Choosing a turbo means finding a compressor whose map covers the airflow and pressure ratio your engine needs at peak power, ideally in the efficient middle of the map. This follows the method published by turbo manufacturers: first find the airflow needed for the power, then the manifold pressure needed to push that air into the engine at that rpm.
How to use the Turbo Size Calculator
- Engine displacement
- Target horsepower
- Rpm at peak power
- Air–fuel ratio
- BSFC
- Volumetric efficiency
- Intake temperature
- Atmospheric pressure
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the engine displacement, target crank horsepower, the rpm of peak power, air–fuel ratio, BSFC, volumetric efficiency and intake temperature after the intercooler.
Tips
- Use your own measured figures – fuel economy, speed and prices vary a lot with driving style, traffic, load and season.
- Pick the unit in each field’s menu (miles or kilometres, mpg or L/100 km); the calculator converts everything for you.
- Fill up to the brim twice and divide the distance by the fuel added to measure real fuel economy.
- For vehicle modifications, treat results as a starting point and confirm with the parts maker or a professional mechanic.
Common mistakes to avoid
- Mixing US and imperial (UK) gallons – a UK gallon is about 20% bigger.
- Using the advertised fuel economy instead of real-world figures.
- Forgetting tolls, parking and wear and tear when comparing driving with public transport.
Inputs explained
| Field | What to enter |
|---|---|
| Engine displacement | Units: L, cc, in³ |
| Target horsepower | In hp |
| Rpm at peak power | A number |
| Air–fuel ratio | A number |
| BSFC | In lb/hp·h |
| Volumetric efficiency | In % |
| Intake temperature | In °F |
| Atmospheric pressure | In psia |
What each result means
- Airflow needed = [Target horsepower]×[Air–fuel ratio]×[BSFC]÷60
- Airflow (kg/s) = [Airflow needed]×0.45359237÷60
- Manifold pressure needed = [Airflow needed]×639.6×(460 + [Intake temperature])÷([Volumetric efficiency]÷100×[Rpm at peak power]÷2×[Engine displacement]×61.0237441)
- Boost = [Manifold pressure needed] – [Atmospheric pressure]
- Compressor pressure ratio = ([Manifold pressure needed] + 2)÷([Atmospheric pressure] – 1)
Example values
How the main result changes when Engine displacement changes and everything else stays at the example values above:
| Engine displacement (L) | Airflow needed (lb/min) |
|---|---|
| 1 | 36.9 |
| 1.5 | 36.9 |
| 2 | 36.9 |
| 2.5 | 36.9 |
| 3 | 36.9 |
| 4 | 36.9 |
Formula
Airflow (lb/min) = hp × AFR × BSFC ÷ 60Manifold pressure (psia) = airflow × 639.6 × (460 + intake °F) ÷ (VE × rpm/2 × displacement in³)Pressure ratio = (MAP + losses) ÷ (atmospheric − inlet loss)Worked example
Inputs — Engine displacement: 2 L; Target horsepower: 350 hp; Rpm at peak power: 6,500; Air–fuel ratio: 11.5; BSFC: 0.55 lb/hp·h; Volumetric efficiency: 90 %; Intake temperature: 130 °F; Atmospheric pressure: 14.7 psia.
Results — Airflow needed = 36.9 lb/min; Airflow (kg/s) = 0.279; Manifold pressure needed = 39 psia; Boost = 24.3 psi; Compressor pressure ratio = 2.99; Compressor size class = Medium (35–55 lb/min).
Key terms
- Pressure ratio
- Absolute compressor outlet pressure divided by absolute inlet pressure.
Frequently asked questions
How do I size a turbo?
Work out the airflow (lb/min) and pressure ratio needed at peak power, then choose a compressor whose map places that point in its efficient region.
How much airflow for 400 hp?
About 10 lb/min per 100 hp with typical turbo AFR and BSFC – roughly 40–45 lb/min.
Why include intake temperature?
Hot air is less dense; an intercooler lowers temperature so less boost is needed for the same airflow.
What formula does the Turbo Size Calculator use?
Airflow (lb/min) = hp × AFR × BSFC ÷ 60.