SS and G6 discs are Sprung Centered for noise reduction on solid ACT Flywheel. Must use ACT Flywheel with Sprung Hub Discs for proper clearance. (Flywheel not included) Perf Street Sprung Disc Smooth engagement, quicker shifts, OE to mild gear rattle; Premium organic composite friction blended for high heat tolerance, increased burst strength, faster recovery from overheating and less wear of mating surfaces; Spring retainers feature heat-treated wraparound spring windows that provide support and reduce stress concentrations that are common causes for failure; Recommended for street and race use; Steel-backed linings for higher burst strength, reduced marcel for quicker shifts 3000601 is a performance street disc and the most popular disc purchased. It features premium organic materials with high copper content for better heat transfer. Performance street discs allow for quicker shifting and smooth engagement. These discs feature a reduced marcel for faster shift action to complement ACT pressure plates. ACT spring-centered assemblies use bushings and cone springs similar to factory clutch discs. This creates frictional dampening, which improves clutch life and reduces noise. Quiet operation with increased strength and reliability.