DaveSmed

It may not vent into the atmosphere, but some turbo cars have control solenoids that vent into the suction side of the turbo, after the air meter. If the port for that vent is not capped, it would pull air in causing a lean condition. That brass valve in that picture looks like a Gills boost valve. That should be plumbed between the compressor outlet and the wastegate. It should not vent anywhere. You can use that valve to fine tune your boost level by loosening the locknut, and turning it in for more boost.