How much cfm per diameter?

**Pure Stock** · 05-08-2007, 10:45 PM

Does anyone know a formula that can calculate maximum CFM of a specific diameter opening? e.g., a round opening like a throttle body or mass air flow sensor

Not sure *if* the length of the tubing can change max. cfm?

**Ares** · 05-08-2007, 11:44 PM

This may help.

http://www.nfsrps.com/airflow_formulas.html

**yeahloh95** · 05-09-2007, 12:09 AM

there are many things that will factor in to the maximum cfm like entrance shape, surface finish and even air temp , and does it taper or stay the same size thru out. i can dig up my fluid dynamics notes if you need me too.

**Pure Stock** · 05-09-2007, 12:17 AM

Originally Posted by yeahloh95

there are many things that will factor in to the maximum cfm like entrance shape, surface finish and even air temp , and does it taper or stay the same size thru out. i can dig up my fluid dynamics notes if you need me too.

Hey Troy I remember us touching on this subject out at Cecil. :) I would really like to flow the cylinder head(s) and intake as one to discover what that combo flows. The reason(s) I threw this thread out there was to try and evaluate M.A.F. size I currently employ on the combo.

Does the length of tubing slow air-flow some?

**somethingclever** · 05-09-2007, 10:26 AM

Q=v*A

Where:

Q = flow
v = velocity
a = area

So with a velocity in ft/min and an area in ft^2 you can multiply to get cfm.

Typically heads are flowed at 28" w.c. i believe.

From a book I have...airflow through a 3" (75mm) orifice at 1" w.c. is 195.6 cfm.

Using Bernoulli's square root law for flow and pressure drop calculations:

flow2 = delta p^.5
flow1 delta p^.5

flow2 = 195.6cfm x 28^.5/1^.5 = 1034cfm

So theoretically...a 75mm MAF can flow 1034cfm

302 ci spinning 6500 rpm would use 568.75 cfm with 100% VE

Adding a long pipe for or aft of the intake/head would yield different flows...possibly better...possibly worse...depends on swirl and someother stuff way above my head. Pressure drop through a pipe increases with length, but if the airflow becomes less turbulent due to the addition of the test tube, flow may increase.

Basically i would send the heads and intake out to get flowed together...and then request flow info from your MAF manufacturer and compare the two. :)

**yeahloh95** · 05-09-2007, 10:35 AM

Could it be as easy as to add a vacuum gauge before and after the meter, i know they have tried that before to find out the inlet restriction, and i would think speed density would flow better than any meter

for ex. aem