This thread has two formulas:

1) how to calculate Air needed by engine
2) how to calculate airflow (CFM) of k&n Filters.

1) To determine the maximum volume of air your engine will use, take the displacement in cubic inches and multiply by the maximum RPM you operate the engine at. If the engine is supercharged or turbocharged, divide the pounds of boost by 14.7, add 1 to this answer, and multiply this value by the CIDxRPM value. Divide by 3,456 for a 4 stroke engine, or 1,728 for a two stroke engine. This will calculate the maximum airflow required by the engine.

here is an example. lets say a 3.0L 4th gen s/c @ 13 psi with stock rev limiter. here is how its calculated

1 liters = 61.0237441 Cubic Inches
3 Liters= 183.071 Cubic Inches

[183.071 x 6500 x [ (13/14.7) + 1]] / 3456 = 648.8 CFM



2) To determine what volume of air a given filter is capable of flowing, multiply the diameter of the filter by its height, multiply by pi (3.14), and multiply by 6. If the filter is a tapered cone (different diameters at each end), add the diameter from the top and base together and divide by two for an average diameter.

EX: K&N RU-1785, Round Straight

5 x 8.5 x 3.14 x 6 = 800.7 CFM

EX: K&N RE-0920 Round Tapered
[(4.625+6)/2] x 9 x 3.14 x 6 = 900.78

 

If you're going to start speweing formulas, you should define and explain what the variables mean/where they came from.....

For example, in your first formula, explain the 14.7, adding the +1, and 3,456 etc. I know what they mean, but I'm sure a lot of people don't.

In your second formula, what does 6 represent?

Volume of a cylinder = (pie)*(radius)^2*(height)

 

not sure. i got the forumlas from k&n and did a few calculations as examples. just thought others might find the info usefull

 

I'm @ ~ 590 - 620 torr vs the standard 760.

 

In case anyone wants to know where the first formula came from..

It's just a variation of the volumetric efficiency formula.

VE = flow/(displacement * cycles/time)

Trying to get flow you have:

Flow = VE * displacement * cycles/time

VE for a supercharged car should be over 100% or 1. That's where this part of the equation comes in: [ (#/14.7) + 1]
Atmospheric pressure is 14.7 PSI or 1 ATM. You’re basically adding your boosted PSI (#/14.7) to atmospheric (14.7/14.7). Units will cancel and you’re left with a number greater than 1 or 100+%. This gives you an estimated VE based on the additional pressure.

Now the displacement. You know the displacement in cubic inches but you have to convert it to cubic feet to end up with cubic feet per minute. There are 1728 cubic inches in 1 cubic foot so you have:
(displacement in^3)(ft^3)/(1728 in^3)= displacement in ft^3

And finally the cycles per time needs minutes in its unit to end up with CFM. RPM has this, but only 1/2 of the 4 stroke cycle occurs during 1 crank shaft rotation. So, you have to divide the RPMs by 2 to get the number of cycles.

To bring it all together,

You have flow in CFM = (displacement in ft^3 * RPM * assumed VE)/ (1728 * 2)

 

Correct me if i'm wrong but a VQ with a supercharger would have the same VE, the only change is the air density.

 

not sure what the 6 is about, but it was multiplied to the surface area. it all seemed good until the 6. volume, however, is not in play here.

ok, youre wrong.

 

6 is # of cylinders i belive

 

The surface area is simply height X diameter X 3.145.

Just for reference a typical pleated gauze element type filter with 100 sq. in. of surface area will flow about 400 cfm not 600, so I don't know what is up with the 6.

 

I found this post very informative. Thanks!

 

I'm confused with formula #2. What are all the units of the variables? Inches x inches x pi (unitless) x 6 (mystery number/mystery units) = CFM (Feet^3/minute). Huh????

It seems there should be some <1.0 multiplier just due to the use of a filter. So what if someone had no filter? What do you use as surface area - the 3.5" opening?! That would result in a smaller 'CFM' than even a small filter would. Right?