Carburetor CFM Calculator
Size a carburetor or throttle body from displacement, max RPM and volumetric efficiency.
The most common carb-selection mistake is buying too big. An engine can only inhale what its displacement, RPM and breathing allow — a carb beyond that just kills throttle signal and low-end drivability.
Carb CFM Formula
Use the RPM you'll actually see, not the tach's optimistic redline. Street engines rarely exceed 85% VE; a well-built performance engine reaches 90–95%; only race engines with tuned intakes exceed 100%. Estimate yours with the volumetric efficiency calculator.
Vacuum vs. mechanical secondaries: vacuum-secondary carbs self-limit airflow, so mild oversizing is forgiving. With mechanical secondaries (double pumpers), size strictly by the math.
Quick Sizing Table (85% VE, street)
| Engine | @ 5,500 RPM | @ 6,500 RPM |
|---|---|---|
| 302 CID | 409 CFM | 483 CFM |
| 350 CID | 473 CFM | 559 CFM |
| 383 CID | 518 CFM | 612 CFM |
| 454 CID | 614 CFM | 726 CFM |
Round up to the next common carb size (500, 600, 650, 750, 850 CFM) — but only one step. To see what horsepower a given airflow supports, use CFM to HP.
Worked Example
Frequently Asked Questions
A street 350 shifting at 6,000 RPM needs about 516 CFM — a 600 CFM vacuum-secondary carb is the classic right answer. A 750 belongs on a built engine revving higher with better VE.
Air velocity through the venturis drops, weakening the fuel-metering signal: bogging off idle, poor low-end torque, bad mileage. Too small merely costs a little top-end power — the safer error.
Yes — airflow demand is the same. EFI throttle bodies tolerate oversizing better because fuel isn't metered by venturi vacuum, but grossly oversized ones still hurt throttle resolution.