Is a Flame Thrower Exhaust Kit Legal in the US?

Why?

Flame kits work by sending raw fuel into the exhaust, usually with the catalytic converter removed so the fuel survives to the tailpipe, and igniting it with a spark unit. That combination fails on two independent grounds: defeating the cat is tampering under the federal Clean Air Act in every state, and deliberately projecting flame from a vehicle on a public road is a safety hazard that state equipment codes and local fire ordinances treat accordingly.

Even factory-style "pops and bangs" overrun tunes sit in murky territory in the US; a purpose-built flame kit is far past that line. A car shooting flames in traffic gives an officer multiple things to write up at once, from illegal equipment to reckless operation, and any resulting fire is on the owner.

As with other extreme mods, the show circuit is the honest home for this: a trailered show car flaming on private property with permission is an event matter, not a traffic one. On the street, there is no legal version of a flame kit.