Airflow Conversion Chart
Published cylinder head airflow figures are not equal. It is often difficult to compare cylinder heads and other components if the provided specification data uses varying flow test values. Flow figures are compiled with the help of a flow bench. However, the technician collecting the data may use non-standard measurement values due to the geographic testing location, capacity of the equipment used, or some other consideration or limitation.
Manufacturers, engine builders, or testers avoid standard measurement values for a variety of reasons, but there is a solution! When the need arises to compare advertised or tested airflow data between products, it is possible to convert the test data to identical values if we know the proper mathematical multiplier. A better choice is to convert the values to the industry standard of 28″ of water. Either is possible, and allows for equal comparison of quoted airflow values.
One simple solution is to use the table provided below to calculate corrected airflow data from provided, non-standard values.
To use the chart below, simply find the “tested inches of water” value that the cylinder head ports, intake, carburetor, or other items were tested at in the left hand column. This value is often provided on the flow data sheet that was included with the cylinder heads or other component. Note the example below.
Next, decide the proper output value from the upper row. Again, 28″ is the standard water column value. Then, follow the tested value row horizontally until meeting the vertical intersection with the output value to determine the correct multiplier.
An intake port was tested to have 183 cubic feet per minute (cfm) flow at 10″ of water. To find out the corrected airflow at 28″ of water, multiply 183 by 1.670 (shown in the chart below). The calculation provides the answer that this intake port flows 305.6 cfm at 28″ of water.
183 x 1.67 = 305.6
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