It's the area of the scroll into the turbine (a1-a6), nothing to do with a/f
"The turbine housing A/R (area/radius) ratio is the area (A) of any turbine inlet scroll cross-section divided by the distance from the center of that cross-section to the center of the turbine shaft (R). For any given turbine housing, A and R vary in the same proportions, so all As divided by their corresponding Rs yield the same dividend--which is the A/R ratio.
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
The R in A/R is the distance from the center of the cone's cross-section to the center of the turbine shaft. A smaller R imparts a higher rotating speed to the turbine; a larger R gives the turbine shaft greater torque to drive the compressor wheel (because the lever arm R is longer)."
