Carburetor Cfm


maximum rpm due to mass = 0.0056(cubic inches)2 - 8.89(cubic inches) + 11527

carburetor cfm = (0.00021)(max rpm due to mass)(number of cylinders)(stroke)(bore)2


Carburetor_istock

Pictured is a 4-Barrel carburetor. It is called 4-Barreled because it has 4 passages the air goes through. The total volume of air that can pass through the 4 passages is the cfm (cubic feet per minute) of the carburetor. Fuel is mixed in as the air passes through.

What you do:

Choose a carburetor cfm which is appropriate for your engine.


Factors to consider:

1. Carburetor cfm is the maximum cubic feet per minute of air a carburetor can handle. The carburetor cfm influences the velocity of the air. Since the velocity of the air is limited to the speed of sound , which is 767.58 mph (miles per hour) or 67547.4 fpm (feet per minute), carburetor cfm could be a limiting factor for air flow in the engine. If the carburetor is too large for a particular engine, the air will move too slowly and reduce power.

The HiPerMath equation to start with is:

carburetor air velocity in fpm = (volumetric efficiency)(air flow coefficient)(number of cylinders)(2)(stroke)(rpm)(3.14)(bore / 2)2
                                                                                            carburetor cfm

Next, fill in volumetric efficiency of 80% and the air flow coefficient of 11.32.

carburetor air velocity in fpm = (0.8)(11.32)(number of cylinders)(2)(stroke)(rpm)(3.14)(bore / 2)2
                                                                                            carburetor cfm

which reduces to:

carburetor air velocity in fpm = (14.22)(number of cylinders)(stroke)(rpm)(bore)2
                                                                               carburetor cfm

Keep in mind, the air velocity cannot exceed 67547.4 fpm, so if your fpm comes out above that, the engine is not capable of whatever rpm you chose. To calculate maximum rpm for a particular carburetor, replace carburetor air velocity in fpm with the speed of sound in fpm and solve for rpm, which has been done in the equation below:

maximum rpm =                  (67547.4)(carburetor cfm)                 
                              (14.22)(number of cylinders)(stroke)(bore)2

Now let's switch things around again, solving for carburetor cfm and get something more useful.

carburetor cfm = (14.22)(max rpm)(number of cylinders)(stroke)(bore)2
                                                                  67547.4

So after all these calculations and after choosing the maximum rpm due to engine mass as the max rpm, you can finally get to an equation you can use, which is below.

carburetor cfm = (0.00021)(max rpm due to mass)(number of cylinders)(stroke)(bore)2

In case you need it again, here is the equation for max rpm due to mass.

maximum rpm due to mass = 0.0056(cubic inches)2 - 8.89(cubic inches) + 11527

2. Carburetor cfm ranges from 0 - 2000. If you need more than 2000 cfm for your engine, use more than one carburetor.

If an engine has a throttle body and no carburetor, it is assumed the engine is fuel injected. If an engine has a carburetor, no throttle body is needed. If an engine has a throttle body and a carburetor, the smaller of the two will dictate air velocity.

Definitions:

fpm = feet per minute