Turbo sizing is about matching the compressor's airflow capability to your engine's needs. Too small and the turbo chokes at high RPM. Too large and you get turbo lag.
CFM (Cubic Feet per Minute) is the volume of air the compressor can move. lb/min is the mass flow rate, which accounts for air density and is what turbo manufacturers actually rate their compressors by.
At sea level, 1 CFM of air weighs about 0.069 lb. The calculator converts your engine's airflow needs to lb/min to help you match a real turbo from a manufacturer catalog.
Compressor A/R (0.42–0.70): Smaller A/R = faster spool, less top-end. Larger A/R = more top-end, more lag.
Turbine A/R (0.48–0.96): Smaller A/R = quicker spool, more backpressure. Larger A/R = less backpressure, slower spool.
Small engines (1.6L–2.0L) with quick spool goals: 0.48–0.63 turbine A/R. Larger engines (2.5L+) or top-end builds: 0.63–0.82 turbine A/R.
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