This pic is a summary graph of what I've been able to come up with so far. If anyone can post additional data or links to where I can find it, I'll put together some additional charts that can be used for easily comparing various filters.

In this graph I estimated the potential extended CFM flow capability for the 6637 by using the theoretical CFM vs Restriction relationship for air flow through a length of straight pipe. This relationship gives an additional CFM increase that's proportional to the square root of the increase in restriction. This estimate for the 6637 is shown as the red dotted line. I also gave the CFM vs Restriction for a 30-ft length of 3.5" diameter straight pipe (in green) as a reference. The estimated CFM for the 6637 overlaps that of the AIS, and both parallel the flow for a pipe.

Also shown on the graph are two Donaldson Power Core filters that might be suitable candidates for high HP applications, but they'd have to be custom fitted for a 7.3L PSD. Details on those filters can be found here... http://www.donaldson.com/en/engine/support/datalibrary/034267.pdf

It should be noted that CFM vs Restriction is not the complete story, and in most cases isn't even the most important parameter to consider when selecting an air filter. I've seen air filter specifications ranging from 99.5% to 99.99% capture efficiency. This can make a big difference in the # grams of dust per 1000 miles that goes through your engine. One of these days I'm going to do an analysis of this, and try to estimate how much dust flows through a typical engine, and how much it takes before showing up on an oil analysis report.

This graph is based on data for brand new clean filters. As these filters begin to collect dirt, their CFM curves will move to the right so that a given CFM will result in higher restriction. Typically, a good filter minder will indicate it's time for a new element at a restriction of about 20" H20. Taking the AIS as an example, at 500 CFM it can collect and hold about 1300 grams of dirt as its restriction doubles from about 10" to 20" H20. However, if I ever see a restriction of 15" H20 on my air flow gauge (which would only occur when towing a grade), I'll change my AIS at the next opportunity.

Filters like K&N, etc... might look attractive because of their low initial restriction when clean, but they can't hold very much dirt, and for that matter capture it very well to begin with! As soon as these filters get a dirt load of even 10 to 20 grams, their restriction rapidly increases to more than that of a plain paper filter.

I've seen many references stating that both MPG and dyno HP are independent of restriction, as long as the restriction is less than about 20" H20 or so. Here's a couple of links, the second one has a RWHP vs Restriction curve.

http://www.filterminder.com:80/faq.asp
Does horsepower decrease as air filter restriction increases?
Dynamometer testing of horsepower at various filter restrictions has shown that horsepower remains constant as air filter restriction increases up to the maximum level of filter restriction recommended by engine and vehicle manufacturers. Engine horsepower can decrease when air filter restriction exceeds the engine or vehicle manufacturer's recommendations.

Does fuel consumption increase as air filter restriction increases?
Fuel consumption remains constant as air filter restriction increases, until it reaches a certain point. Testing by independent companies suggests a slight increase of fuel consumption with a diesel engine when air filter restriction reaches 17-18 inches water vacuum. However, the fuel consumption may not be severe. The 17-18 inches restriction is only seen at full load conditions, which may be an infrequent condition. Full load condition includes hard throttling, pulling a trailer, passing another vehicle, etc. Air filter restriction reduces when these conditions end. The engine may be operating most of the time at lower restriction levels cruising at highway speeds, which would not affect fuel consumption. If there are concerns about excess fuel consumption, the air filter can be changed at lower restriction levels.

http://www.filterminder.com/bulletins/hpvsrestriction.pdf
Dyno testing at various air filter restrictions has shown that horsepower remains constant as filter restriction increases within engine manufacturers specifications.

 

These reasons don't apply to a dyno run, at the drag strip, or for a sled pull, but if you tow a lot, they apply in spades!

1) It lets in too much hot air which makes the turbo work harder trying to flow more CFM to make up the difference in the lower MAF due to the lower air density. You can read the whole story on that here https://www.ford-trucks.com/forums/645117-getting-ready-to-do-some-6637-testing-6.html#post5098431 starting with my post# 78! If you tow hot and heavy, you definitely will be better off using a cold air intake of any type vs an open element filter of any type. Measurements, computer models, and my almost 9 years as a fulltime RVer, which included several years towing with an open element filter, have demonstrated conclusively that ingesting hotter than necessary under the hood air is detrimental to towing performance

2) Without an air box and an inlet snorkel leading to a Zoodad hole, not only do you give up the benefit of the cooler ambient air, but you also give up the benefit of 2" H20 or more of RAM air pressure which helps force cooler outside air past the filter and into the turbo inlet tube. This might not sound like much, but when you're struggling to get to the top of the Grapevine on a hot day, every little bit helps, if only a little bit.

3) An open element filter is subjected to all the dirt and dust that gets into the engine compartment. It gets coated with oil fumes, dirt, and grime, just like everything else in a typical engine compartment. A filter in an air box with an inlet leading directly to ambient air, only has to deal with the dust that's in the air actually being used by the engine. I can wipe the top of my air box clean, and after 2 or 3 hours of towing pull into a rest stop, and the air box is dirty again. The way I see it, that's an extra amount of dust that would've wound coating my filter if it hadn't been protected by an air box.

4) Condensation! Yes, I mean inside the engine compartment as well as all over the outside of the truck and RV. It's happened to me many times when camped out near the ocean and gulf. This might not be a big problem for an oiled type element but it's not good for a paper one. Even high humidity has made papers inside my RV so limp that I've had to hold them with both hands to read them.

 

Since people seem reluctant to post on this thread themselves, I'll do it for them! I received these questions via a PM, so the author shall remain anonymous, but I'm posting the questions and the answers to help better explain my CFM vs Restriction chart.

"Gene, I was looking at your chart for restriction, and if I read it correct, the AIS is more restrictive then stock? Or was that at the manufacturers spec for "fully dirty capacity"? Also was that model reflecting just the filter or the system? And one final thing, if it was a system, did that include the fender sleeve on the AIS?"

The only data point I have so far on the stock filter is 277 CFM at 2" H20 restriction, and I found that # in an add for the AFE which gave 582 CFM for the same 2" restriction for the 7.3L AFE. I'm pretty sure AFE wouldn't be comparing their clean AFE vs a dirty stock filter, and that's a pretty low restriction anyway, so I'd say the # is for a clean filter, but I don't even know if the AFE # given for a stock filter is correct to begin with.

I found the dirt capacity #'s in an add for the 7.3L AIS that was comparing the AIS to a stock filter. I got my AIS CFM vs Restriction data from Clux, who referenced posts on Dieselstop from a DONALDSON rep, and those #'s appear to be for the entire system using a standard installation without a Zoodad. I'm still looking for additional data on the AIS because that's how I calibrate the air flow measurements on my truck.

Several years ago I was told by a source in LA who runs a diesel performance shop and is a Banks dealer, that Banks recently (then) flow tested the AIS, and it did much better than they expected, but he couldn't get the data for me. I plan to stop by the Banks main office this winter and see if I can talk them out of some data by showing my CFM measurement setup and explaining why I need the data.

As I explained to begin my thread, this is the best I've come up with so far. For all the other filters on the chart, I could only find 1 to 3 data points, so I included 0 CFM at 0" H20 restriction as an additional point, and plotted what I had. I started this thread to try and enlist the help of others to ferret out CFM vs Restriction data on all the filters of interest and post it, so I can put together a revised chart that can be used to compare choices at a glance. It would also be useful to compare dirt capacity, capture efficiency, and how the filters respond as their restriction increases as a function of their dirt load.