Pressure Ratio is the total absolute pressure produced by the turbo divided by atmospheric pressure.
Pressure Ratio= (14.7+boost)/14.7
(14.7+15)/14.7 = 2.02 This means that with 15 lbs of boost, 102% more air will be going through the system.
Airflow rate (cfm)= (cid*rpm*0.5*Ev)/ 1728
Ev= volumetric efficiency
(250*5000rpm*0.5*80%)/1728 = 289.35 CFM
I'm guessing on the volumetric efficiency of the 250. I think 80% is kinda generous though.
The pressure ratio times the basic engine flow rate then becomes the approximate flow rate under boost.
Airflow rate= pressure ratio*basic engine cfm
2.02 * 289.35 = 584.487cfm
This is taken directly from Corky Bell's book "Maximum Boost." It's a VERY useful book. Hope this helps. John
'68 C-10 - 250 with Edelbrock 600cfm 4 barrel, Offy intake, Hedman headers and true duals, HEI, MSD 6A box, relocated gas tank
Soon to have: T-5 tranny
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