How to size a turbo
How to pick a turbocharger from a power target: required airflow in lb/min, pressure ratio, and how to read a compressor map without getting lost.
Turbo choice looks like black magic, but it reduces to one question: can this compressor deliver the airflow your power target needs, at your boost, efficiently? Answer that and the catalogue shrinks to a handful of options.
Step 1 — power target → airflow
Horsepower is made from air mass. The fuel your engine burns per horsepower (BSFC) times the air needed to burn it (AFR) gives the airflow:
airflow (lb/min) = hp × BSFC × AFR ÷ 60
For a well-tuned boosted petrol engine (BSFC ≈ 0.55, AFR ≈ 12:1), that works out near the classic rule of thumb: about 1 lb/min per 10 hp. A 450 hp target needs roughly 45–50 lb/min.
Step 2 — boost → pressure ratio
Compressor maps don't speak "psi of boost"; they speak pressure ratio (PR) — absolute outlet pressure over inlet:
PR = (boost + atmospheric) ÷ atmospheric
18 psi of boost at sea level (14.7 psi ambient) is a PR of 2.22. At altitude the ambient falls, so the same boost needs a higher PR — one reason mountain cars work their turbos harder.
Step 3 — put the point on the map
A compressor map plots airflow (x) against PR (y), with efficiency "islands" in the middle, the surge line on the left and choke on the right. Your (airflow, PR) point should land inside the 70%+ efficiency island — ideally slightly right of centre, so spool-up (which approaches from the left) doesn't clip surge.
- Point near/left of the surge line — turbo too big in the housing or the map too narrow: it will surge on transients.
- Point off the right edge (choke) — turbo too small: it'll spool fast but run hot, inefficient and out of breath up top.
- Point mid-island — right-sized: efficient at target with headroom.
The trade-off you can't escape
Bigger compressors flow more but spool later. Chasing a dyno number pushes you toward a big frame; chasing street response pushes you smaller. Be honest about where the engine lives — a turbo perfect at 7,000 rpm and miserable at 3,000 makes a slower road car. Modern twin-scroll and ball-bearing units shift the compromise, not remove it.
FAQ
How many lb/min do I need per horsepower?
Roughly 1 lb/min of compressor airflow per 10 hp for a typical boosted petrol engine — so 400 hp needs about 40 lb/min. Efficient engines need slightly less, rich or old designs slightly more.
What pressure ratio is 20 psi of boost?
At sea level: (20 + 14.7) / 14.7 ≈ 2.36. The same 20 psi at 2,000 m altitude, where ambient is about 11.5 psi, is a PR of 2.74 — noticeably harder work for the compressor.
Is a bigger turbo always capable of more power?
More peak power, yes — but it spools later, so the car can be slower in real driving. Size to your power target with modest headroom, not to the biggest map you can find.