CFM question is a good one, Gary. That is not a subject I have seen brought up, but when you think about it, the CFM that passes a filter will vary depending on rpm of engine and also rpm of turbo. As the cfm goes up, the pressure drop across the filter increases in a logarithmic curve which makes it harder for the turbo to pull in air. This can cause cavitation, just like in a water pump. So at the higher end of the flow, there will be a higher percentage restriction.
Larry

 

Yes,, and those of us who have experienced a dusted compressor wheel know that you need good filtration PLUS adequate surface area or all your $$performance upgrades wont realize their potential. Hoping some of the guys like Tugly and others will weigh in and educate some of us.

 

The filter media is hydrophobic; it doesn't absorb water.

 

The rated flow is at a certain restriction level.

The 6637/2326 filters are both listed as flowing 470 CFM at 8 inches water column restriction.

The severe duty AIS is listed as flowing 650 CFM, BUT it does it at a much higher restriction; 15 inches water column.

The AFE element is listed as flowing 850 CFM on Clay's site, but there's no mention of the restriction at that flow level so it makes the flow number meaningless. I can't find that element listed on AFE's site to look for any specs.

 

sounds to me like the AIS severe filter is a bunch better than the 6637.

not sure if the measurement of water column is linear or not but
it appears the AIS flowing 650 at 13 in WC is almost twice the flow of the 6637.

I know they, the AIS, cost more and now you see the reason why..

 

That is excellent information. I wish the manufactures would provide this level of detail in their ads and web sites, but rare to find. I need to make some calls tomorrow and see what the wc restriction is for my Air Raid 401-246. Interesting topic.

 

Actually the numbers show the AIS flowing only 38% more air at 88% more restriction than the 6637. It would be a much better comparison if they were tested at either the same flow rate or restriction level.

 

well for the sake of argument there is what of a level playing field as both state the restriction in WC in.

so looking at the raw number and you posted the difference, 38% better flow at 88% more restriction. I just stands to reason if the restriction is reduced the flow will go up.. I was wondering what the factor might be.

But i do agree apples to apples is better, but we have apples to oranges that we can turn in to apples maybe.

just sayin the AIS if better flow...

 

Carl, I don't think it works like that...I think what is being said is if you draw 650 cfm of air through the ais, there will be 15" of water column restriction with a clean filter. Who knows how much air is being drawn through when there is only 8" water column restriction....

 

Water repellency on an air filter? Be very careful how you think about this issue. Take a step back and think this issue through very, very carefully. Seriously. Chill your beans for a few minutes and just think rationally about the following issues. This issue is NOT keeping water from inside your intake, but more of keeping water from damaging your filter media. I'll state up front that I don't believe it is either necessary or helpful for 99.99% of our trucks, and the explanation below reveals how I've come to that conclusion.

(Please forgive the Eugenish length, but I've thought a lot about this issue over the past 6 years... eve since I began making and selling filter covers).

That said, there are really just two questions that you really have to consider, which are as follows:
1) How do you limit the ability for liquid water (liquid) to pass through the filter media without simultaneously limiting the ability for air to flow through the same media?
AND THEN
2) The what happens when liquid water is on the surface of the filter media when the filter is under vacuum conditions while you're engine is running?

Question 1 - response...

Since the molecules of liquid water are smaller than those of air (particularly oxygen) - yes... SMALLER than oxygen (2.75 Angstroms for water as compared to 3.6 Angstroms for oxygen... there is really no way to limit water intrusion without seriously reducing oxygen flow at the same time. What you CAN do, though, is treat the filter media so that the media itself does not absorb liquid water, and there are a few good reasons for using water resistant media, but it will NOT keep water from getting into your intake system, as I explain below!

Question 2 - response, two parts...

Q2, Part "i" - When you test your newly purchased filter for water repellency in your kitchen or back yard, what you do is run water over the filter media while it is not on the vehicle, RIGHT? Of COURSE! Who would EVER try to pour water over the filter on a running vehicle!!?? The answer to that question is, "Only the person who wants to ingest water into his engine's air intake system would ever pour water over the filter while it is on the engine and the engine is running." The reason for that is really already answered above for Question 1... it's a simple matter of real world physics... if your filter is so "tight" that water molecules cannot pass through, then the larger air (oxygen) molecules will also NOT be able to pass through. So, then, what about something like Gore-tex? I'll get to that shortly, but let's finish dealing with Question 2 for now.

Q2, Part "ii" - Well, when you try this test to verify your filter suppliers' claims, you will generally see water beading up on the surface and running off, right? Of course you do, and that's because of two reasons... the filter media has been treated with something to keep the media from absorbing liquid water, and there are no forces being applied to either "push" or "pull" the water through the filter media. Does this REALLY mean that the water cannot or will not pass through your filter? Really? REALLY? Well, maybe it does hold true for the filter in in your test conditions, but once your filter is back on your vehicle and the engine is running (sucking air through the surface), the conditions are completely different than when you were "testing" your filter.

PAUSE this train of thought to remember a few other facts about water. The water molecules for both water vapor and liquid water are exactly the same size, but when allowed by temperature and pressure, liquid water molecules typically really like to hang together in groups that we can call "drops". Depending on the amount of energy (heat, pressure, etc.) applied to liquid water, you can make drops larger and smaller, but they still want to hang together. Also, by the very nature of being a fluid, liquid water will try to conform to the shape of its surrounding surfaces. A lot more could be said, but these are sufficient for this particular discussion. Now, back to the previous train of thought... water... liquid water... running across the surface of your air filter...

Q2, Part "ii", continued... When you install your filter on your vehicle and start the engine, the engine begins pulling air through the filter media. The pressure inside the air intake system is lower than that outside the filter. Under THOSE conditions, which are the real world in which we drive and function with our vehicles and air filters, if you choose to pour water over your air filter while the engine is running, you are CERTAINLY going to suck liquid water THROUGH the filter and into your intake. This will happen because the air flow from outside the filter will begin to "press" the water against the filter surface. This results in a rapid reduction in "open" surface area on your filter (through which the air has to flow), and this will then increase the amount of external pressure pushing the liquid water against the filter surface. You see how this spirals into a situation where the water begins to seal the surface of the filter? This sealing effect simultaneously reduces air flow into your combustion chambers. The result will be ingesting water into your intake, and it will continue until one of two things happen... the flow of water onto the filter decreases or the engine chokes from the reduced flow of oxygen into the engine. Think of it like water boarding your engine, and if enough water is used, you will either choke your engine or collapse your filter, just as if you put a plastic bag over the filter.

Hey, wait a minute, I remember hearing a tale from one of our fellow brothers who accidentally left a bag on his filter after washing the engine bay, and TOTALLY collapsed his brand new filter! OWW.... And that there, my brothers, can also create potential for turbo damage!

Just how much water passes through will vary based on a number of other variables... how the media was or was not treated for repellency, how much water gets splashed onto the filter, the geometry of the filter, the total amount of surface area on the filter, how much load your engine is under when it is doused, etc.

The bottom line, though, is that even if the filter repels water in your "test", it will still pass water into your engine while the engine is running.


Okayyyy.... Gore-tex? Gore-tex works. It really does. The operating conditions are completely different, though, between your rain jacket and your air intake filter. I guarantee that if you pulled a vacuum through a Gore-tex fabric, you could pull water through the fabric... it's just the nature of real physics at work.

So do you want a Gore-tex filter element? I don't think so... it would have to be so huge to allow enough air to pass that you couldn't fit it under your hood.


So for you naysayers who take issue with anything above, why do people like Baldwin, Racor, Airraid, AFE, etc. make water repellent filters (i.e. marine applications)? Because if you get water into either a non-woven cellulose fiber media or other fabric which would otherwise hold water, you can either damage your filter element or choke your engine.

Do I believe, then, that a water repellent pre-filter or filter element is beneficial or necessary... only in rare circumstances and to a very limited degree. I've stated before that I've driven through the hot southeastern summer days here in Alabama during extended runs through frog-strangling down pours, and I've not yet had a wet 6637 under my engine bay. Would I think differently if I were mudding all the time... I'd probably just rely on a "splash shield" under the 6637 or use a cold-air intake box, and not worry a moment further or invest in any further protection at all (unless I was going to go submerged, for which I would need a snorkel kit).

 

I'm sure that the filter media manufacturer has a pressure-vs-flow curve, but I believe that getting your hands on that kind of engineering detail would be a major "inside job". The alternative would be to go to a shop where they have something like a flow bench (like a head machining shop) and get them to make an adapter to actually test the air flow at various pressures. I know that there is reputable shop in TN who will do that (can't remember or find their name), but they charge something like $100/hour for that kind of specialized testing.

 

very well said pete!!!

 

Maybe so, Tom. I DO know, though, that I was completely out of breath when I finished typing!! LOL

 

i bet it's gonna take you a week to recuperate from that long post.

 

got it,
that make sense. that is a lot of resistance to flow.. guess air filter info is like trying to find info about the price of diesel on the open market..