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Turbo Aspect Ratio - AR

Aspect Ratio (AR) is commonly used to indicate a turbocharger housing size. The Aspect Ratio (AR) is the ratio of the area of the exhaust turbine inlet to the radius from the turbine impeller center. The AR ratio should remain the same as the Volute gets smaller as it gets closer to the impeller.

Aspect Ratio effects turbo spool up time (Turbo Lag) and maximum air flow. Increasing the AR will increase spool up time, but increase top end performance by allowing more air to flow. Decreasing the AR will decrease spool up time, but reduce top end performance.

Turbine housing size (aspect ratio) increases or decreases engine inlet air flow and can be varied in order to gain low end, midrange or top-end power from the engine. A smaller turbine housing can provide excellent low-end and midrange HP gains but won't carry the torque curve to a high RPM, limiting the amount of peak horsepower.

Decreasing the aspect area, using a smaller turbine housing, decreases the maximum airflow to the turbine wheel. A smaller turbine housing builds pressure and turbine speed quickly. The faster turbine speed increase (spool up) allows the compressor to increase air pressure at lower engine speeds which results in higher low speed power. The trade off with quick pressure build up and turbine spool up is that it causes an increase in exhaust back-pressure. The back-pressure results from the exhaust flow restriction caused by the small housing. The back pressure limits engine volumetric efficiency (air flow through the engine). This limits RPM increase (engine airflow increases) and a torque curve drop off and maximum horsepower.