Competition Clutch Ultra Lightweight Steel Flywheels (STU suffix) are recommended for forced induction andor 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. All flywheels are SFI certified. Not all applications have the ST and STU versions available. Flywheel weights vary from 8lbs to 18lbs depending on application.