SS, G6 and G4 discs are Sprung Centered for noise reduction on solid ACT Flywheel. Must use ACT Flywheel with Sprung Hub Discs (Flywheel not included) HD/Race Sprung 6 Pad Light to moderate pedal effort, aggressive engagement, OE to mild gear rattle; Moderate friction life, extended by ACT's exclusive diaphragm design; Carefully engineered, precisely assembled, accurately balanced, uses select ceramic friction materials for high heat tolerance and maximum performance; Recommended for high horsepower street or race use; 100% clamp load tested and dynamically computer balanced for smooth high RPM reliability BM2-HDG6 kit features ACT's most popular Heavy Duty pressure plate. ACT Heavy Duty 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 6-pad, spring-centered race disc features a spring-center to properly damp torsional vibrations and resist failure under harsh conditions. ACT spring retainers utilize wraparound spring windows and thicker retainer stampings to provide spring support, reduce stress concentrations, and increase strength and reliability. With the 6-pad, spring-centered race disc, engagement is harsher, but shifting is faster. The 6-pad, spring-centered disc is recommended for larger clutch sizes to absorb more heat.