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What is A/R Ratio of Turbocharger
When choosing a turbocharger, you often see an A/R number, and if you’ve taken a closer look, you’d find out that A/R applies both on to compressor cover and turbine cover. A/R stands for Area over Radius. Both compressor and turbine housings use the same formula to describe the housing size. A/R is a critical parameter in the design and performance of turbocharger.
Understanding A/R
This is the cross-sectional area at the narrowest point of the turbine or compressor housing.
Radius(R):This is the distance from the center of the turbine or compressor wheel to the center of the cross-sectional area.
(photo📷 @hmudaart)
Significance of A/R
Performance Characteristics
Choosing the proper size A/R housings for your turbocharger is very important to reach the optimal performance of your build.
- Higher A/R Ratio: A higher A/R ratio indicates a larger housing, meaning it has a bigger air passage that allows more exhaust gas to flow through the turbine and reduce backpressure. Back pressure occurs when the air cannot get through the air passage fast enough. Higher A/R housings result in better top-end power although it may increase turbo lag.
- Lower A/R Ratio: A lower A/R ratio indicates a smaller housing with smaller air passage, which in turn allows less exhaust gas to flow through the turbine, increasing back pressure. This results in quicker spool and better low-end torque, but with limited top-end power.
Turbo Lag
Turbo lag is the delay between the moment when the throttle is applied and when the turbocharger starts to provide significant boost pressure to the engine. The engine needs to produce enough air pressure to spin the turbine and force compressed air to the engine intake manifold. A lower A/R ratio housing helps in reducing turbo lag as the turbocharger spools up faster due to smaller turbine housing.
(photo📷 @baxleysspeedshop )
Engine Matching
Matching proper A/R ratio turbo to the engine’s characteristics and desired performance is very crucial. A turbocharger with an A/R ratio of 0.63 spools up faster and provide strong mid-range performance while a turbocharger with an A/R ratio of 1.06 provides better performance at higher RPMS with more power available at the top end of the rev range. This means a high-performance engine designed for high RPM will more likely to benefit from a higher A/R Ratio turbo.
(photo📷 @jxkehxlmes)
Choosing A/R
Main purpose of your vehicle is also one of the significant factors in choosing the A/R, various circumstances require corresponding A/R ratio turbos.
(photo📷 @skulldrift )
Daily Drive
When you’re tuning your vehicle mainly for daily drive, you need a turbocharger that can provide you the better spool under low-range and mid-range along with better low-end torque. In this case a smaller A/R ratio turbo is a better choice.
For Races and Events
If you’re building a vehicle for high speed or race events, your engine often operates under high RPMs, therefore the turbocharger needs to be able to provide optimal compressed air pressure and efficiency to comply with the engine. A higher A/R ratio turbo is therefore a better choice for higher performance requirements.
A/R ratio of a turbocharger is a key parameter in determining how the turbocharger will behave in terms of spool time, power delivery, and overall engine performance. Understand and choosing the proper A/R ratio ensure turbocharger to provide the best overall performance under various circumstances.
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