Is an Exhaust Cutout Legal in the US?

Not legal

An exhaust cutout is not street-legal when open. US states require an effective muffler in constant operation, and opening a cutout routes the exhaust around the muffler; if the cutout sits ahead of the catalytic converter, opening it is also federal emissions tampering.

Why?

A cutout is a valve that, when opened, lets exhaust exit before the muffler for maximum noise. State vehicle codes universally require a working muffler at all times and prohibit exhaust modified to increase noise, so running with the cutout open breaks that rule the moment you open it.

There can be a federal problem too. If the cutout is located upstream of the catalytic converter, opening it can route exhaust around the cat, which is tampering with emissions equipment under the Clean Air Act.

Kept permanently closed it is just pipework, but the purpose of a cutout is to open it, and open on the road it is not legal. It is track or off-road hardware.

What decides if it's legal

Every state requires an effective muffler in constant operation.
An open cutout bypasses the muffler, which is not street-legal.
If the cutout is ahead of the catalytic converter, opening it is also emissions tampering.
It fails state noise and equipment rules.

Does it depend on your state?

The muffler requirement exists in every state, so an open cutout is not street-legal anywhere. The emissions angle (if it bypasses the cat) is federal on top of that.

Related legality guides

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Sources

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