CONVERTING CFM TO MASS AIR, PART II

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

STP isn't very "real world."

Standard temperature and Pressure (STP) provides a means to account for the fact that air density and air volume are not linked.

STP - commonly used in the Imperial and USA system of units - as air at 60 [degrees]F (520 [degrees]R) and 14.696 pounds per square inch absolute (psia) (15.6oC, 1 atm) See, Engineering Toolbox.

Of course, engines in the real world seldom if ever operate at STP. Underhood temperatures, heat radiation, convection, and conduction, humidity and altitude variations help all assure that. That's one of the reasons why the SAE and others have developed more realistic standard references for automotive use.

Recall that we learned turbo manufacturers use 85 degrees F as "standard" (A. Graham Bell).

Is there really much difference between the various methods of converting CFM to lbs/min? Let's compare them:

Assume we're trying to convert 500 CFM to lbs/hr:

Warner method: divide 500 CFM by 13.7 = 36.49 lbs/hr

A. Graham Bell method: multiply 500 CFM by 0.07 = 35 lbs/hr

Miller method: multiply 500 CFM by 0.069 = 34.5 lbs/hr

STP method: multiply 500 CFM by 0.076 = 38 lbs/hr

Note that Jeff Hartman in Turbocharging Performance Handbook (2007) also uses the STP method.

Although the spread among the various methods is not that large, it appears that Miller  and A. Graham Bell's conversion factors produce the most conservative results. Moreover, because they track with turbocharger industry standard practice, we'll use the 0.07 conversion factor.

But don't forget that these all of these factors are based on picking a somewhat arbitrary air temperature and pressure as a measuring point. Actual results may vary.



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?