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) XT/Perf Street Sprung Moderate to heavy pedal effort, smooth engagement, OE to mild gear rattle; Longer friction life, extended by ACT's exclusive diaphragm design; Carefully engineered, precisely assembled, accurately balanced, with premium organic friction materials for heat tolerance and reduced fade; Recommended for street and race use; 100% clamp load tested and dynamically computer balanced for smooth high RPM reliability TS4-XTSS kit features an Xtreme pressure plate for the most demanding driver. ACT Xtreme pressure plates use exclusive diaphragm design to increase clamp load, reduce deflection and maximize clutch life. All diaphragms endure a four-stage, heat-treating process for unparalleled performance. ACT pressure plates have precision cover stampings to provide rigidity and consistency. ACT's Performance street discs are the most popular disc purchased. They feature premium organic materials with high copper content for better heat transfer and steel-backed lining for greater burst strength and durability. These discs have reduced marcel for faster shift action to complement ACT pressure plates. Quiet operation with increased strength and reliability.