An engine is an air pump: at full throttle it moves half its displacement every revolution (four-stroke). Choose a carburettor that flows about this much and the engine is neither starved nor bogged down.
How to use the Carburetor CFM Calculator
- Engine displacement
- Maximum engine rpm
- Volumetric efficiency (%)
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the displacement, the maximum rpm you will use and the volumetric efficiency (≈ 80% stock, 85–90% performance, 95%+ race).
Inputs explained
| Field | What to enter |
|---|---|
| Engine displacement | Units: in³ (CID), L, cm³ (cc) |
| Maximum engine rpm | A number |
| Volumetric efficiency (%) | A number |
What each result means
- Required airflow = [Engine displacement]×[Maximum engine rpm]×[Volumetric efficiency (%)]÷100÷3456
- In litres per second = [Required airflow]×0.4719474
Example values
How the main result changes when Engine displacement changes and everything else stays at the example values above:
| Engine displacement (in³ (CID)) | Required airflow (CFM) |
|---|---|
| 175 | 236.7 |
| 262.5 | 355.1 |
| 350 | 473.5 |
| 437.5 | 591.8 |
| 525 | 710.2 |
| 700 | 946.9 |
Formula
CFM = CID × RPM × VE / 3456
Worked example
Inputs — Engine displacement: 350 in³ (CID); Maximum engine rpm: 5,500; Volumetric efficiency (%): 85.
Results — Required airflow = 473.5 CFM; In litres per second = 223.4 L/s.
Key terms
- Volumetric efficiency
- Actual air drawn in versus the swept volume.
Frequently asked questions
Why divide by 3456?
1728 in³ per ft³ × 2 revolutions per intake cycle.
Is a bigger carb better?
No – too large a carb reduces air velocity and low-rpm response.
What VE should I use?
About 80% for stock engines, 85–90% for mild performance.
Does it work for two-strokes?
No – they draw a full displacement every revolution (divide by 1728).