Here is a good place to start:

http://www.honda-tech.com/zerothread?id=1024174


"A/R stands for area ratio and is a mathematical description of the cross sectional area of the turbine housing inlet divided by the radius described by the center of the turbine wheel to the center of the turbine housing inlet."

"A large A/R housing has good flow, less backpressure and creates more top end power at the same boost pressure by improving volumetric efficiency. It will also spool slower. Conversely, a small A/R housing has less flow and more backpressure but quicker turbo spool."