Tuesday, February 22, 2011

FIRST PRINCIPLES: AIR VOLUME IS RELATIVE


By 460-BBF-Turbo-In-CC (From the legendary Car Craft Turbo Thread)


Most Car Crafters are more familiar with CFM [Cubic Feet Per Minute) than they are with measuring air in pounds/minute. After all, the time-honored measure for engine size is cubic inch displacement. And a cubic foot of air is 12 x 12 x 12 = 1,728 cubic inches. That's easy to understand.

Many of us have also read that it takes roughly 1.67 CFM for every horsepower on gasoline and ~ 1.47 CFM per pony on alcohol fuel. And some flow bench companies publish CFM charts such as this:
Flow**      Horsepower potential (V8 engine)

100 CFM   205.7 h.p.
150 CFM   308.6 h.p.
200 CFM   414.4 h.p.
250 CFM   514.3 h.p.
300 CFM   617.1 h.p.
350 CFM   720.0 h.p.
400 CFM   822.8 h.p.
450 CFM   925.7 h.p.
500 CFM   1028.6 h.p.

**cylinder head port measured at 28"




 So why do we even need to think about air in lbs/min?
Because volume of air is relative. For example, a cubic foot of air at sea level has a lot more mass than a cubic foot of air in Denver, Colorado. And a cubic foot of air at 30" hg has more mass than a cubic foot of air at 14" hg.

NEXT: Converting volume to mass.








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posted by Editor at 06:27

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