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As an example, think of A/R as a straw and you are trying to blow air through it. The smaller the A/R, the smaller the diameter of the straw, or air passage. If you take a coffee straw and try to blow air through it, you quickly reach the flow limits for volume and velocity of the straws air passage. You may have also noticed pressure in your cheeks as you were not able to get the air through the straw fast enough. In turbo talk we call that back pressure which can be really bad for an engine especially in the quest for higher horsepower. Finding a way to decrease engine backpressure helps promote flow from the cold side of the engine (intake) to the hot side (exhaust). More Flow = More Horsepower. Now imagine the same example with a larger diameter straw. The straw’s larger air passage can flow more air and you are able to exhaust your breath easier without as much pressure (back pressure) in your cheeks. This is very similar to how an engine and turbo work together. The engine produces exhaust air that flows through the turbine housing and propels the wheel assembly. So, selecting the right size A/R for your turbine housing is very critical to reaching the optimum performance for your build.
A/R stands for Area over Radius. It is defined as the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area. 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.
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