The turbine must make the compressor spin fast enough to produce the required airflow at the specified boost level. A small turbine spins faster than a larger turbine (which reduces lag), but develops more backpressure (which restricts exhaust flow). The goal is a turbine that spins fast enough to generate the necessary response and airflow while minimizing backpressure in the exhaust.


But brute size is not all that matters. The turbine's A/R (area/radius) ratio basically determines where the turbo starts to accelerate. A turbine housing looks kinda like a big snail shell. Unwrap the shell and it resembles a cone. Cutting off the tip of the cone leaves a hole--the cross-sectional area of this hole is the A in A/R. The hole size is important since it determines the velocity at which the exhaust gases exit the turbine scroll and enter the turbine blades. For a given flow rate, the smaller the hole, the higher the velocity--but the greater the restriction to exhaust-gas flow