If anyone has anything to add please do so and provide technical data to better serve the DB members!

Trim
The term trim is used when talking about or describing turbochargers. But what is 'Trim?' Trim is a term to express the relationship between the inducer and exducer of both turbine and compressor wheels. Also know as an Area Ratio
The inducer diameter is the diameter where the air enters the wheel, whereas the exducer diameter is the diameter where the air exits the turbo wheel.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 changing the airflow abilitys of the charger. A higher trim will usually flow MORE, but not in all cases.

Housing Sizes or A/R
A/R (Area/Radius) is 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.
Larger A/R housings are sometimes used to maximize performance of low boost applications, and smaller A/R are used for higher boost applications.
Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing is used to alter the flow capacity of 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. BUT a small A/R also causes the flow to enter the wheel more tangentially, which reduces the flow capacity of the turbine wheel. This will increase exhaust backpressure and therefore reduce the engine's ability to "breathe" effectively at high RPM, affecting peak engine power.
Using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's flow capacity, leading to lower backpressure and more power at higher engine speeds.
When choosing an A/R be realistic with the intended vehicle use and use the A/R to “tune” performance toward the desired powerband .

What is surge? AKA the funny noise when I let off the throttle
Surge is most commonly experienced when one of two situations exist. The first and most damaging is surge under load. It can be an indication that your compressor is too large. Surge is also commonly experienced when the throttle is quickly closed after boosting. This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.
Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region. This situation is commonly addressed by using a Blow-Off Valves (BOV). A BOV functions to vent intake pressure to atmosphere so that the mass flow ramps down smoothly, keeping the compressor out of surge.

 

thanks, this was helpful for me to understand a turbo just a lil more.

funny noise when I let off the throttle