I looked in the archives for a formula for the square inches needed for a air cleaner surface,no luck Do any of you know if there is a formula based on HP and if there is what it is? The reason I'm asking is I'm getting ready to dyno test a motor and I want to test several different intakes and carb combination with the air cleaners on, including a tunnel ram with dual carbs, 4150 and 4500 Holleys I have several nine inch daimeter 4150 air cleaners assembly but no small diameter dominator air cleaners yet, I want to order the nine inch diameter dominator air cleaner but not get to tall of ones that won't fit in the hood scoop on my tunnel ram hood or order to short either EDIT: I called K&N on Monday and talk to oneof there tech guys, he gave me several formulas for figuring out how much surface area is needed based on C.I. and RPM (like already posted on one of the comments on here now ) and how to figure the area on the round and falt filters. Now I need to dyno the motor to find out where it makes peak power and go from there, once I know the results I wlll make sure the surface area is bigger than the minimums suggest by the formula. It looks like, based on past motors with simialr C.I. and peak power RPMs, that two nine inch diameter by 4 inch tall with the Extreme lids will be more than enough surface area, time will tell My 512 six pak motor made 8 hp more by adding a second six pak element on top of the first on back to back test. that motor made the most power with no elements on the air cleaner base, not much but more(2 to 3 hp maybe, I can't remember the exact amount now, that was in 2003. Thanks to all for your help

Cab, I know of no such formula for air cleaners. Its probably because they can be so different in there flow abilties.

So no One formula will work for all the different types. But there could be formulas for each and every Specific type of air filter.

The best help I can give is to make sure the filter you use flows more then the carb and the engines need at rpm. Witch with some of todays modern filters doesnt seem to be a problem. mike

All I could suggest is buy a filter element and cut
it open and stretch it out and measure it. But with
all the different papers they are made with you could
see flow numbers all over the place. I use to flow
air cleaners years ago for work BUT I never got into
the numbers on them.... they were all restrictive to
some point

K&N site has one.

one attempt at a formula for the air inlet ducting:

http://www.hotrod.com/techarticles/marlan_davis_technical_advise/cold_air_inlet_sizing.html

The Bosch 'little Blue book' has the formulas for the hp loss for any manifold pressure, as well as the formula for hp loss for any air intake 'absolute' temperature

http://www.amazon.com/Automotive-Handbook-Robert-Bosch/dp/0768006694

if you don't want to buy one your local library can certainly get one through inter-library loan for free

I don't have any formulas either but, have this information
I've had written down for several years:

WIX 46945R 14" x 3" 860 CFM @ 5" H2o (100%)
WIX 46944R 14" x 4" 907 CFM @ 5" H2o (100%)

I've never tried flowing a carburetor with and without
an air filter but, should be pretty straight forward.

Edit:

Just for added information........

Most 4-barrel carburetors are given their flow values
based on 1.5" of mercury(hg) or 20.37" H2o so, 5" H2o is
.368" hg........which means the filters I have listed above
were flowed at only 36.8% of the pressure differential as
the carburetors they go on.

Interesting article, but duct size is definitely variable with race venue.
Drag racing requires the input speed to be a compromise between max power with very little MPH - top of 1st gear was used, and max power at trap speed. I suspect that fast cars (210) are already going too fast to use that, and that more area may help, or just use 50 MPH instead of "top of 1st gear".

Bonneville is almost entirely max power at MPH. Search http://tinyurl.com/bmzhtj for various opinions on how to size the scoop.

Road racing has the same upper limit, but the lower is based on how much time is spent in low gear in tight corners and will vary between tracks.

Since ideally the filter is under light pressure or STP only, the size is more related to power than speed.

K&N formula:
engine size × VE × RPM ÷ 20,839

To get the effective height of a K&N subtract 3/4" from the measured height.