An important term to know when talking about turbo housings is it's aspect ratio (AR). It's not important to know for a non modified car but it's still useful to know what an AR is. AR is the ratio of the area of the cone to radius from the center hub. If you were to measure the cross section on any point on the turbo and divide by the distance from the center of that cross section to the center of the turbine wheel, you would get the AR ratio. Ideally, this ratio should remain the same as you move in and out of the turbo housing because the housing gets smaller as you get closer to the center. This spiral shaped cone is called a volute. It begins where the flange opening changes to the shape of the volute. Basically, it concentrates airflow at a point on the turbo wheels through the spiral shape. Comparing only AR mean much because engine displacement, tune, supporting mods, and rpm, etc., also effect turbo spool up. But everything else being equal, increasing the AR will reduce spool up but increase top end performance by allowing more air to flow. Decreasing the AR will increase spool up but reduce top end performance. A larger AR will allow more air to flow through its passages.
