Wheel "trim" refers to the squared ratio of the smaller diameter divided by the larger diameter times 100. Generally, the larger the trim number the more flow the wheel has.

For compressor wheels , larger trim tends to mean slightly lower efficiency.

For "families" of turbine wheels (those with the same inducer diameter), larger trim usually means better flow with less backpressure but longer spool time.

A/R is a ratio of the exhaust discharge area vs the distance from the center of turbine wheel to the center of the cross sectional area. Smaller A/R housing has faster spool up. Bigger A/R housing has less back pressure, more flow for the top end.

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