this post was on another forum, i cant say HOW accurate anything is, but its worth reading for sure!!!!
Here's some info from Dale Isley at TYMAR on this subject:
"Testing on a flow bench will show you how much the filter is ABLE to flow, not how much it ACTUALLY flows on the vehicle. Case in point, if you restrict the intake to the filter in any means by dictating what air comes to it, you will restrict flow regardless of the ability of the filter to flow more cfm's. So, if you have one filter that flows 500 cfm, and another filter that flows 2500 cfm, but you restrict the flow to come from a 3" diameter opening, they will both flow with the same restriction even the better flowing filter has an ability to flow more.
Flow bench results are done backwards. They go with a restriction level and then dictate an air flow derived from pulling that restriction through the filter. Trucks work opposite. The dictate a flow and you get a restriction from the filter when the flow is needed. If they want to compare filters, have them give you comparative restriction levels on the same truck using two different filters. That will take into account the whole intakes level of design - not just the filter - but the routing of the air and ability of the air to get to the filter without restriction as well as the filter and the tubes leading to the turbo as well. A flow bench test will give you a filter cfm, but they have the perfect environment to draw from. It is a filter sitting in open space and not an intake installed in an engine compartment.
I have both K&N's and AFE's flow bench and air filter efficiency tests. I think the Air Raid uses the K&N filter? In short, K&N is the easiest, they don't filter. In an SAE J726 filtration efficiency test the initial filtration rate at which the K&N conical filter was rated at was 97%, so 3% of all the dirt that hit the filter went through it
. This was at 240 cfm pulling 10" of water column restriction. The cumulative rate (after the filter was partially packed) was 99.1%. In comparison, the Donaldson went with an initial rate of 99.9% and a cumulative rate of 99.9%. Take into consideration that the 240cfm is, at best, only half the flow the engine needs. A bone stock truck will take in 550 cfm to 600 cfm. Add a chip, downpipe, etc, and flow rates in the 700 cfm's are not hard to compute. The more the flow, the more the restriction, the worse the filtration efficiency of the K&N's get. As well as the flow rate difference, the J726 test was cut at 10" of restriction. Our trucks pull 24" to 28" in stock format. Even with my intake installed they pull 17" of restriction. Again, more restriction the worse the efficiency rating for the K&N gets.
It is a size matter. If a 5" x 5" area will flow the same rate as a 9" x 9" area, you will be able to filter through the larger area at a smaller particle rate with no loss of flow. Every filter also has a maximum filtration capacity. The restriction at which the filter fails. That will always happen on the smaller area if the filtration quality is the same.
But, again, it is much easier than facts and graphs. For some reason everyone thinks I am "the Donaldson guy". I am not. If anyone can show me another filter that will operate better, I will use it. I am not selling the Donaldson because I don't have options. It is just the best option I have found. I also believe in not overcharging for the intakes. As much as they say they will save you money, I can get you an intake and filters for over 5 years just for the initial cost of the other guys set-up.
Did you know that Ford, Dodge, and Chevy have voided warranties for the gasser trucks when K&N filter is installed by the truck owner? Turns out that the oil leaches into the intake tube and coats the MAF sensor which burns it out. The volumetric efficiency of a gasser motor is 70% to 80%. The volumetric efficiency of a turbo diesel is 140% to 170%. For an intercooled turbocharged diesel the volumetric efficiency rate is 200%. Can you imagine the oil leaching that would happen in an environment that is twice to two-and-a-half times the flow rate? If the oil leaches, than the filtration efficiency is compromised by eliminating the correct level of hydraulic fluid in the filter.
The correct level of oiling is also of major concern for me. If you oil it too much, you get too much restriction. If you oil it too little, you don't get filtration efficiency. You don't have to worry about any of that with a paper type element.
AFE is a little harder to prove because they use units of measure interchangeably. You measure pressure in psi. In vacuum they use inches of water column in a mercury scale (Hg). In restriction you use inches of water column in a water scale (H2O). There are 17.2" H2O for every 1" Hg. Using the Hg scale allows for too much leeway in measurements to really specify amounts and rates. For the two years I have had contact with them they have stated that their filter would outflow the Donaldson. In the latest test results they show that they do not, but flow very similar numbers. The only problem is that they did not use a Donaldson filter. They used a Fram filter that is a cross referenced to the Donaldson. I don't know if they knew that Donaldson (and Baldwin since they paid the patent fees to Donaldson) has a patent on the specific filter media used in the Donaldson and Baldwin filters, but testing a similar filter sharing the same design but using different filter medias isn't really testing the same filter.
Outside of using the wrong filter, they used a computed hp rating of 698 hp with a cfm flow rate of 987 cfm. I don't think this accurately represents real data. Not many trucks will produce 698 hp or use 987 cfm. Also, according to their results, at 1" of water column (they mistakenly use H2O references instead of Hg) the stock filter and stock intake only flows 220 cfm. If that was true, when I measured 26" H2O with a stock filter in place on my restriction test, that would mean a PSD at 3000 rpm would only require 220 cfm of air, which we all know is pretty absurd. For whatever reason, the numbers just don't add up and no one over their caught it.
Again, it is too easy to hook up a restriction gauge. But, again, the filtration efficiency will always be in question since the oil level will be dependant on the customer being able to reproduce factory levels. The filters are not "life time" no matter what warranties the manufacturer gives, and initial filtration levels are never published."
Hope that helps. Cheers!