Turbo Systems 102 (Advanced)

Please thoroughly review and have a good understanding of Turbo Systems 101- Basic prior to reading this section. The following areas will be covered in the Turbo System 102 - Advanced section:
1. Wheel trim topic coverage
2. Understanding turbine housing A/R and housing sizing
3. Different types of manifolds (advantages/disadvantages log style vs. equal length)
4. Compression ratio with boost
5. Air/Fuel Ratio tuning: Rich v. Lean, why lean makes more power but is more dangerous
Wheel trim topic coverage
Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say "I have a GT2871R ' 56 Trim ' turbocharger. What is 'Trim?' 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. 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.
Understanding housing sizing: A/R
A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as: the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area.
The A/R parameter has different effects on the compressor and turbine performance, as outlined below.
Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.
Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.

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