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.
Turbo sizing chart
Airflow and pressure ratio across the range, generated by running the calculator above so a row here cannot disagree with the tool.
Compressor airflow needed, by power target
At 0.55 BSFC, 12:1 AFR and sea level — the defaults above. Airflow scales with power and barely cares about boost; pressure ratio is the part boost decides.
| Power target | lb/min | Pressure ratio at 15 psi |
|---|---|---|
| 200 hp | 22.0 | 2.02 |
| 250 hp | 27.5 | 2.02 |
| 300 hp | 33.0 | 2.02 |
| 350 hp | 38.5 | 2.02 |
| 400 hp | 44.0 | 2.02 |
| 450 hp | 49.5 | 2.02 |
| 500 hp | 55.0 | 2.02 |
| 600 hp | 66.0 | 2.02 |
| 700 hp | 77.0 | 2.02 |
| 800 hp | 88.0 | 2.02 |
| 1,000 hp | 110.0 | 2.02 |
| 1,200 hp | 132.0 | 2.02 |
Pressure ratio by boost and altitude
Pressure ratio is what you read off a compressor map's vertical axis. Thinner air raises it for the same gauge boost, which is why a turbo that is comfortable at sea level can run out of map in the mountains.
| Boost | Sea level | 1,000 ft | 3,000 ft | 5,000 ft | 7,000 ft |
|---|---|---|---|---|---|
| 5 psi 0.34 bar | 1.34 | 1.35 | 1.38 | 1.41 | 1.44 |
| 8 psi 0.55 bar | 1.54 | 1.56 | 1.61 | 1.65 | 1.71 |
| 10 psi 0.69 bar | 1.68 | 1.71 | 1.76 | 1.82 | 1.88 |
| 12 psi 0.83 bar | 1.82 | 1.85 | 1.91 | 1.98 | 2.06 |
| 15 psi 1.03 bar | 2.02 | 2.06 | 2.14 | 2.23 | 2.32 |
| 18 psi 1.24 bar | 2.22 | 2.27 | 2.37 | 2.47 | 2.59 |
| 20 psi 1.38 bar | 2.36 | 2.41 | 2.52 | 2.64 | 2.76 |
| 25 psi 1.72 bar | 2.70 | 2.76 | 2.90 | 3.04 | 3.20 |
| 30 psi 2.07 bar | 3.04 | 3.12 | 3.28 | 3.45 | 3.65 |
Reading a compressor map
The chart above gives you the two numbers a compressor map is drawn in, and that is the whole point of sizing a turbo on paper before buying one. A 400 hp target at 15 psi needs about 44.0 lb/min at a pressure ratio of 2.02. Find 44 on the map's horizontal axis, 2.02 on the vertical, and look at where they meet.
Three things decide whether that point is any good:
- Is it inside the map at all? Off the right-hand edge means the compressor is choked — it physically cannot flow that much, and more boost will not help. Past the left-hand surge line means the wheel is too big for the airflow and will surge, which sounds like fluttering and shortens the turbo's life.
- Which efficiency island is it in? Every ring on the map is an efficiency percentage. Landing in the 70–76% region means reasonable charge temperatures; landing at 60% means the same boost arrives much hotter, which costs you power and knock margin no intercooler fully recovers.
- Where does the rest of the curve sit? Your engine does not live at one point. Plot the airflow at a few engine speeds, not just peak power, and check the whole line stays in useful territory — that is what separates a turbo that drives well from one that only looks good on a dyno graph.
Why altitude moves the answer
Boost gauges read pressure above ambient, and ambient falls as you climb. At sea level, 15 psi of boost is a pressure ratio around 2.0. Take the same car to 5,000 ft and the compressor has to work through thinner air to reach the same gauge reading, so the pressure ratio climbs — the second table above shows exactly how much.
That is why a turbo sized comfortably at sea level can run out of map on a mountain pass, and why the same 15 psi makes noticeably less power up there: you are compressing less dense air to begin with. The density altitude calculator gives the equivalent figure for a specific day's temperature and humidity, which matters more than raw elevation.
The inputs that move the number most
- BSFC — brake-specific fuel consumption, how much fuel the engine needs per horsepower-hour. 0.50–0.55 is typical for a modern boosted petrol engine; older or less efficient combustion runs 0.60 and up. Raising it raises the required airflow proportionally, so a guess here is the single biggest source of error in the whole calculation.
- AFR — richer targets need more fuel for the same air, which changes how much air a given power figure implies. 11.5–12.5 is the normal boosted range.
- Power target — airflow scales almost linearly with it. Doubling power roughly doubles lb/min, and that is the number that decides compressor size.
- Boost — barely touches the airflow requirement, but sets the pressure ratio. Two builds making the same power at different boost levels need similar flow at very different heights on the map.
Once you have a compressor sized, the fuel system has to keep up with it — run the same power target through the injector size calculator, and check what the boost does to your effective compression with the boost & compression calculator.
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.