What you need to know:
As the volume decreases in the volute of the housing, the exhaust gas is able to maintain velocity and a high energy level thus increasing turbine wheel speed. A small turbine housing A/R can also be a choke point with too small a size limiting the efficiency of the system by increasing backpressure and preventing total horsepower capability.
If the boost pressure to backpressure remains equal (1:1) the engine essentially thinks it is naturally aspirated. The boost pressure can continue to be turned up higher and higher until the backpressure climbs too high (above 1.75-2.0:1) or the strength limitations of the engines components are reached. Some backpressure can be a good thing for street/driving use as the pressure differential helps with turbine wheel speed and transient boost response. For racing applications it is critical to maximize the turbine housing as much as possible to keep backpressure low and efficiency high while still providing the necessary response time. There are no written rules to sizing turbine housings and as such professional recommendations and testing are often the best way to start.
Let’s say you have selected a Turbonetics turbocharger and have successfully installed it on your setup - you love the power it delivers but would like to improve low-end torque (or high-end power). It’s an easy task with a large selection of Turbonetics turbine housing A/R options. Simply change to another A/R turbine housing and fine tune your setup! Utilizing a smaller A/R will amplify the exhaust gas energy to the turbine wheel. This will allow quicker spool up. However, this will reduce the maximum power potential on high RPM. On the other hand, a larger A/R will delay the delivery of the exhaust gas energy to the turbine wheel. This will allow high RPM power at expense of low-end torque. If you are having traction issues due to high power/boost in low RPM, a larger A/R turbine housing will help you to achieve the optimal power band. On another note, compressor housing A/R options are almost nonexistent as they are not critical for turbocharger performance.