The turbine housing has a spiral design and it is typically made of steel or cast iron. Its main function is to collects the exhaust gases from the engine, before directing them into the turbine wheel. As a result of this the wheel rotates. Turbine housing increases exhaust drive pressure and also controls boost and exhaust pressure. The size and radial shape of the housing impact to the performance of the turbocharger.
The turbine housing is an essential part of a turbocharger because if it cracked or worn out the turbo will not generate boost pressure correctly.
On the other hand, using a small A/R reduces the flow capacity which leads to increase exhaust backpressure and impacts peak engine power.
The A/R of the housing adjusts the flow capacity of the turbine, therefore, using bigger or smaller A/R will impact turbine performance. For example, if you use a larger A/R, the amount of the exhaust gas decreases which causes boost at a higher engine speeds and more power overall. Also, there are less engine backpressure and wastegate flow.
To summarise, the size of a turbine housing is crucial for a turbocharger’s performance. Larger turbine housings lead to more power at the top end but longer time to generate it due to a lag, while smaller turbine housings offer quicker spool, less lag but they can create a ‘’choke’’ point at high engine RPM which will restrict power. There is no right or wrong option, it is up to your preference, engine displacement and vehicle use for the best fit for your application.