The key advantage to a DLC coating is the superior sliding surface it creates between two moving parts. This allows significant reductions in friction, and far greater durabiltiy due to reduced reliance on the lubricity of the fuel.
Standard Needle and Nozzle
A standard needle is uncoated and relies on the end result of the precision machining. This leaves very slight imperfections in the surfaces that increase the reliance on the fuel to prevent wear. Also this slightly increases the friction and thus lowers the response time of the needle consequently lowering its overall efficiency.