There is a lot of hullabaloo about air filters and which one is better for this, that, or the other reason. The fact of the matter is that which filter is better depends on what you are intending to do with your Jeep and what your Jeep is made of.
For example, if you've got a stock engine, basically any filter will flow enough air to keep it happy. And you aren't going to pick up any power from a filter swap alone. The restriction in the stock Jeep is the stock air-intake system, not the filter.
However, if you've got a super-modified mud-runner or just a high-horsepower trail machine, the stock filter won't cut it and you'll need to upgrade.
The Test: ISO-5011
The International Organization for Standardization sets up uniform testing procedures, conditions, and equipment so that test results can be reproduced anywhere. Why do you care? Well, the ISO-5011 test that we are talking about deals with dust and the air filter. Basically, what they do is flow air through the filter while suspending dirt particles in the air stream from 1-120 microns (.001mm) in size. The ISO-5011 standard sets guidelines for the range and size of particles involved. The old testing standard was an SAE J726, but this test has been adopted by ISO and improved upon.
OEMs typically use the coarse dust standard for normal passenger vehicles, but since we are talking about heavier-duty usage and dirtier conditions, we focused on the fine dust test.
Dust Capacity
We aren't too concerned about the actual flow numbers of a clean filter, as aftermarket filters flow more than 99.99 percent of what our Jeeps can handle, we are more worried about dirty flow numbers - or how much dirt it can accumulate and still flow enough to keep the engine happy.
The capacity of the filter in the ISO-5011 test is when there is more than 10 inches of water difference across the filter. Ten inches of water is a measure of pressure with the equivalent of 52 pounds per square foot or enough pressure to collapse a non-supported conical filter.
Basically, what this means for you is that while your Jeep will still run, it most likely won't be getting enough air in the higher rpm ranges, and you should clean the filter. How dirty your filter gets totally depends on your driving conditions.
Cumulative Efficiency
This is the big one that everyone talks about. If they aren't talking about it using the ISO-5011 fine dust standard, you'll be comparing apples to oranges, and the numbers you are bench racing with will be useless.
The cumulative efficiency basically tells you how much of the dust shot at the filter was caught in the filter media and how much got through. For example, an efficiency number of 98 percent would indicate that 98 grams of dirt were caught while two grams got past the filter.
Restriction
OK, you're probably thinking, "Well what about horsepower increases?" You got us. The less restriction a filter has, the more horsepower potential your engine can realize. We ran into so many problems here trying to get numbers that matched across the board, we couldn't get too far into it.
Most companies market their filters by horsepower ... which is fine if you are talking about an intake kit. Instead, you want to know the restriction of the filter at a certain cubic foot per minute of flow. That is the big reason we are swapping the paper element out, after all. It filters great, but the restriction is very high.
The technology
When you get down to it, there are four basic types of air filters:
1) Foam: An open-cell foam medium that's normally oiled. Not to be used in Jeep applications because it won't take the abuse or filter out enough dirt. Often used as a prefilter for paper air filters.
2) Paper (or cellulose): the typical filter used in OE applications. Filters fairly well, but doesn't flow that well. Also, as it accumulates dust, whatever flow it had goes straight down the tubes. Good for a Jeep that never sees dirt. Not reusable, not cleanable, and falls apart really quick in water.
3) Cotton gauze: Basically, a cotton ball spread out really thin, oiled, and held between two layers of window screen, this filter flows very well. However, without multiple layers of cotton it just doesn't filter well enough to protect your engine. Seven-layer is what we've found to be the minimum for adequate protection. Cleanable, reusable, and stands up to water.
4) Dry element: A synthetic medium that is reusable, designed to catch smaller particles than cotton, and so does not need to be oiled. Doesn't depend on oil to filter air, and there is no oil to potentially harm sensors.