Tuesday, March 01, 2011

CONVERTING CFM TO MASS AIR (PART I)

By 460-BBF-Turbo-In-CC (From the legendary Car Craft big-cube turbo thread)

"Sometimes things aren't as simple as they first seem."
The problem with converting CFM to lbs/min. is that the density of air varies with changes in altitude, temperature and even moisture content.


That's causes some to just ignore the whole thing. For example, Corky Bell (Maximum Boost) avoids the whole thing during turbo selection by using the "semi-incorrect term 'CFM.'" (Pg. 27).


Mark Warner suggests that to convert CFM to lbs/min, you should divide CFM by 13.7, but points out that the conversion assumes 85 degree (F) inlet temps. (Street Turbocharging, pg. 35)


A. Graham Bell in Forced Induction Performance Tuning (2002) explains that "Most compressor maps are corrected for 85 [degrees] F/28.4 in Hg or 20 [degrees] C/98 lmB air conditions; so to convert from CFM to lbs/min, multiply by 0.07." (pg. 90)


Jay Miller concurs, stating that multiplying CFM by .069 at "standard density" yields lbs/min. (Turbo: Real World High-Performance Turbocharger Systems, pg. 40)

But other sources claim that you should multiply CFM by .076 at Standard Temperature and Pressure. (STP)


Next: What is STP and which conversion method is best?

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posted by Editor at 11:10

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