Air Filter Flow Calculations
Chuck Leeper with permission from K&N
Introduction
Below is a series of formulas to determine air filter size for a given air flow capacity. Chuck was trouble shooting a problem when he found that air flow he needed was more than his 9" filter could provide. K&N shared this information with him to help trouble shoot his problem.
Formulas
  1. Square inches of filter required for a motor = CID x Maximum RPM / 20839
  2. CFM required for a 4 stroke motor = CID x Maximum RPM / 3456
  3. CFM required for a 2 stroke motor = CID x Maximum RPM / 1728
  4. CFM required for a turbocharged or supercharged motor = Pounds of Boost / 14.7

    Add 1.0 to this answer. This relates to #2 and #3

    Example: (CID x maximum RPM x #4 ) / 3456 or 1728
  5. To find circumference of a circle multiply diameter by 3.1416
  6. To find diameter of a circle multiply circumference by .31831

CFM Formulas for Filters

  • Round: Diameter x Height x 6 x 3.14
  • Flat Panel: Length x Width x 6
  • Universal Round Taper: (Base + Top) / 2 = A
    Take A x Length x 6 x 3.14
  • Universal Round Straight: Diameter x Length x 6 x 3.14


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