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Selecting an incorrectly sized carburetor costs power and fuel. An oversized carburetor starves low-RPM airflow velocity, killing throttle response and atomization. An undersized unit caps peak horsepower by choking the engine above its torque peak. The correct size depends on three measurable quantities: engine displacement (CID), maximum operating RPM, and volumetric efficiency (VE). This calculator applies the standard airflow equation used by carburetor manufacturers to determine the cubic feet per minute (CFM) your engine demands at wide-open throttle. The divisor constant 3456 accounts for the two-revolution intake cycle of a four-stroke engine and the cubic-inches-to-cubic-feet conversion factor.

Results assume sea-level atmospheric pressure (14.7 psi) and standard air density. Forced-induction applications require multiplying the result by the boost ratio. A naturally aspirated street engine typically achieves 80% volumetric efficiency; a well-ported race head reaches 95% or higher. Use measured dyno VE when available rather than estimates. This tool approximates ideal carburetor size. Real-world selection should also consider venturi design, fuel metering range, and intake manifold runner volume.

Formulas

The required carburetor airflow in cubic feet per minute is determined by the engine's volumetric demand at wide-open throttle:

CFM = CID × RPM × VE3456

Where CID = engine displacement in cubic inches, RPM = maximum engine speed in revolutions per minute, and VE = volumetric efficiency expressed as a decimal (e.g., 0.80 for 80%).

The constant 3456 is derived from the four-stroke cycle requirement of two crankshaft revolutions per intake event combined with the unit conversion from cubic inches to cubic feet:

3456 = 2 × 1728 in³/ft³

For metric displacement input, the conversion factor is:

CID = cc16.387

For forced-induction applications, multiply the naturally aspirated CFM result by the pressure ratio:

CFM_boosted = CFM_NA × P_abs14.7

Where P_abs = 14.7 + boost pressure in psi.

Reference Data

Engine Type	Displacement	Typical RPM	VE %	Approx. CFM
Small Block Chevy 305	305 CID	5500	80	389
Small Block Chevy 350	350 CID	5500	80	446
Small Block Chevy 383 Stroker	383 CID	6000	85	565
Big Block Chevy 454	454 CID	5500	80	578
Big Block Chevy 502	502 CID	6000	85	741
Ford 289	289 CID	6000	80	402
Ford 302 Windsor	302 CID	5500	80	385
Ford 351 Windsor	351 CID	5500	80	447
Ford 351 Cleveland	351 CID	6500	85	561
Ford 390 FE	390 CID	5000	80	451
Ford 427 FE	427 CID	6500	90	722
Chrysler 318	318 CID	5000	80	368
Chrysler 340	340 CID	6000	85	502
Chrysler 360	360 CID	5500	80	459
Chrysler 440	440 CID	5500	80	561
Chrysler 440 Six Pack	440 CID	6000	90	687
Pontiac 400	400 CID	5500	80	510
Pontiac 455	455 CID	5000	80	527
Buick 455 Stage 1	455 CID	5500	85	616
AMC 360	360 CID	5000	80	417
AMC 401	401 CID	5500	80	511
Mild Street 4-cyl	122 CID	6000	80	170
Sport Compact 4-cyl	122 CID	7500	90	238
Inline 6 (250 CID)	250 CID	4500	75	244
Pro Street / Race 632	632 CID	7500	95	1304

Frequently Asked Questions

An oversized carburetor reduces air velocity through the venturi at part-throttle and low RPM. Lower velocity weakens the pressure differential that draws fuel from the boosters, causing lean stumbles, poor atomization, and degraded throttle response. The engine may idle roughly and exhibit a flat spot off idle. For street-driven vehicles, it is better to err slightly undersized than oversized.

Air density decreases approximately 3% per 1,000 feet of elevation gain. At 5,000 feet, atmospheric pressure is roughly 12.2 psi versus 14.7 psi at sea level. The engine ingests less air mass per cycle, so the actual volumetric efficiency drops relative to the sea-level baseline. You do not need a smaller carburetor - the same unit flows less mass at altitude - but jetting must be leaned to match reduced oxygen content.

Stock production engines with cast iron heads and a single exhaust typically achieve 75-80% VE. Engines with aftermarket aluminum heads, a performance camshaft (230°+ duration at 0.050"), headers, and an intake manifold upgrade reach 85-90%. Purpose-built race engines with individual runner intakes, large-valve heads, and tuned exhaust can exceed 95%. Use dyno-measured VE when available; estimated values introduce ±5% error in the final CFM figure.

No. The divisor 3456 assumes a four-stroke cycle where each cylinder fires once per two crankshaft revolutions. A two-stroke engine fires every revolution, so the divisor becomes 1728. For a Wankel rotary, each rotor completes one power stroke per crankshaft revolution with a chamber volume equal to the displacement per rotor. Consult rotary-specific airflow tables.

Yes. Multiply the naturally aspirated CFM result by the absolute pressure ratio. For example, at 8 psi of boost the ratio is (14.7 + 8) / 14.7 ≈ 1.54. A 500 CFM naturally aspirated requirement becomes approximately 770 CFM under boost. Many turbo applications use throttle-body fuel injection rather than a carburetor, but draw-through and blow-through carb setups still require this correction.

The constant 3456 is the standard for four-stroke engines and equals 2 × 1728 (cubic inches per cubic foot). Some references divide by 2 separately and use 1728 as the conversion factor, arriving at the same result. Variations appear when authors pre-incorporate a typical VE (e.g., dividing by 3456 and then stating the result is for 100% VE). Always verify whether the formula expects VE as a decimal or a percentage to avoid a 100× error.