Airflow (lb/min) a turbo must move for your target power - and a rough frame size.
Crank horsepower you are aiming for.
Petrol makes roughly 10 hp per lb/min of air; E85 a little less per lb/min but tolerates more boost.
Match the lb/min to a compressor map to pick a turbo that is efficient at that flow.
As a rule of thumb, petrol makes about 9-10 horsepower per lb/min of airflow. So a 500 hp target needs roughly 50 lb/min. Match that figure on the compressor map at your target pressure ratio.
Around 50 lb/min of airflow, which is typically a ~62-67 mm compressor wheel frame on petrol - but the right choice depends on engine size, RPM and how much lag you accept.
No. An oversized turbo is laggy and inefficient at low flow; a turbo sized to your real airflow target spools sooner and stays in its efficient zone.
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