Carburetor CFM Calculator
Work out how much air your engine can use at its top RPM, then see how every Autolite 1100, 2100 and 4100 size and the common aftermarket sizes compare with it. It works for any carbureted engine, Ford or not. Two-barrel and four-barrel ratings are measured differently, so the calculator puts them on the same basis before comparing.
Note: the formula is a starting point, not a rule. Verify against your engine's actual setup before buying parts.
Engine
Use the highest RPM the engine actually sees at full throttle, not the redline on an aftermarket tachometer. For an engine sized in liters, multiply by 61.02 for cubic inches.
Carburetor
Results
Airflow needed at 5,000 RPM
334 cfm
Four-barrel rating (1.5" Hg)
As a one- or two-barrel rating (3.0" Hg)
473 cfmClosest Autolite sizes
- 4100 with 1.06 venturi (465 cfm, approx.)139% · Over the formula
Ford used it on: 1958 Mercury 383 low torque. Uncommon: only a few Ford part numbers
- 4100 with 1.08 venturi (480 cfm)144% · Over the formula
Ford used it on: 1964½–66 289, some 390
Ford’s four-barrels usually came out over the formula, since the secondaries only open fully at wide-open throttle.
Nearest common sizes
- 390 cfm four-barrel117% · Over the formula
Every Autolite Size Compared
One- and two-barrel ratings converted to the four-barrel basis (divided by 1.414), as a percentage of your engine's 334 cfm. Within 15% either way counts as near the formula. Uncommon sizes appear on four or fewer of the Ford part numbers in the identification guide.
| Carb | Venturi | Rated CFM | 4-bbl basis | Of need | Fit | Ford used it on |
|---|---|---|---|---|---|---|
| 1100 | 1.00 | 120 (approx.) | 85 | 25% | Under the formula | 144 six |
| 1100 | 1.10 | 150 (approx.) | 106 | 32% | Under the formula | 170 six, 1968–69 200 six |
| 1100 | 1.20 | 185 | 131 | 39% | Under the formula | 1965–67 200 six |
| 2100 | 0.98 | 190 | 134 | 40% | Under the formula | 221 V8 (uncommon) |
| 2100 | 1.01 | 240 | 170 | 51% | Under the formula | 260 V8 |
| 2100 | 1.02 | 245 | 173 | 52% | Under the formula | 272/292 Y-block, early 289 |
| 2100 | 1.08 | 287 | 203 | 61% | Under the formula | 302 and 304 V8 |
| 2100 | 1.14 | 300 | 212 | 63% | Under the formula | 1965–67 289, 352 |
| 2100 | 1.21 | 351 | 248 | 74% | Under the formula | 1971–75 351, 360, 390, 400 |
| 2100 | 1.23 | 356 | 252 | 75% | Under the formula | 1967–70 351, 352, 390 |
| 2100 | 1.33 | 424 | 300 | 90% | Near the formula | 1964–66 Mercury 390 (uncommon) |
| 4100 | 1.06 | 465 (approx.) | 465 | 139% | Over the formula | 1958 Mercury 383 low torque (uncommon) |
| 4100 | 1.08 | 480 | 480 | 144% | Over the formula | 1964½–66 289, some 390 |
| 4100 | 1.12 | 600 | 600 | 179% | Over the formula | 289 HiPo, 352, 390, 428 PI |
| 4100 | 1.19 | 669 | 669 | 200% | Over the formula | 1958 Mercury 383 high torque (uncommon) |
To find which size your carburetor is, look up its tag number in the Autolite carburetor identification guide.
Convert a Carburetor CFM Rating
Compare a two-barrel with a four-barrel by moving its rating to the other test vacuum.
500 cfm at 3.0" Hg is about 354 cfm as a four-barrel rating (1.5" Hg).
Carburetor CFM Formula
The calculator uses the standard four-stroke airflow formula:
CFM = cubic inches × RPM × volumetric efficiency ÷ 3,456
3,456 is 1,728 (cubic inches in a cubic foot) times 2, because a four-stroke engine fills each cylinder once every two turns. Volumetric efficiency is how full the cylinders actually get compared with their swept volume. The answer is on the four-barrel basis; multiplying by 1.414 gives the equivalent two-barrel rating, since flow rises with the square root of the test vacuum (√2 ≈ 1.414).
Example (the calculator's defaults): a 289 at 5,000 RPM and 80% volumetric efficiency. 289 × 5,000 × 0.8 ÷ 3,456 = 334 cfm on the four-barrel basis, or about 473 cfm as a two-barrel rating. Ford put a 480 cfm 4100 or a 300 cfm 2100 (about 212 cfm on the four-barrel basis) on the 289.
Frequently Asked Questions
One- and two-barrel carburetors are flow-tested at 3.0 inches of mercury and four-barrels at 1.5. The higher vacuum pulls about 41% more air through the same carburetor, so a 2100 rated at 300 cfm flows about the same as a four-barrel rated at 212. Divide a two-barrel rating by 1.414 before comparing it with a four-barrel.
The formula gives the most air the engine can use at full throttle and its top RPM. Ford sized its two-barrels well under that: a smaller venturi keeps air speed up at part throttle, where these engines spent nearly all their time, which helps throttle response and economy. Its four-barrels often came out over the formula because Ford used the same one or two sizes on engines from the 289 to the 428, and the secondaries only open fully at wide-open throttle.
About 75% for a stock six-cylinder or two-barrel V8, and 80% for a stock four-barrel V8. A mild cam, intake and exhaust bring a street engine to about 85%. 90% and above is for well-built performance engines; only a race engine with excellent heads and tuning gets near 100%.
Not on the street. A carburetor much larger than the engine needs has slow air through its venturis at low RPM, which weakens the signal that draws fuel and makes it sluggish off idle. Going a little under the formula costs a few horsepower at the very top; going well over usually costs drivability everywhere else. When the airflow falls between two common sizes, the smaller one is usually the better street choice.
Yes. The formula only needs displacement, RPM and volumetric efficiency, so it works for any carbureted four-stroke engine. Pick its make and engine, or choose Other and enter its cubic inches. The Autolite comparison is Ford-specific, but the common sizes and the two-barrel conversion apply to any make, since one- and two-barrels are conventionally rated at 3.0 inches of mercury and four-barrels at 1.5.