Carburetor CFM calculator
A 350 cubic-inch four-stroke at 6,000 rpm with 85 % volumetric efficiency needs about 516 CFM of carburetor airflow. Enter displacement in cubic inches, litres or cc, the maximum rpm and the efficiency to size the carburetor. Free in your browser.
- Free
- Runs in your browser
- No sign-up
- Your data stays on your device
Run — free
Worked example
- Displacement
- 350
- Rpm
- 6,000
- Volumetric efficiency
- 0.85
- Displacement unit
- Ci
- Displacement ci
- 350
- Cycle
- Four stroke
- Constant
- 3,456
A carburetor CFM calculator answers a practical engine-building question: how much air, in cubic feet per minute, must the carburetor flow so the engine can fill its cylinders at a chosen maximum RPM. The textbook identity multiplies displacement in cubic inches by RPM and volumetric efficiency, then divides by 3456 for a four-stroke or by 1728 for a two-stroke.
How required carburetor CFM is computed from CID, RPM and volumetric efficiency
Required carburetor airflow is the volume of air an engine would ingest if every cylinder filled to the stated volumetric efficiency at the stated maximum crank speed. Start from displacement. Convert litres or cubic centimetres to cubic inches with the exact inch definition one inch equals 2.54 centimetres, so one cubic inch equals 2.54 cubed cubic centimetres and one litre equals one thousand divided by that cube.
That cubic-inch figure is CID. In one minute the crank turns RPM times; a four-stroke engine opens an intake valve only every other revolution, so the theoretical uncorrected volume in cubic inches per minute is CID times RPM divided by two. Divide by 1728 cubic inches per cubic foot to obtain cubic feet per minute, which is the same as dividing CID times RPM by 3456.
The compact identity is therefore CFM equals CID times RPM times VE divided by 3456. A two-stroke takes a charge every revolution, so the factor of two disappears and the divisor is 1728.
Where a carburetor CFM calculator earns its keep
Engine builders and restorers use a carburetor CFM calculator before they order a four-barrel for a 350, a 454, a 5.0 litre Windsor, or a 125 cubic-centimetre two-stroke, so the chosen venturi area matches the engine’s air demand at the RPM where power is made rather than at idle. Dyno shops and calibration notebooks pin the Holley-style identity in tests so a change of VE assumption from 0.85 to 0.95 does not hide an arithmetic slip when they compare a square-bore carburetor to a spread-bore one.
Motorsport schools and vocational classrooms auto-grade the 3456 versus 1728 choice and the litre-to-cubic-inch conversion without standing up a spreadsheet that silently uses 61.0 as a rounded factor. Failed validation does not consume balance. Keep VE honest: a street engine that never sees 85 percent volumetric efficiency will look over-carbureted if you size at 1.00, and a tunnel-ram race piece will starve if you size at 0.80.
Use a dedicated carburetor-selection landing for those questions so this search intent stays the CFM number.
What you can do with it
Street and race engine carburetor sizing
Convert CID or litres, redline RPM, and a VE fraction into required CFM before ordering a four-barrel for a small-block, big-block, or 5.0 litre build.
Classroom and dyno-notebook checks
Auto-grade the 3456 versus 1728 divisor and the litre-to-cubic-inch conversion against a deterministic oracle instead of a rounded spreadsheet factor.
FAQ
How do I calculate required carburetor CFM?
Four-stroke CFM = (CID × RPM × VE) / 3456; two-stroke uses 1728. Displacement may be ci, litres, or cc.
What happens if displacement or RPM is zero or negative?
Required carburetor airflow is not defined for a non-positive engine size or a non-positive crank speed, and those failures are not billed.
Should volumetric efficiency be 85 or 0.85?
Enter a fraction of 1: 0.85 means 85 percent. A value such as 85 is rejected so a percent is not treated as 8500 percent. Typical street VE is 0.80–0.85; race or boosted combinations can exceed 1.00, up to 2.
How are litres and cubic centimetres converted?
One litre equals 1000 / 2.54³ cubic inches; one cc equals 1 / 2.54³ cubic inches. Then CFM = (CID × RPM × VE) / 3456 or 1728.
What volumetric efficiency should I assume?
A stock street engine often sits near 0.80 to 0.85, a well-tuned naturally aspirated one near 0.95 to 1.00, and forced induction can exceed 1.00. Sizing a street engine at 1.00 over-carburets it; sizing a tunnel-ram race engine at 0.80 starves it.
Related tools
For developers — API access
Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.
API endpoint
Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.
Call it from your stack
curl -X POST https://api.kit.forhosting.com/phys/carburetor-cfm \
-H "Authorization: Bearer $KIT_KEY" \
-H "Content-Type: application/json" \
-d '{"displacement":350,"rpm":6000,"volumetric_efficiency":0.85}'const res = await fetch("https://api.kit.forhosting.com/phys/carburetor-cfm", {
method: "POST",
headers: {
"Authorization": `Bearer ${process.env.KIT_KEY}`,
"Content-Type": "application/json"
},
body: JSON.stringify({
"displacement": 350,
"rpm": 6000,
"volumetric_efficiency": 0.85
})
});
const { task_id } = await res.json();import os, requests
res = requests.post(
"https://api.kit.forhosting.com/phys/carburetor-cfm",
headers={"Authorization": f"Bearer {os.environ['KIT_KEY']}"},
json={
"displacement": 350,
"rpm": 6000,
"volumetric_efficiency": 0.85
},
)
task_id = res.json()["task_id"]<?php
$res = file_get_contents("https://api.kit.forhosting.com/phys/carburetor-cfm", false, stream_context_create([
"http" => [
"method" => "POST",
"header" => "Authorization: Bearer " . getenv("KIT_KEY") . "\r\nContent-Type: application/json",
"content" => '{"displacement":350,"rpm":6000,"volumetric_efficiency":0.85}',
],
]));
$task = json_decode($res, true);body := bytes.NewBufferString(`{"displacement":350,"rpm":6000,"volumetric_efficiency":0.85}`)
req, _ := http.NewRequest("POST", "https://api.kit.forhosting.com/phys/carburetor-cfm", body)
req.Header.Set("Authorization", "Bearer "+os.Getenv("KIT_KEY"))
req.Header.Set("Content-Type", "application/json")
res, _ := http.DefaultClient.Do(req)Example request
{
"displacement": 350,
"rpm": 6000,
"volumetric_efficiency": 0.85
}Example response
{
"task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
"type": "phys.carburetor_cfm",
"status": "queued",
"_links": {
"result": "/tasks/tsk_…/result"
}
}The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.
Pricing
Published price — no tokens, no invented credits. A failed task is never charged.
Errors
| HTTP | Code | Meaning |
|---|---|---|
401 | unauthorized | Missing or invalid API key. |
402 | insufficient_balance | Your balance doesn't cover the task price. |
404 | unknown_type | That task type doesn't exist. |
429 | rate_limited | Too many requests. Use the webhook instead of polling. |
Technical notes
This endpoint accepts displacement, an optional unit of cubic inches, litres, or cubic centimetres, maximum RPM, volumetric efficiency as a fraction, and an optional cycle, then returns required airflow in CFM with the cubic-inch equivalent and the formula used. Displacement and RPM must be strictly positive; a zero or negative value is rejected as invalid_input rather than a hollow zero-flow success.
How required carburetor CFM is computed from CID, RPM and volumetric efficiency
Multiply by volumetric efficiency VE, a fraction of one that captures how completely the cylinders fill relative to geometric displacement: a stock street engine often sits near 0.80 to 0.85, a well-tuned naturally aspirated combination near 0.95 to 1.00, and a forced-induction or high-overlap race piece can exceed 1.00. This endpoint evaluates that identity only. It does not recommend a catalog Holley or Edelbrock part number, apply altitude or temperature density corrections, or model booster size, vacuum-secondary spring rate, or plenum volume. Those belong on dedicated landings so this search intent stays the sizing formula people actually type.
Input contract, units, response shape, and error behaviour
If displacement is not strictly positive the task fails, because a zero or negative engine size is not a carburetor-sizing input. If rpm is not strictly positive the task fails for the same reason: a parked or reversed crank does not define a required airflow. Volumetric efficiency must be a positive fraction no greater than two; enter 0.85 for eighty-five percent. A value such as 85 is rejected so a percent is not silently treated as eighty-five hundred percent. Send three required finite numbers: displacement, the engine size in the unit named by displacement_unit; rpm, the maximum engine speed in revolutions per minute; and volumetric_efficiency, the fill fraction VE. Aliases are accepted: cid, cubic_inches, and engine_displacement for size; max_rpm, maximum_rpm, and engine_rpm for speed; ve and ve_fraction for efficiency. Optional displacement_unit is an enum of ci (cubic inches, the default), l (litres), or cc (cubic centimetres). Optional cycle is four_stroke, the default, or two_stroke. Each numeric field may arrive as a JSON number or as a numeric string. Missing fields, empty strings, NaN, and Infinity fail with invalid_input naming the field. On success the body echoes cleaned displacement, rpm, volumetric_efficiency, displacement_unit, and cycle; returns displacement_ci in cubic inches; returns the divisor constant 3456 or 1728; and returns cfm rounded to twelve decimal places together with formula and equation strings and unit_cfm ft3/min. Example: displacement 350 cubic inches, rpm 6000, volumetric_efficiency 0.85, four-stroke yields about 516.5 cubic feet per minute, the classic small-block figure that points a builder toward a 650 CFM catalog carburetor rather than a 750. Pricing for successful API calls is the published base of $0.002 per item; validation failures do not consume prepaid balance. Pair the free browser form with the same module the API uses so classroom demos and parts-counter scripts never diverge.
Where a carburetor CFM calculator earns its keep
This endpoint reports only the required airflow in cubic feet per minute; it does not pick booster style, jet, power-valve rating, or throttle-bore diameter.
Integration use cases
Parts-counter and catalog automation
Store the same Holley-style CFM figure the public widget shows, so inventory scripts and the browser calculator never disagree on a 350 at 6000 RPM.
Technical questions
What does the carburetor CFM calculator cost?
The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.