Turbo Sizing Guide – Hot Side Turbine Housing AR Numbers and Wheel Dimensions Explained

Turbo sizing guide for diesel performance builds – today we cover turbine housing AR numbers, cubic centimeter sizing, turbine wheel dimensions and why the same AR number on two different turbos produces completely different results.

Cold Side Sizing – The Simple Part

The compressor side of a turbocharger is relatively straightforward to select. Garrett, BorgWarner and Holset all publish power ranges for their compressor wheels. A GTX2860 covers approximately 200-350 horsepower. Experience across many builds refines this further. For most builders selecting the cold side based on manufacturer power ratings is adequate.

One important note on Garrett naming convention – when Garrett calls a compressor a GTX2860 the 60 refers to the exducer diameter, the large end inside the compressor housing. The inducer, where the air filter connects, is smaller. When drag racing media talk about 91mm turbos they are referencing the inducer – the small end. Garrett references the exducer – the large end. Keep this in mind when comparing specifications across sources.

Hot Side Sizing – Where It Gets Complicated

The turbine side is where most builders get confused and where correct sizing makes the biggest difference to boost response and power delivery.

AR Numbers and Cubic Centimeters

Turbine housings are rated in either AR numbers or cubic centimeters depending on manufacturer. Holset uses cubic centimeters. Garrett uses AR.

AR stands for Area over Radius – specifically the discharge area of the turbine housing divided by the radius from the centre of that area to the centre of the turbine wheel. The calculation is complex but the number is useful as a comparison tool.

Holset to AR approximate conversions:

• Number 8 housing ≈ 0.55 AR
• Number 10 housing ≈ 0.73 AR
• Number 12 housing ≈ 0.89 AR

The HX35 typically comes standard with a number 12 housing – 0.89 AR. This is a very large turbine housing which is why HX35 equipped builds often feel sluggish and why swapping to a number 10 is a common improvement after running the standard setup.

For Garrett turbos 0.63 and 0.64 AR are common turbine housing sizes for diesel performance builds in the 400-700 horsepower range.

Why Turbine Wheel Size Changes Everything

Here is where it becomes genuinely complicated. The same AR turbine housing on two different turbos produces very different results because the turbine wheel size determines how much exhaust gas flows through regardless of housing AR.

Turbine wheel dimensions for common turbos:

GTX2860:

• Turbine inducer: 54mm
• Turbine exducer: 47mm
• Small turbo – good to approximately 400 horsepower with 0.63-0.64 AR housing

GTX3076:

• Turbine inducer: 60mm
• Turbine exducer: 55mm
• Six millimetres larger inducer, eight millimetres larger exducer than GTX28
• Good to approximately 600 horsepower with the same 0.63-0.64 AR housing

The GTX30 flows 50% more power than the GTX28 through the same AR turbine housing because the larger turbine wheel flows more exhaust gas regardless of housing restriction. This is why you cannot simply compare AR numbers between different turbos – wheel size is the primary variable.

HX30:

• Turbine inducer: 65mm
• Turbine exducer: 52mm
• Five millimetres larger inducer than GTX30 but three millimetres smaller exducer
• Typically runs 6-9 cubic centimetre housing (approximately 0.41-0.65 AR)
• Excellent performance from relatively small housing due to wheel size

HX35:

• Turbine inducer: 70mm
• Turbine exducer: 60mm
• Ten millimetres larger inducer than GTX30, five millimetres larger exducer
• Despite massive turbine wheel typically runs number 12 housing (0.89 AR)
• Opportunity exists to improve response significantly by fitting smaller turbine housing

BorgWarner S200:

• Turbine inducer: 70mm
• Turbine exducer: 62mm
• Similar inducer to HX35, two millimetres larger exducer
• Excellent flow capacity, very capable turbo for high power diesel applications

BorgWarner S300:

• Turbine inducer: 80mm
• Turbine exducer: 73mm
• Enormously large turbine wheel
• No turbine housing in the S300 range is small enough for optimal use on most diesel builds tested
• Solution used in practice: S200 turbine housing fitted to S300 turbo
• When this turbo is spinning at 200 km/h the results are impressive

Practical Conclusions

Larger turbine wheel equals more exhaust gas flow capacity at any given AR number. This means:

A GTX3076 with 0.63 AR housing flows more than a GTX2860 with 0.63 AR housing despite identical AR numbers. The wheel does the work, the housing controls the velocity.

Go as small as possible on turbine housing AR for your turbine wheel size. The HX35 at standard number 12 housing is significantly underperforming its potential. Number 10 or smaller transforms response while maintaining top end flow because the 70mm turbine wheel can flow the exhaust gases regardless of increased housing restriction.

Holset turbos as a family flow exceptionally well despite appearing large. They are an exception to many general rules – they spool better than their size suggests and respond well to turbine housing reduction.

Summary

• Turbo sizing guide: cold side from manufacturer power ratings, hot side requires understanding turbine wheel dimensions and AR numbers
• Garrett names compressors by exducer size, drag racing media uses inducer size – keep this distinction clear
• AR numbers: Holset number 8 ≈ 0.55, number 10 ≈ 0.73, number 12 ≈ 0.89
• Larger turbine wheel flows more exhaust gas at any given AR – wheel size is primary, AR is secondary
• GTX2860: 54/47mm turbine – 400hp with 0.63 AR
• GTX3076: 60/55mm turbine – 600hp with same 0.63 AR
• HX35: 70/60mm turbine – significant improvement possible by reducing from number 12 to number 10 housing
• S300: 80/73mm – use S200 turbine housing for best results on diesel builds

Cover the hot side, understand the numbers, choose correctly. See you in three days!