What does “A/R” mean in relation to Turbochargers?
It is the area to radius ratio. Think of a long ice cream cone. Now think of that cone wrapped around a shaft kind of like a snail shell. Cut a little bit off the closed tip of the cone. This hole is the discharge area for the exhaust gases. The area of this hole is the “A” of A/R ratio. The “R” is the distance from the center of the area (A) to the center of the shaft. No matter where along the cone you make this measurement the A/R for a give cone will be the same. Back to the small open end of the cone. This determines the velocity of exhaust gases exiting the cone and therefore the velocity of exhaust gases as they pass over the turbine wheel. Overly simplified, the smaller the A/R , the quicker the turbo will spool up, but also the more it will contribute to higher back pressure and reversion into the combustion chambers. The smaller A/R also limits max turbine speed. This means that the .48 A/R turbine side (T3) will spool a little quicker than the .63 A/R and will have higher ba
