AR, or A/R refers to the ratio between a turbine or compressor’s inlet area and the radius between the center of the inlet and the center of the turbine shaft. While both the turbine and compressor have an AR, it is generally the turbine AR that people refer to in discussion.
A smaller turbine AR generally provides a lower boost threshold?, but also lower maximum horsepower.
A higher turbine AR generally provides a higher boost threshold?, and also higher maximum horsepower.
For more information:
http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech102.html
