Turbo A/R Ratio

A comes from the word AREA and means the cross-sectional area of the inlet throat. R comes from the word RADIUS and means the distance measured from the center of the area to the axis. A/R is the ratio of these quantities. Typically, A/R ratio is 0.35–1.50.

With a smaller A/R ratio (all else being the same), the turbo starts spinning at a lower RPM. The downside is that a small housing clogs early, causing exhaust pressure and temperature to rise more than the benefit from the turbo.

A/R = cross-sectional area / distance to axis. Example: Throat Ø 2.6 cm → A = 1.3² × 3.14 = 5.31 cm². R = 5.0 cm. A/R = 5.31 / 5.0 = 1.06. Smaller housing from same turbo: Ø 2.2 cm → A = 3.80 cm². R = 5.6 cm. A/R = 3.80 / 5.6 = 0.68.