Are our air cleaners adequate for our engines??

[DocDirk]

I would welcome any helpful input regarding the impact of air filter surface area and engine CFM demands please!

I took my car to a chassis dyno shop last week and the owner said that my engine would not perform with as restrictive an air filter as I had on it. My engine builder recommended an 850 CFM carb, that's what I'm running.
I've been researching just how to provide the optimal CFM flow for my 533 BB Ford engine and if the filtration numbers I've found online are correct, my filter is restrictive for my engine except at low RPM's. What I've found in online searches sums up to the following:

"K&N" type cloth filters are said to flow about 6.5cfm/in sq.
Standard paper filters are said to flow about 4.5cfm/in sq.
Foam filters are said to flow even less than 4.5cfm/in sq.

I started with this fornula I found online:

Required engine CFM = (CID x max RPM)/3456

Example: 533 CID x 6000RPM / 3456 = 925 max CFM requirement

(Volumetric efficiency of between 80 -90% modification
would make this between 740 and 832 max CFM req.)

So I calculated the actual flow capacity of my air filter, which is 8 1/2" wide, 15 1/2" long and breathes through a 1.75" open side (the filter heighth measures 2" but that includes the top and bottom rubber parts.)

If we generously figure the side area using a rectangle of 15 1/2" by 8 1/2" and height of 1.75" the total filtering square inches equals the product of
2 x (15 1/2 + 8 1/2) x 1.75 = 84 in sq. area.

Since mine is a paper filter it would flow at 4.5cfm/in sq and give the filter a breathing capacity of 378 cfm. (I think that the CFM figure doesn't give the maximal flow rate for my filter, but instead it's the maximal flow rate without restriction.) I have been given to understand that restriction in the flow through the filter creates a problem with the air/gas mixing in the carburetor (reverse vacuum?) - I'll leave it to the carburetor gurus to explain that.

This suggests to me that if I need 850 CFM for 6000RPM on my motor then with a paper filter in my current air filter my engine only breathes without restriction only up until 2668 RPM!??!

Using the optimistic figure of 6.5cfm/in sq listed for cloth filters the total CFM capacity of my filter would be

84 in sq. x 6.5cfm/in sq = 546CFM

This would allow my engine to breathe well without restriction up to 3854 RPM..... decent enough for most of my street cruising.

But if I want to use my engine to the max and have it breath optimally nearer to redline I would have to obtain 850CFM which means I'd need

850CFM / 6.5 cfm/in sq = 129 in sq. of cloth filter to avoid restriction
on engine breathing.

I haven't found any filters than have that kind of in sq. filtering area except the "extreme" filter top ones that use both the top and sides to filter. K&N makes them in a circular but not in an oval shape Summit carries an oval one and it figures out approximately to 150+ in sq. filter surface area, is made of the multilayer cotton and might even breath as well as K&N ..... I don't know. If it even breathes at 6 cfm/in sq. than it would provide engine air demands up to over 900 cfm without restriction. Heck even at paper filtration levels it would provide 675CFM.

This seems a bit crazy since most all the cobra engines I've seen photos of have side filtering air cleaners that would appear to restrict flow by these numbers - where am I going wrong? Are the flow numbers actually much higher?

I have not found out how much the "restrictive" qualities of my air filter affect performance / HP / torque. Has anyone got these numbers? Anyone got comparison Dyno information with and without a restrictive filter? Anyone know what filters are routinely used on engine dyno testing if any?

Thanks for any answers and for putting up with the eternal post!

Regards!

Dirk