HD fan clutches tuned to flow more more air throughout the operating range to better cool the engine and lower the temperature of the AC during idling times.
There are 3 variants:
Modified Blue hub for NA engines
Modified 1HD Orange up for Diesel engines.
Modified SC Orange hub for Supercharged engines. These clutches replace the stock clutch. They allow you to use the stock cooling fan without the spacer installed which provides better cooling.