Created By : Vaibhavi Kumari

Reviewed By : Rajashekhar Valipishetty

Last Updated : May 17, 2023

We can help you avoid destroying your engine by using our carburettor CFM calculator. Carburettors (also called "carburettors" in some regions or simply "carb") are intelligent devices that create the proper air-fuel mixture in your engine. The question is whether we know the necessary CFM for our carburettor as well as how to calculate your engine's CFM. This engine CFM calculator will help you figure out what size carburettor your engine requires!

Engine size
Max rpm

What is the function of a Carburettor?

Isn't it amazing that planes and birds can fly? All of this is due to the Bernoulli equation, which is the same principle at work in carburettors. Carburettors are like a piping system that pumps more air into the engine as you accelerate. This pump is connected to the fuel intake by way of a venturi.

A venturi is a section of pipe that is slightly narrower than the rest of the pipe, causing air to flow faster through it. An increase in airspeed must be accompanied by a decrease in pressure to conserve mechanical energy, and this decrease in pressure is sufficient to pull fuel from the fuel intake into the airflow.

The faster the air moves at first, the lower the pressure becomes, allowing more fuel to enter the airflow and engine. We've accomplished what we wanted when the fuel is sucked into the engine, but how do we adjust the air-fuel mixture? Above and below the venturi there are two valves. The choke valve, located at the top, controls the amount of air that can enter. When the choke is closed, less air enters the engine, reducing combustion due to oxygen deficiency. The throttle is the valve at the bottom.

The air inside the carburettor expands when the throttle valve is opened, increasing the amount of air and fuel in the engine. Because there is now enough oxygen to burn all of that fuel, more energy is released to power the engine, which accelerates the vehicle. The throttle is controlled by the gas pedal.

How does the CFM calculator for carburettors work?

The formula we use in our engine CFM calculator is shown below:

Carburetor size = Engine size x Max rpm x Efficiency / 3456


  • Engine size (CID) is the measurement in inches of the size of your engine. The size of an engine is often expressed in litres or millilitres (mL) (e.g. 1.2 L, 1400cc);
  • The term "max rpm" refers to the maximum number of revolutions per minute. This information is easily gathered from the tachometer screen - it is the highest value displayed; and
  • Engine efficiency - we can estimate that any stock engine will be around 80% efficient. Race engines typically have a volumetric efficiency of 95% to 110%, whereas most reconstructed street engines have a volumetric efficiency of around 85%.

Since 1 foot is 12 inches, the constant "3456" converts inches to feet. We can remember this conversion constant by remembering that a cubic engine has two identical compartments and is 12 inches 3 in size. So, if we want to convert inches to feet, we must perform the following calculation:

Volume in cubic inches = 12 inch x 12 inch x 12 inch x 2 compartments = 3456 inches of volume

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FAQs on Carburetor CFM Calculator

1. How do you find out the CFM of a carburettor?

The volumetric efficiency of the engines is a measurable value, and a good carburettor will help you get the most out of your engine. CFM= Cubic Inches x RPM x Volumetric Performance 3456 is the formula for calculating how much CFM (cube feet per minute) a motor requires.

2. How do I calculate engine CFM?

To calculate the CFM (Cubic Feet per Minute) for a 4 cycle motor, multiply the cubic inch displacement by the maximum rpm (Revolutions Per Minute) and divide by 3456. The results are then multiplied by the motor's volumetric efficiency.

3. Is it possible for a carburettor to be too big?

You're too big for a street engine, according to all the carb charts. A Holley 715 CFM was used in race 289. To need a 750, you'd need a lot of RPMs. Too big a carb can cause issues with throttle response and acceleration smoothness.

4. What if your carburettor isn't the right size?

Airflow into the engine is restricted when the carburettor is too small. The cylinder will not be able to be filled. This depletes the engine's fuel supply, resulting in sluggish acceleration and low top-end power. More air and fuel will be required by a larger displacement engine running at a higher rpm.

5. How many CFM do I need for a range hood?

For every 12 inches of stove width, a range hood fan should move at least 100 CFM of air. This means you'll need to have a range hood with a fan that rotates at least 250 CFM of air if your stove is 30 inches wide.