Carburetor Size Calculator
Enter the engine size, the highest RPM you will use and how well the engine breathes. The calculator works out the air flow in CFM and suggests the nearest common four-barrel carburetor.
- CFM from cubic inches
- Litres or CID
- Street to race tune
Use the RPM where you shift or the engine stops pulling, not the redline you never reach.
A carburetor should match the air the engine can actually pull in. Too small and the engine runs out of breath at high RPM. Too large and air speed through the venturis drops, so fuel atomises poorly and throttle response suffers at low speed. Most street engines are happiest slightly under the theoretical number.
Carburetor CFM chart
Mild street engine, 85% VE. Search for an engine size.
| Engine (CID) | Litres | 5,000 RPM | 6,000 RPM | 7,000 RPM |
|---|---|---|---|---|
| 250 | 4.1 | 307 | 369 | 430 |
| 302 | 4.9 | 371 | 446 | 520 |
| 327 | 5.4 | 402 | 483 | 563 |
| 350 | 5.7 | 430 | 517 | 603 |
| 383 | 6.3 | 471 | 565 | 659 |
| 400 | 6.6 | 492 | 590 | 689 |
| 454 | 7.4 | 558 | 670 | 781 |
| 502 | 8.2 | 617 | 741 | 864 |
CFM = CID × RPM ÷ 3,456 × VE. Common four-barrel sizes are 390, 465, 600, 650, 750, 850 and 950 CFM.
RPM and carb size
350 CID, three tunes. Slide the RPM.
Air demand rises in a straight line with RPM. A 350 that revs to 6,500 needs roughly 30% more carb than one that stops at 5,000.
How to size a carburetor
Air first. Step through it.
Find the CID
Litres × 61.02 if needed.
The 3,456 comes from a four-stroke engine filling its cylinders every second turn, and 1,728 cubic inches in a cubic foot.
Vacuum or mechanical secondaries
Two styles of four-barrel. Compare them.
Rear barrels open by engine vacuum. Smooth and forgiving.
Rear barrels linked to the pedal. Sharp but can bog if too big.
Simple and economical for smaller engines.
Vacuum secondaries open only when the engine asks for air, so they tolerate a slightly oversized carb better.
Before fitting a carb
Make it match. Tick each item.
Tick each item as you go.
A new carburetor usually needs its jets and idle mixture adjusted. Read the plugs after a run to check the mixture.
What you get
The number on the carb. Each tile explains one result.
At your RPM and tune.
Nearest common size.
In both CID and litres.
At 100% efficiency.
A safe window around it.
Two-barrel ratings are measured at a higher vacuum than four-barrels, so they are not directly comparable.
Frequently Asked Questions
No question matches that search.
What size carburetor for a 350?
About 600 CFM for a mild street 350 at 5,500 RPM. 650 to 750 for one that revs higher.
How do I calculate carburetor CFM?
CID × maximum RPM ÷ 3,456 × volumetric efficiency.
What happens if my carb is too big?
Air speed drops, fuel atomises poorly and the engine bogs or feels lazy low down.
What size carb for a 454?
About 750 to 850 CFM for a street big block.
What is volumetric efficiency?
How well the cylinders fill compared with their swept volume. Stock engines run about 80%.
How do I convert litres to cubic inches?
Multiply by 61.02.
Does a bigger carb make more power?
Only if the engine needs more air. Otherwise it often loses power and response.
The right carb for your engine
Work out the air your engine really pulls at the RPM you use, and choose the closest carburetor. A well-matched carb pulls cleanly from idle and still breathes at the top.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.