction and/or track and rally racing applications. This flywheel is machined with less material to assist in quicker revving capabilities. Well-suited for Stage 3 and up clutch applications. Made of 4140 forged steel and are a one-piece design. Not all applications have the ST and STU versions available. Flywheel weights vary from 8lbs to 18lbs depending on application.