How this instrument works
Carburetor CFM is a demand calculation dressed up as a shopping figure: how many cubic feet of air per minute the engine will pull through the carburetor's throat at a chosen RPM, if the carburetor itself were not the bottleneck. Each cylinder displaces its swept volume once per intake stroke, but a four-stroke crankshaft turns twice for every one of those strokes, so raw displacement times RPM overstates true demand by a factor of two — hence dividing by 3456 rather than 1728. No engine actually fills its cylinders completely; volumetric efficiency is the correction, the fraction of a full theoretical charge that friction, valve timing, and intake-tract geometry actually let in.
The constant itself is just bookkeeping: 1728 cubic inches per cubic foot, doubled for the missed intake stroke, giving 3456. SAE technical paper 520259, 'The Volumetric Efficiency of Four-Stroke Engines,' laid out the underlying physics decades ago — cylinder filling depends on piston speed, valve flow area, and intake-tract tuning, not on displacement alone — and the shorthand formula here is the practical distillation racers and engine builders have used ever since to translate a spec sheet into a part number.
The formula returns a single-RPM snapshot, and that is its real limit. Volumetric efficiency is not one fixed number for an engine; it rises and falls across the RPM range depending on camshaft profile, header design, and intake-runner length, often peaking well below redline and falling off above it. Feed in redline RPM and a VE measured near peak torque, and the result overstates real airflow demand at that speed. A dyno sheet or flow-bench figure specific to the RPM you care about is worth far more than a guessed constant plugged into every RPM.
- Enter Engine displacement, cubic inches — the total swept volume of every cylinder, printed on most spec sheets or computed from bore and stroke.
- Enter Max RPM — the engine speed where you actually want peak airflow, typically your intended shift point or redline, not an idle figure.
- Enter Volumetric efficiency, % — 75-85% suits a stock or mildly modified engine, 90% and up suits a well-built performance engine.
- Read Required carburetor CFM and round up to the nearest size a carburetor manufacturer actually catalogs.
Worked example — sizing a 350 V8 for a 6,000 RPM redline
A 350-cubic-inch small-block V8 revving to a 6,000 RPM redline, running at 85% volumetric efficiency — a reasonable figure for a mildly built engine with stock-style heads — sets Engine displacement to 350, Max RPM to 6000, and Volumetric efficiency, % to 85.
Working the formula: CFM = (350 × 6000 × 85) ⁄ (100 × 3456) = 178,500,000 ⁄ 345,600 = 516.493055556, which the instrument displays as 516.5 CFM. That figure is the airflow floor the carburetor must clear at redline, not a number to round down.
A 500 CFM carburetor would pinch the engine right where wide-open throttle needs the least restriction, while a 750 CFM unit flows the air easily but drops signal velocity through the venturis at part throttle, hurting fuel atomization and drivability. Rounding 516.5 up to a common catalog size such as 570 or 600 CFM is the sizing decision this number actually supports.
Questions
Why does the formula divide by 3456?
3456 converts a cylinder's swept volume in cubic inches into cubic feet of airflow per minute for a four-stroke engine. It is 1728 — the number of cubic inches in a cubic foot — multiplied by 2, because a four-stroke cylinder draws in one fresh charge only every second crankshaft revolution; the other revolution is spent on compression, combustion, and exhaust. A two-stroke engine, which inhales every revolution, uses 1728 instead.
What volumetric efficiency should I actually enter?
Somewhere between 75% and 85% covers most stock and mildly modified engines with unremarkable heads and camshafts. Ported heads, a matched intake, and a performance camshaft can push a naturally aspirated engine to 90-95%, and well-developed race engines occasionally clear 100% using intake-runner tuning to ram-charge the cylinder near its resonant RPM. Without a dyno or flow-bench number, 80% is a defensible, conservative default.
Is a bigger carburetor always the safer choice?
No — oversizing carries a real cost. A CFM rating well above what the engine can flow drops air velocity through the carburetor's venturis at low RPM and light throttle, which weakens fuel atomization and hurts idle quality and throttle response. Undersizing chokes peak-RPM power by restricting the one thing the engine is trying to breathe. Treat the formula's result as a target to match, not a floor to clear by a wide margin.
Does this formula work for turbocharged or supercharged engines?
No, it sizes a carburetor for a naturally aspirated engine, where atmospheric pressure alone pushes air through the intake tract. Forced induction shoves pressurized air into the engine ahead of the carburetor, so airflow demand and carburetor or throttle-body selection follow separate rules, usually set by the boost system's own supplier. Entering figures from a forced-induction engine here will understate what the engine actually needs.
Should I use redline or the RPM of peak horsepower?
Peak-horsepower RPM is the more accurate choice, since that is where the engine genuinely demands maximum airflow; redline usually sits a few hundred RPM higher and pulls the required CFM up with it. Many builders enter redline anyway as a margin of safety, accepting a slightly larger carburetor in exchange for one less number to track. Either approach is defensible — stay consistent about which one you used when comparing against a catalog spec.
Why cubic inches instead of liters for displacement?
Because the constant 3456 is built specifically for cubic inches and revolutions per minute — 1728 cubic inches per cubic foot, doubled for the four-stroke cycle. Convert a metric displacement to cubic inches first (1 liter equals 61.0237 cubic inches) before entering it; the arithmetic will still run, but a displacement typed in liters and read as cubic inches produces a CFM figure roughly sixteen times too small.