In order to check the match of a turbocharger compressor for a custom application, or to find the best compressor match it is first necessary to calculate:

(1) The correct Pressure Ratio, and;

(2) Turbocharged AirFlow (in Ibs per minute)

Once these factors have been derived they can be plotted a Compressor maps (section 6) to determine which compressor to use.

Pressure Ratio
Before calculating the compressor Pressure Ratio, you must decide what maximum boost pressure will be utilized (7Ibs P.S.I will be used for your example). Then, follow the formula #1

• NOTE: Subtract .5 p.s.i. from 14.7 p.s.i. for each 1,000 ft. above sea-level to determine approximate atmospheric pressure at altitudes.
Turbocharged AirFlow
Next, it is necessary to determine the volume of air or “AirFlow” (Ibs/min) which the turbocharged engine will require.
The following formula is used to calculate the required C.F.M. for 4-cycle engines formula #2

EXAMPLE: 100 C.I.D. engine operating up to 6,000 RPM and 10 Ibs. p.s.i. boost pressure (at sea level) formula #3

EXPLANATION: 125 C.I.D. is the engine size (divide c. c. by 16.387 to get C.I.D.) This displacement is divided by 2 (or, multiplied by 1/2) because the cylinders in a 4-cycle engine take in air on every other revolution.

We then multiply 1/2 C.I.D by the engine’s maximum RPM to get cubic inches (of air) per minute. Now we divide this figure by 1728 to convert cubic inches per minute to cubic feet of per minute (C.F.M.). However, average engines only fill the cylinders approximately 80%, so we must multiply the cubic feet of air per minute by .80 (V.E.)

We now have the figure which represents the “naturally aspirated” AirFlow. It must be multiplied by D.R. (Density ratio) to find the turbocharged volumes of air. To find the turbocharged volumes of air. To find the D.R., first locate the 1.7 Pressure Ratio (as determined earlier) on the “Pressure Ratio vs. Density Ratio” chart. Now move up to the 70% Compressor Efficiency point then move left to the Density Ratio, which is 1.25 in this case. Multiply the D.R. times the “naturally aspirated” AirFlow to find the turbochaged air requirements in C.F.M.
Now we can complete the formula by dividing that C.F.M. amount by 14.5 to get the engine Ibs per minute airflow requirements.

Using the Compressor Map
On the displayed compressor map locate the 1.7 Pressure Ratio point and move across to the 16 Ibs/m point. If the point falls between the surge line and efficiency line, the compressor is a good match. If the Ibs/m point falls to the left of the surge line the compressor could be damaged if used for your application.
If the point falls to the right of the choke line, the compressor will be less efficient than desired for your application.