Engine CFM Calculator — Carburetor, Turbo & Throttle Body

It applies the same displacement math this page uses: theoretical airflow = CID × peak RPM ÷ 3456 × VE, then multiplies by a duty factor — about 85% for street, up to 95% for race. Edelbrock's own tuning guide expresses the same idea: a carb rated 85–90% of the engine's theoretical peak gives the strongest throttle response, while sizing at 100% only pays at wide-open throttle on a combination that can breathe. The '350 with a 600 CFM Edelbrock' pairing that has sold millions is this formula at work: 473 CFM theoretical × 0.85 ≈ 402, and the 600-class carb with vacuum secondaries covers it with signal to spare.

No — the underlying formula is the industry-standard displacement method, and Summit's sizing guide states it the same way. Where sellers differ is the recommendation band: some round up to the next catalog size, others round down for drivability. Use the calculator's street target (85%) as the decision line: if demand lands at 520 CFM, both a 500 and a 600 will run, and the honest tiebreaker is the cam — a long-duration cam that idles poorly prefers the bigger carb, a stock converter prefers the smaller.

CC describes displacement; CFM describes airflow per minute — the bridge is RPM and volumetric efficiency, not a direct conversion. Two steps: CID = CC ÷ 16.387, then CFM = CID × RPM ÷ 3456 × VE. A 5,700 cc engine at 5,500 RPM and 85% VE moves about 473 CFM. There is no fixed CC-to-CFM number because two engines of identical displacement move different air at different RPM — that is why the calculator asks for all three inputs.

Start with the naturally aspirated figure — CID × RPM ÷ 3456 × VE — then apply the pressure-ratio correction: CFM × (14.7 + boost PSI) ÷ 14.7. That 350 making 6 PSI of boost at 5,500 RPM demands 665 CFM instead of 473. Two cautions the correction hides: a turbo heats the charge, so sizing an intercooler and piping uses the same CFM at higher temperature, and compressor maps are read in lb/min — 1 lb/min of air ≈ 14.3 CFM at standard density — so convert before matching a wheel.

The demand correction is identical — × (14.7 + boost) ÷ 14.7, because both raise manifold pressure and the engine ingests what the pressure ratio delivers. The difference is upstream: a roots-style blower displaces a fixed volume per revolution (a 6-71 moves roughly 411 CFM per 1,000 blower RPM), so its 'calculator' also asks for blower-drive overdrive, while a turbo's flow comes from the compressor map. For engine demand at the manifold, this page's correction covers both — what changes is which machine you match it against.

About 1,300 CFM by the area rule of thumb (0.125 × 102²), and flow-bench numbers for popular 102 mm LS-style bodies land close to that at 28 in.H2O. It is more airflow than any naturally aspirated small-block under 500 CID demands at street RPM — which is why 102 mm bodies appear on LS7s, boosted builds and big-cam engines. On a mild 350, the same body is dead flow area: the idle-air port dominates, response goes soft, and a 90 mm unit would make more power everywhere except the top 500 RPM.

Use the duty-based rule: 5 CFM per CID street, 7 per CID for towing, hot climates or stop-and-go duty. A 302 needs ~1,510–2,110 CFM; a 345 CID tow rig wants 1,725–2,415. Electric fans advertise free-flow CFM — through a radiator core and condenser, expect 60–70% of the box number, which is exactly the gap the ×7 towing figure exists to cover. A mechanical clutch fan that moves 2,500+ CFM is why stock trucks rarely overheat; the electric-swap mistake is under-sizing for the real duty.

The 3.6 L Pentastar is 220 CID, so street duty needs 220 × 5 ≈ 1,100 CFM and towing duty 220 × 7 ≈ 1,540 CFM. The factory fan clutch assembly on that truck moves on the order of 1,500–2,500 CFM at full lock — a useful reality check on aftermarket electric claims. A single 16-inch electric fan rated 1,600–1,800 CFM (delivering ~1,100–1,300 through the core) handles street duty; for towing at GCWR, run dual fans or a 2,000+ CFM rated unit with a proper shroud, not an open blade on a bracket.

It is two unit conversions in one: 1,728 cubic inches per cubic foot, times 2 because a four-stroke engine fills its displacement only every second revolution. CID × RPM ÷ 1,728 gives cubic feet per minute at 100% VE with one fill per revolution; dividing by another 2 corrects for the four-stroke cycle — hence ÷ 3456. VE then scales the ideal to the real engine. Knowing the constant's anatomy matters when the inputs change units: liters or CC must convert to CID first, or the formula silently returns a number 6–17% wrong.

Same physics, different fluid circuit. The homepage calculator sizes building airflow: rooms, ACH, ducts and fans in CFM of air for ventilation. This page sizes an engine's induction: the same CFM unit applied to how much air an engine consumes, corrected for boost pressure and matched to carbs, throttle bodies and cooling fans. The homepage carburetor FAQ covers the crossover question in one paragraph; this page is the full engine-side treatment — displacement method, boost correction, throttle sizing and fan duty.