So I've been doing some reading and I've come across the issue that the 6637 flows about 430-450 CFM while the stock filter (Motorcraft) is rated at 630-650 CFM.

Apparently a STOCK 7.3 will consume a max of 631 CFM. So what's the deal here? I always hear of people noticing an improvement with the 6637, but in reality it is supposed to flow less air than stock? I've even heard that the 6637 is good for 700+ HP.
On the other hand the AIS is rated at 775 CFM but only good for about 450HP.

So whats the deal here? Supposedly even Jody thinks the 6637 flows less but I want to know what you all think?
So let me have it!

 

I don't know how to link a thread but if you use the search bar & type in AIS vs. 6637 u can read for hours. Good Luck

 

Just so you know for future reference, to link a thread to a word like "here" you highlight the word then click the chain icon and then paste the url and the link will be there.

Do you think you can at least paste the link, I can copy it into my browser. I ask just because when I search here I always get the same damn results no matter what I type.

 

https://www.ford-trucks.com/forums/9...vs-6637-a.html


https://www.ford-trucks.com/forums/1...th-photos.html

 

There is no fast abnd simple answer to your question aside from "It depends!"

OK. Here's a little physics for you to mull over.

You have to be very careful with referring to air flow, because the real... VERY REAL issue is cfm at what pressure?

Pressure? Why does pressure have anything to do with it? Simply, it is the pressure differential across the filter which is the sole driving force for getting the air through the filter element. Pressure, therefore, is EVERYTHING in terms of how to rate a filter's flow capacity. If you increase the differential pressure, you get more air flow. It's that simple.

The differential pressure is simply the difference in pressure between the filter inlet and outlet. So what is it that determines this differential pressure? There are several factors.
1) The porosity of the filter element itself, which is affected by the type of material (paper versus foam versus fabric, etc.) and the pore size openings (paper's porosity, foam density, oiled versus dry fabric, type of fabric, fabric thread count density, etc.).
2) The amount of surface area of your filter element.
3) The elevation where you are located greatly influences the air pressure on the inlet side of the filter element, as does the size of the duct openings through which the inlet air flows to get to the filter.
4) Your turbo's ability to generate suction between the filter element and the turbo inlet, and this is affected by several more things like
a) your turbo design (size, style of turbo wheel, etc.)
b) the tightness of your turbo inlet and outlet piping
c) the condition of your turbo wheel
d) the velocity of exhaust gases leaving your engine heads
e) etc.
AND
5) Ambient air conditions such as humidity and temperature have a small effect on the entire situation.

So, what were the differential pressure conditions for each of the reported cfm ratings you reported? Without that information, it is impossible to say a whole lot except that there were probably different differential pressure conditions leading to what seem to be discrepancies in your cfm data.

Case in point. I know for a fact that the 6637 can flow over 1,000 cfm at a DP of 12" W.C. as determined on Flow Bench testing system at near sea level in California. The question then becomes, can your turbo generate that kind of DP? Probably not.

So, in the end, we are left with experiential input from those who say things like "the 6637 is good for up to 700 hp" (depending on the setup for your turbo/injectors/programming, of course, because a stock setup simply won't do that kind of work), or "the AIS is only good for about 450 hp" (what is the DP here for their reported 775 cfm?, and was that with a clean element or one which had been run for several years and was beginning to plug?).

If you don't test every element under the exact same conditions, you just can't make a clean comparison of their reported (advertised) flow ratings.

One example of a very limited comparison is linked here>>> http://dieselinnovations.com/techfiles/airflow02.pdf

And here is one more link to a decently performed comparison. Although it does not address your specific filter inquiry, it demonstrates what I've been talking about and most of the issues which influence real world perfomance.>>> What You Should Know About Aftermarket Air Filters | Diesel IQ

Hope that doesn't muddy the issue too much for you.

 

Great post Pete !! I think you hit the nail on the head with all the variables that effect cfm flow. But what I am wondering how would someone be able to do real world testing or back yard R&D on this subject. I was wondering if you had part of equation like lbs. of boost and you used a millibar gauge or vacuum gauge on the intake tube if you could figure out the cfm. Or maybe you need to gauge pre and post filter millibars??
I'm not smart enough to have a clue. I'm just spitting in the wind here.

I do find the whole subject very interesting.

 

Ya! great post Pete! Helped me understand a lot more.

 

Ed...

Method 1, you need air velocity, inside pipe diameter, air temperature, and air velocity to calculate flow.

Method 2, you could only use differential pressure for the calculation if you had a long pipe run with no restrictions inside the pipe (basically back-calculating flow based on the friction losses inside the pipe). We simply do not have near enough pipe length to use that type of DP for the calculation.

Link to an online calculator for Method 1 above>>> Calculator: Air Flow Rate through Piping | TLV

 

Wow the detail in that post, more than I could have imagined lol. I'm convinced that it flows more air, I just think its so confusing since the filter is rated 430 CFM.

 

Thanks Pete I guess I will leave it to the pro's since all this info is like pouring a 12oz beer into a shot glass as far as me being able to get a handle on it. It really does get deep !! It makes me glad I have a stock E99 and a AIS since it is really a none issue for me. But like I said I do find this all very interesting. I guess I'll just reread every thing and try to get a 1oz shot worth of knowledge out of it. Honestly I think I am spilling more than I am drinking ...LOL

 

That AIS flowing at 775 CFM and good for 450 HP: That's another "depends"... as in it depends on whether you're comfortable with the filter minder sucking in it's gut for a photo, or if you like to roll coal.

7.3L/2 (one intake stroke per two revolutions) X 3200 (RPM) = 11,680 liters of air per minute at atmospheric pressure = about 412 cubic feet per minute.

Say the barometer reads 14 PSI, then 14 PSI boost will theoretically double the CFM, - so we end up with 824 CFM (minus inefficiencies). 28 PSI boost adds another theoretical 412 cubic feet per minute - for 1236 CFM in a perfect world. I can only estimate my HP, based on those with my sticks hitting the dyno - about 400 HP. I limit my boost to 35 PSI, but I see no reason why I can't "blow" past 1000 CFM on Stinky - even with efficiency losses.

Factoring in efficiency losses, humidity, temperature, filter performance, and barometric pressure leads to one thing - my brain melts. I leave these calculations to those with a hotter-burning desire to know precise CFM, because even my OCD has limits.

 

That's a good baseline approach to the cfm calcs, Rich. However, you really do not end up with quite the total of 1,200+ cfm because as the air is being compressed, it is being heated quite a bit as well, and that's why Ford designed the system to include the air-to-air intercooler between the turbo discharge and the intake plenums. The hotter temperature of the compressed air offsets some of the "doubling" effect associated with the increased pressure. The actual increase in cfm is something like 70-90% of the doubled number, depending on how much actual temperature increase takes place. With gases, both temperature and pressure are vital elements to the "volume" equation and they each have the equal impact. My basis for this is the Ideal Gas Law (PV=nRT), where "P" is pressure, "V" is volume, "n" is number of moles, "R" is the ideal gas constant, and "T" is temperature. And THEN there is the issue of Volumetric Efficiency for our engines....

 

Yup - that's why I gave a very conservative number of 1000 CFM at 35 PSI boost with 3200 RPM.

 

I love learning but holy cow all this stuff is like reading chinese, there's so many different variables that it sounds pretty much impossible to have all the same constants to run a test. Even then we have to have an expensive machines to run a test unless someone is crazy enough to rig up something that would work, without them going crazy too!

 

Racing engine shops will sometimes have a flow bench device, as well as some engine head rebuild shops. The trick then is building a leak-free adapter jig to connect between the machine and the various filter element sizes, and these shops typically don't have any justification for doing so. Obviously, air filter companies may or may not have the equipment as well, but we all know that would not be the best way to go about getting objective results for competing filter suppliers (can you spell "fox guarding the hen house"?)!

At one time, I had contacted a shop in central Tennessee (about 3-4 hours from me) who was prepared to do some of this air flow testing for me on their machine, but I ran out of any real reason for paying for the testing as well, and never got the work completed. I would love to do it and know the results and share the info with everyone, but I simply cannot justify the time and expense just for the sake of knowledge, even though it would be a very fun project.