“In this turbo size calculator, we will show you how to calculate the airflow your engine needs in just a few steps.
We will also help you understand the turbo sizing formula, what each part of it means, and why airflow is such an important number.
What is Wa in turbo sizing?
How does horsepower affect airflow?
How do you know the right turbo size for your build? Find all the answers here!”
Turbo Size Calculator
Calculate estimated airflow rate (Wa) in lb/min for selecting turbo sizing.
Turbo Size Formula — Explanation, Applications & Example
Understand the airflow formula for turbo sizing, see typical real-world values, and follow a worked calculation example.
The Formula
Where:
• Wa = Airflow rate (lb/min) — the key number for turbo sizing.
• HP = Target horsepower.
• A/F = Air/Fuel ratio (use ~12 for turbo gasoline engines).
• BSFC = Brake Specific Fuel Consumption (use ~0.60 for turbo gasoline engines).
Multiplying HP × BSFC gives fuel flow (lb/hr). Multiply by A/F to get air flow (lb/hr). Divide by 60 to convert to lb/min.
Shortcut with defaults: Wa = 0.12 × HP (lb/min).
Real-World Applications
| Application | HP | Wa (lb/min) | Notes |
|---|---|---|---|
| Daily Driver | 150 | 18.0 | Small turbo, fast spool |
| Hot Hatch | 200 | 24.0 | Mild performance build |
| Street Performance | 300 | 36.0 | Balanced street/track |
| Track Car | 400 | 48.0 | Requires mid-large turbo |
| High Output | 600 | 72.0 | Large turbo setup |
| Race Build | 1000 | 120.0 | Extreme / multiple turbos |
Worked Example
Target: 400 HP turbo gasoline engine, with A/F = 12 and BSFC = 0.60.
- Step 1 — Given Data HP = 400, A/F = 12, BSFC = 0.60
- Step 2 — Fuel Flow Fuel = 400 × 0.60 = 240 lb/hr
- Step 3 — Air Flow per Hour Air/hr = 240 × 12 = 2880 lb/hr
- Step 4 — Convert to Minutes Wa = 2880 ÷ 60 = 48 lb/min
- Step 5 — Quick Check Wa = 0.12 × 400 = 48 lb/min ✔
FAQs
What is Wa in turbo sizing?
Wa is the airflow rate in lb/min. It tells you how much air your engine needs and helps you pick the right turbo.
How does horsepower affect turbo size?
More horsepower means more air is needed. As HP goes up, Wa increases, and so does the turbo size.
What A/F ratio should I use for turbo engines?
For most turbo gasoline engines, an air/fuel ratio of 12 is a good safe value.
How do I quickly estimate airflow?
Just multiply your horsepower by 0.12. That gives you Wa in lb/min in one step.