Is a Blow-Off Valve Legal in the US?

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A blow-off or diverter valve is legal in the US, with a catch on turbo cars that measure intake air: a recirculating valve is the safe, compliant choice, while a vent-to-atmosphere valve releases already-metered air and can upset running and emissions.

Why?

On a turbocharged car, the factory diverter valve recirculates pressurized air back into the intake when you lift off. A recirculating aftermarket valve does the same and keeps the car running and metering correctly, so it is the straightforwardly legal option.

A vent-to-atmosphere (VTA) valve dumps that air outside instead. On a car that measures intake air with a mass-airflow sensor, that released air was already counted, which can make the car run rich and, in emissions terms, is not ideal. In California and CARB states, emissions-relevant intake changes are expected to be CARB-compliant.

So a recirculating valve is the clean, road-legal choice everywhere; a VTA valve is best treated as an off-road or track item, especially in CARB states or on any car where it affects emissions.

What decides if it's legal

A recirculating diverter valve keeps metered air in the system and is the safe choice.
A vent-to-atmosphere valve releases metered air, which can affect emissions and running.
In CARB states an emissions-relevant valve should be CARB-compliant.
It must not cause the car to run rich or fail emissions.

Does it depend on your state?

Whether a valve raises an emissions concern depends on how the car meters air and whether you are in a CARB state. A recirculating valve avoids the issue on any car; a vent-to-atmosphere valve is where the state and setup matter.

Related legality guides

Find your car in the garage

Sources

This page is general guidance, not legal advice, on the rules in the US for blow-off valve. The detail varies by exact vehicle and changes over time - confirm with your insurer and the EPA, NHTSA and your state DMV before modifying.