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.
What Actually Goes Wrong When a Carb Is Too Big
"Bigger carb, more power" is the most persistent myth in engine building, and the failure mode is specific rather than vague.
A carburetor is not a valve that meters fuel electronically — it relies on air velocity through the venturi to create the pressure drop that pulls fuel from the bowl. That pressure drop is the signal. Oversize the carb and at low and mid rpm the air moves too slowly to generate a strong signal, so:
- Throttle response goes soft. A stumble or bog off idle as the engine waits for velocity to build.
- The mixture goes lean then rich. Weak signal under-fuels at part throttle, then the accelerator pump has to mask it, which over-fuels.
- Low-end torque falls away. Exactly the range a street car lives in.
- Tuning becomes fiddly. Jetting that works at one rpm is wrong at another.
The trade is real but lopsided: an oversized carb buys a small gain at peak rpm — a range you visit for seconds — and costs drivability everywhere else. Undersizing hurts too, but only at the very top end, and it is the more forgiving error on a street engine.
Choosing a Realistic VE Figure
Volumetric efficiency is where estimates go astray, because it is the one input people guess optimistically.
| Engine type | Typical VE | Notes |
|---|---|---|
| Stock street engine, mild cam | 75–80% | Restrictive intake and exhaust, emissions calibration |
| Mild performance build | 80–85% | Better intake, headers, modest cam |
| Well-developed performance engine | 85–95% | Ported heads, matched cam and intake |
| Race engine, tuned intake | 95–110% | Above 100% only where intake resonance genuinely helps |
| Forced induction | Over 100% | This formula does not apply — boost changes the air mass entirely |
That last row matters: for a turbocharged or supercharged engine, airflow demand scales with the pressure ratio, so a naturally aspirated CFM formula understates it badly. Size induction for boosted engines from the boost horsepower calculator instead.
Vacuum vs Mechanical Secondaries
| Vacuum secondary | Mechanical secondary | |
|---|---|---|
| How it opens | Engine demand pulls it open | Linkage opens it with the throttle |
| Oversizing tolerance | Forgiving — it self-limits | Unforgiving — size by the maths |
| Best for | Street cars, heavier vehicles, automatics | Light cars, manual gearboxes, drag use |
| If wrongly sized | Mild loss at top end | Immediate bog when secondaries open |
Practical rule: on a street car with a vacuum-secondary carb you can round up one size safely. On a double pumper, take the calculated figure seriously — a 750 on an engine that wants 600 will fall flat every time the secondaries snap open.
Why Carb CFM Ratings Aren't Comparable Across Types
A four-barrel carburetor is rated at 1.5 inHg of pressure drop, while a two-barrel is rated at 3.0 inHg. They are not measured the same way, so the numbers are not directly comparable — a 500 CFM two-barrel flows roughly the same air as a 350 CFM four-barrel would at the same test pressure.
The calculation above gives the engine's airflow demand. Match it against a four-barrel rating directly; if you are comparing two-barrel carbs, be aware their published numbers are inflated relative to four-barrel figures by roughly a factor of 1.4.
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.
Air velocity through the venturi drops, so the signal that pulls fuel from the bowl weakens. You get a soft throttle, a bog off idle, unpredictable mixture at part throttle and lost low-end torque — in exchange for a small gain at peak rpm.
About 473 CFM at a 5,500 rpm shift point with 85% VE, or 559 CFM at 6,500 rpm. A 600 CFM carb suits most street 350s — not the 750 that gets recommended on forums.
75–80% for a stock street engine, 80–85% mildly built, 85–95% for a well-developed performance engine. Above 100% only applies to race engines with tuned intakes.
No. Boost raises the air mass well beyond what displacement alone would draw, so this naturally aspirated formula understates demand badly. Use the boost horsepower calculator instead.
They are measured at different test pressures — four-barrels at 1.5 inHg, two-barrels at 3.0 inHg. A 500 CFM two-barrel flows about the same as a 350 CFM four-barrel, so two-barrel numbers run roughly 1.4× higher for the same real airflow.