All in One: A/R, Compressor Maps, Turbo Lag, Surge, Twin Scroll, VGT, Wastegate +more

Introduction

The A/R (Area over Radius) number on the compressor or turbine housing of a turbocharger simply indicates the housing size. Larger A/R values correspond to larger housings, implying a bigger turbo. For example, Garrett turbochargers typically have A/R numbers as ratios like 0.43, cast onto the housing. This number relates to the cross-sectional area at the compressor inlet. Calculating the A/R number involves measuring the radius from the center of the turbo axis to the center of the cross-sectional area and applying the formula for the area of a circle divided by the radius.

The performance of A/R is analogous to varying the blocking of a hose with a thumb. Smaller A/R numbers enhance responsiveness at low RPM by increasing gas velocity but can choke the engine at high RPM due to reduced volume capacity. Conversely, larger A/R numbers support high RPM performance by increasing volume capacity but reduce low RPM responsiveness.

Boost threshold and boost lag differ: boost threshold is the RPM at which the turbo starts generating significant boost, while boost lag is the delay in torque response after the throttle is opened above the boost threshold. Using a smaller A/R turbine housing can reduce the boost threshold but won't significantly affect boost lag.

Bearings in turbochargers can either be journal or ball bearings. Journal bearings survive longer under consistent oil supply, while ball bearings offer reduced rolling resistance, thus improving boost threshold and reducing boost lag, though at a higher cost.

The Center Housing Rotating Assembly (CHRA) is the turbo's core. Replacing the CHRA can be a cost-effective and quicker way to rebuild a turbo.

The compressor wheel design impacts turbocharger performance. Increasing the inducer size (the part that bites into the air) may boost peak power but increase the risk of surge, whereas increasing the exducer size (the part that expels air) enhances response and supports higher boost pressures. The compressor wheel's trim, calculated by squaring the ratio of the inducer to exducer diameter then multiplying by 100, indicates flow capacity: higher trim numbers mean greater max airflow and higher peak power.

Compressor Maps are visual tools indicating the performance range of a turbo. They plot airflow versus pressure ratio, showing efficiency "islands" where the turbo performs best, and the surge and choke lines, which indicate operational limits.

Surge occurs when the turbo supplies more air than the engine can handle, creating damaging reverse pressure waves. It can be mitigated by using appropriately sized turbos, adjusting boost pressure, or employing an anti-surge housing.

Twin Scroll Turbos separate exhaust pulses to maximize efficiency, improving spool-up and performance by avoiding interference between sequential cylinder exhaust pulses.

Internal and external wastegates manage boost pressure by diverting exhaust gases. Internal wastegates are simpler and integrated into the turbine housing, while external wastegates offer more precise control.

Variable Geometry Turbo (VGT) systems fine-tune the a/r ratios via adjustable vanes in the turbine housing, enhancing performance across a broader RPM range. They are mainly used in diesel engines due to the higher cost of materials needed to withstand gasoline engines' higher exhaust temperatures.

Keywords

• A/R Ratio
• Turbo Lag
• Boost Threshold
• Compressor Wheel
• Compressor Map
• Surge
• Twin Scroll
• VGT (Variable Geometry Turbo)
• Wastegate
• Bearings

FAQ

Q: What does the A/R number on a turbocharger represent? A: The A/R number represents the area over radius of the compressor or turbine housing. It indicates the size of the housing and affects the turbo's performance, with larger values supporting better high RPM performance and smaller values enhancing low RPM responsiveness.

Q: What is the difference between boost threshold and boost lag? A: Boost threshold is the RPM at which a turbo starts producing significant boost, while boost lag is the delay in torque response when the throttle is opened above this RPM.

Q: How do ball bearings compare to journal bearings in turbos? A: Ball bearings reduce rolling resistance, which lowers the boost threshold and lag, but they are more expensive and slightly less durable than journal bearings under ideal oil supply conditions.

Q: How does a twin scroll turbocharger improve performance? A: A twin scroll turbo separates exhaust pulses into different channels, reducing interference and increasing gas velocity, which enhances spool-up and performance, particularly at low to mid RPMs.

Q: Why are VGT turbos more common in diesel engines? A: VGT turbos use delicate vane mechanisms to adjust turbine housing size dynamically. Diesel engines' lower exhaust temperatures allow these materials to function effectively, whereas gasoline engines' higher temperatures require expensive, high-temperature-resistant materials.