To regulate the boost pressure on turbo charged cars a wastegate

Wastegate

A wastegate is a valve that diverts exhaust gases away from the turbine wheel in a turbocharged engine system. Diversion of exhaust gases regulates the turbine speed, which in turn regulates the rotating speed of the compressor. The primary function of the wastegate is to regulate the maximum boost...

 is fitted to allow exhaust gases to bypass the turbine and pass straight down the exhaust down pipe. This can cause turbulent airflow around the turbine so to overcome this problem gases can be re-routed down a separate pipe about thirty centimetres long and vented straight to atmosphere. The reason for the term screamer pipe is the phenomenally loud jet engine noise produced which is popular and highly desirable for car modifiers. Due to the noise and the fact they miss out the catalytic converter

Catalytic converter

A catalytic converter is a device used to convert toxic exhaust emissions from an internal combustion engine into non-toxic substances. Inside a catalytic converter, a catalyst stimulates a chemical reaction in which noxious byproducts of combustion are converted to less toxic substances by dint...

these are not technically for street use.