Turbo Wheel Trim Explained

What is Turbo Wheel Trim

turbo wheel trim

Illustration of the inducer and exducer diameter of compressor and turbine wheels

If you’re in the market for a turbocharger upgrade, comparing new units, or are planning your own DIY turbo kit, you’re going to run across turbo wheel trim. This is a term you will need to get familiar with because it is commonly used when describing turbochargers. For example, you may hear someone say “I have a GT2871R 56 Trim turbocharger”. Turbo wheel trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio.

* The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.

Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter.

  • Turbo wheel trim values are expressed as 45, 50, 55, etc… and can only go from 0 to 100. A value of 100 means Dp = Dg
  • A large trim indicates a large turbine diameter.
  • A trim of 55, gives 10% more caudal than a trim of 50.
  • Trim is used in the same way for turbine wheels as for compressor wheels.

turbine wheel

Real World Example

turbo wheel trim formulaExample #1:
GT2871R turbocharger (Garrett part number 743347-2) has a compressor wheel with the below dimensions. What is the trim of the compressor wheel?

Inducer diameter = 53.1mm
Exducer diameter = 71.0mm

Trim = Inducer diameter divided by outer diameter, squared, and multiplied by 100

or

(in / out) 2 x 100

turbo wheel trim formulaExample #2:
GT2871R turbocharger (part # 743347-1) has a compressor wheel with an exducer diameter of 71.0mm and a trim of 48. What is the inducer diameter of the compressor wheel?

Exducer diameter = 71.0mm
Trim = 48

This can be done in reverse as well.

Conclusion

The difference in flow has to do with the blade speed but issues such as the blade contour, swept angle, and surface area of the blades also impact the overall flow characteristics of the wheel in question. When it comes to compressor wheel selection, lean towards a large trim rating in a smaller wheel instead of going to a larger wheel. Larger wheels are heavier and rotational weight equals lag. The trim of a wheel, whether compressor or turbine, affects performance by shifting the airflow capacity. All other factors held constant, a higher trim wheel will flow more than a smaller trim wheel. However, it is important to note that very often all other factors are not held constant. So just because a wheel is a larger trim does not necessarily mean that it will flow more. If you want to learn more check out the wheel trim article on Garrett’s website.

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