What is A/R?
A/R, also known as Area/Radius, defines the ratio between the cross-sectional area of the exhaust turbine housing and the radius from the turbo centerline to said cross-section.
Why is this important?
The A/R ratio drastically affects your new turbo’s spooling characteristics and potential horsepower by modifying the flow capacity in the turbine housing.
Which A/R ratio is best for my application?
A lower A/R ratio will increase exhaust velocity. The larger radius makes for a more tangent exhaust entry thus improving response and low-end torque. Consequently, this also limits the overall flow capacity (more backpressure) of the turbine and will hinder high RPM engine power.
On the contrary, a higher A/R ratio will decrease exhaust velocity. This will noticeably create more “boost lag” but in turn will provide lower backpressure and increased power in the higher RPM range.
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