Sizing a turbo for an engine is about matching airflow capacity and efficiency to the RPM range you actually use — not the headline horsepower number on the compressor map. A turbo that makes 700 HP at 7,500 RPM but is laggy below 5,000 is the wrong turbo for a street car that lives at 3,500–5,500.
Calculating the airflow
CFM = (CID × RPM × VE × PR) ÷ 3456
Where PR is the pressure ratio (absolute boost pressure ÷ atmospheric pressure). A 350ci engine at 6,000 RPM with 90% VE and 15 PSI boost (PR = 2.02) needs approximately 1,100 CFM. The turbo's compressor map must show an efficiency island that covers your operating range at this flow rate and pressure ratio.
Reading the compressor map — surge vs choke
Compressor efficiency is critical. Operating in the surge zone (left side of the map, too little flow for the pressure ratio) causes the compressor to stall and reverse flow — violent oscillations that destroy the turbo shaft and bearings.
Operating in the choke zone (right side, too much flow) means the compressor wheel can't accelerate the air any further, so adding RPM produces no additional boost. Ideal operating point: the center of the highest-efficiency island on the compressor map.
A/R ratio and the most common sizing mistake
The turbine housing A/R (area-over-radius) ratio controls spool characteristics. A smaller A/R (e.g. 0.63) increases exhaust gas velocity at the turbine wheel, spooling the turbo faster but restricting top-end flow. A larger A/R (e.g. 1.00) allows more exhaust flow at high RPM but spools slower.
The most common turbo sizing mistake is choosing for maximum HP potential instead of the RPM range the engine will actually use. A street car spends 90% of its time below 5,000 RPM — size for the range you use, not the peak number on paper.