Is a Supercharger Conversion Legal in the US?

Why?

Bolting on a supercharger changes how much air the engine moves, which puts it squarely in emissions-related territory. Federally, the rule is the familiar one: the conversion must not remove, disable or defeat any emissions control the car was certified with. A kit that keeps the cats, sensors and evaporative system working, with fueling handled properly, does not violate the Clean Air Act.

California and the states following CARB add the formal step. There, an emissions-related kit must carry a CARB Executive Order for the exact vehicle it is installed on, and the big supercharger names sell EO'd kits for popular applications precisely so the cars pass smog checks. A non-EO kit in a CARB state fails the check regardless of how clean it actually runs.

Outside CARB states, an emissions-intact kit on a well-tuned car is generally accepted, though a check-engine light from crude tuning will still fail any emissions test. Buy a kit with the right paperwork for your state, tune it to run clean, and forced induction is an established legal path to serious power.