How to size a turbo
A compressor map has two axes: airflow (usually lb/min) across the bottom and pressure ratio up the side. Size a turbo by working out the airflow and PR your target needs, then choose one whose map puts that point in its high-efficiency island — not off the edge (choke) or hard left (surge).
Air mass follows the fuel the engine must burn:
airflow (lb/min) = hp × BSFC × AFR ÷ 60
where BSFC (brake-specific fuel consumption) is around 0.50–0.55 for an efficient boosted petrol engine and 0.60–0.65 for a harder-run or older one. Pressure ratio compares absolute manifold pressure to ambient:
PR = (boost + ambient) ÷ ambient
Ambient falls with altitude, so the same boost is a higher pressure ratio up a mountain. Pick a compressor whose map contains your (airflow, PR) point with margin for spool and a bit of headroom.
FAQ
What BSFC should I use?
For a well-tuned boosted petrol engine, about 0.50–0.55 lb/hp/hr. Use 0.60–0.65 for a conservative estimate, an older engine, or one running rich for safety. Diesel is much lower (~0.35).
Is the airflow figure for one turbo or two?
It's the total the engine needs. On a twin-turbo setup, split it between the two compressors — each sees roughly half the airflow at the same pressure ratio.
Does altitude really change turbo choice?
Yes. Thinner air lowers ambient pressure, so hitting the same boost takes a higher pressure ratio and the compressor works harder for the same lb/min. At altitude, size with the local ambient in mind.