Carburetor CFM Calculator

By Avram Aruh
Last updated: Oct 27, 2021

With our carburetor cfm calculator, we will prevent you from ruining your engine. Carburetors (also known as "carburettor" in some countries, or simply "carb") are a smart device that create the correct mix of air-fuel in your engine. The question is, do we know the required cfm for our carburetor, and, if not, how to calculate the cfm of your engine? This engine cfm calculator gives you an idea of the carburetor size you need for your engine!

How does a carburetor work?

Isn’t it incredible that birds and planes can even fly? That's all down to the Bernoulli equation, which is the same principle that carburetors work on. Carburetors can be thought of as a pipe that pumps more air into the engine as you push on the accelerator. This pipe passes through a venturi, which is connected to the fuel intake.

A venturi is a slightly narrower area of pipe which causes air to speed up when passing through it. To conserve mechanical energy, an increase in the air's speed must be met with a decrease in pressure, and this decrease in pressure is enough to draw fuel from the fuel intake into the airflow. The quicker the air's initial movement, the more it's speed increases by, which causes a lower pressure, drawing more fuel into the airflow and engine.

When the fuel gets sucked into the engine, we've achieved what we wanted, but how the air-fuel mixture can be adjusted? There are two valves above and below the venturi. The valve at the top is called the choke
and regulates how much air can flow in. If the choke is closed, less air flows in which reduces combustion in the engine due to lack of oxygen.

On the other hand, the valve at the bottom is called the throttle. When the throttle valve is open, the air inside the carburetor increases, increasing both the air and fuel amount in the engine. As there is now sufficient oxygen for all that fuel, its combustion releases more energy to drive the engine, which accelerates the vehicle. The throttle is what the gas peddle controls.

How does the carburetor CFM calculator work?

You can see the formula below that we use in our engine cfm calculator:

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


  • Engine size (CID) - the size of your engine in inches. Engine size is also commonly given in liters or cc (mL) (e.g. 1.2 L, 1400cc);
  • Max rpm - stands for maximum revolutions per minute. You can easily gather this information from the tachometer screen - it is the maximum value that you see; and
  • Engine efficiency - we can say that any standard stock engine will have about 80% efficiency. Generally, race engines can range from 95% up to 110%, and most rebuilt street engines have a volumetric efficiency of about 85%.

The constant "3456" converts from inches to feet since 1 foot is 12 inches. We can remember this conversion constant by remembering that a cubic engine is 12 inches3 and has two identical compartments. So, if we want to convert from inches to feet, we have to do the calculation shown below;

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

How to use the carburetor calculator

Let's say we have a 340 cubic inch engine with a 5,600 max rpm, working at 80% efficiency. With our engine cfm calculator:

  1. 340 CID engine x 5,600 max rpm = 1,952,500 cubic inches

  2. This is an engine with 80% efficiency, so we have to multiply our result by 0.80;

1,904,000 x 0.80 = 1,523,200 cubic inches

  1. Now we convert the units from cubic inches (CID) to cubic feet per minute (cfm) using the following division:

1,523,200 ÷ 3456 = 440.74 CFM

That's how our carb cfm calculator works!

If you're interested in car and automotive industry, you may also fine our winch size calculator useful.

Avram Aruh
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