reg for reg gas, and premium for premium gas. Set the switch according to what octane gas you run. It compensates for pre-detenation and adjusts the timing accordingly I believe. Maybe someone else can chime in and double check.
2-300higher rpm on the topend which equals to more speed. The way these are overjetted from the factory I always ran mine on premium even running regular fuel. Overall runability is somewhat improved and slightly better fuel economy, but that's just me.
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