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Air Intake Systems | Standard Filters | Cool Air Intake systems | Jeep | Pre Filters | Major Brands

After Market Filters:
You can get after market filters such as K&N, which fit in the same airbox but promise improved flow.
An Air Intake system replaces your air box, tube and filter. In addition to improved flow from the filter they claim advantages from air tube tube and throttle interface design and cooler air. See below.

Some of these systems have been criticized for allowing more fine dust to get by. See discussion below.
Filter only:
Types of filter:

  • Paper (cellulose) - Most common.
    May clog faster than synthetic. Will expand and restrict flow when wet.
  • Paper + foam - Some original equip.
  • Synthetic - Common for after market filters such as AEM Brute Force
  • Oiled Cotton - K&N and others.
    Better flow, less filtering.
  • Combination - Cotton plus synthetic - AIRAID
  • Oil bath - Not used much now.
 Cellulose                Microfiber          Cotton
Red circles represent the diameter of a 2 micron and a 5 micron particle.
micron = µm (micrometer) = 1/1,000,000 m

Filter Type Dust trapped
Paper 30µm
Cotton Gauze 45µm
Unifilter foam 4µm
Synthetic fiber * 2µm
Source: UNIFLOW.com
* Synthetic was not listed in the UNIFLOW page. I couldn't find a standard test for synthetics, so based the numbers on claims by some manufacturers.

An article at quadratec says:

"Dirt trapped by a paper element will impregnate the fibers which will impede air flow at a proportional rate. In other words, performance decreases dramatically as a paper element gets dirty. At the service interval, say 14,000 miles, air flow through a paper element can decrease as much as 70 percent."
(Note: They are trying to sell after-market parts)

Because foam is a full depth filter even as dirt is being stopped airflow can continue at a high rate by simply going around the stopped dirt.

Source: True Flow High Performance Air Filters Facts at PremierHummer.com

Air Intake Systems: or Cold Air Intakes (CAI)

These systems claim several advantages:

  • Increased Air Flow from a less restrictive filter.
  • A panel isolating them from the engine for cooler (thus more dense) air providing more oxygen.
  • Many include a spacer that goes between the throttle body and intake manifold for improved airflow.
  • The above are supposed to give you more horsepower (up to 12 HP) and better mileage (1-2 MPG).
  • They are also cleanable so you save money on replacement filters.
There are dyno tests by the manufacturers of these systems as well as independent tests which show horsepower and testimonials to mileage improvements (I didn't see any formal tests of mileage), but none of them break it down as to which component, the filter material, cooler air, tube and spacer, account for the improvement.

K&N says "We certainly understand why it is theoretically possible for a consumer to experience a mileage increase after installing a K&N air filter or intake system, however, we do not go so far as to make a general claim that our air filters and intake systems will provide an increase in mileage."

Bottom Line:

  • You'll get a 7-10% horsepower improvement; more at high RPM.
    The extra RPM could save you some gas if you drive in mountains a lot and can avoid downshifting and running at higher RPM to make it up the hill.
    It will also give you a little extra acceleration for merging into traffic or passing.
  • Don't expect any increase in mileage.
  • You'll save money with cleanable air filters.
    At $25-45/ filter and changing every 20,000 mi. that's $12-22/yr savings.
With a replacement system cost of $225 you can't justify it on filter savings.
Consumer Reports tested some cars with the air cleaners intentionally restricted and found no change in mileage. There was some power loss.

One person reported it took 3 tank fulls of gas before the onboard computer adjusted the air fuel mixture to give better mileage. see SteveAmbrose.net

K&N was one of the first to produce these and advertised better air flow for increased performance, however they let more dust thru. Dust can cause your rings and valves to wear prematurely.
Also, much of the benefit only comes at high (> 4,000) RPM which will benefit racers but not normal drivers.

The computers in modern engines will adjust the Air/Fuel ratio to compensate for reduced air flow, so the mileage gain is questionable. If you do a lot of driving in the mountains where the extra HP will save you down shifting and running at higher RPM there may be some advantage.

A Duramax owner tired of the lack of scientific evidence pursued a path of testing that eventually ended with a lab performing the ISO 5011 procedure on a number of replacement air filters. The result was one that many might have concluded on their own - for a given size, a filter's ability to trap dust is inversely proportional to its flow rate. If keeping dust from your engine is the goal, a paper filter such as an AC Delco will provide far superior filtering performance, but you'll pay a price in airflow. If WOT (Wide Open Throttle) performance is most important, the K&N filter used in this test provided about 50% less restriction to airflow, with the 1.6" of H20 difference in air pressure drop at 350 CFM making for a minimal difference (~0.5%) in power at this flow rate.

A test of the Tornado system (Which fits in the air intake hose before the intake manifold and creates "a vortex or swirling effect to the engine" resulting in an "improved airflow into the engine's combustion chamber) at autorepair.about.com showed a slight 0.2 MPG increase in some vehicles and a decrease in others and no horsepower difference.

You also have to rethink when and how often you wash your cotton filter. Not only do they clog 3X faster than paper, but if you clean them too much, they let in up to 18X more dirt.
Source: How well do K&N air filters work? at AutoBLog.com

K&N air filters are notorious for passing way more dust than any other filter commonly used on Jeeps.
There is also a rumor of oil in the K&N gumming up the MAF (Mass Air Flow) Sensor.

Fram (AIR HOG), Purolator and other manufacturers make Washable and Reusable replacement filters now for much less than a whole system.

I found a testimonial from a guy who didn't have to shift into 3rd anymore when he went to Tahoe, and the 3 MPG improvement with an AEM kit.

Others say "the TJ's air filter system already breathes freely, unlike many other vehicles." The exhaust is the limiting factor for air flow.

Jerry Bransford, a well known off-road enthusiast, writes that the Jeep Wrangler TJ does not have a restrictive air filtering system,

"The 4.0L's intake does indeed (!) have more airflow capability than the engine is capable of drawing at WOT (Wide open throttle) and high RPM. This information comes literally and directly from the senior-most Jeep engineer known as the "father of the Jeep Rubicon" who has been there since AMC days. In a personal phone conversation I had with him (several, actually) discussing a different issue, I brought up the air intake system and he assured me it was one of the few air intakes on a production vehicle that did have greater air flow capacity than the engine can consume."
However the April 2008 JP Magazine published a dyno test that showed a 12 HP improvement.

bpcompton on jeepForum.com says:

"What was said above is correct. However, there is one point of restriction on a 03 sport - the 'trumpet' at the front of the air box. Take it off, and you will see a marginal but noticeable increase in power above 3000 RPM."
Jp Magazine Tests:
Type Dust
Stock     188 142
NONE     192 150
AEM 129 ‡ 99.4 198 155
AirRaid 126 98.4 201 154
K&N 167 96.1 198 154
Online article "Air Filters Blowout" at JpMagazine.com
Seems to be based on Southwest Research Institute tests sponsored by AEM.
Dust - How much dust (grams) will it hold before engine doesn't run well.
Efficiency - % of dust filtered
Apr. 2008 Jp Magazine article "Intake Shootout" -
  Dyno Test TQ - Torque, HP - Horsepower.
‡ The jeep web site lists the AEM dust Capacity as 74. That was the capacity for a test before washing the filter in 2005. The capacity was 129 for tests after washing the filter. See new tests below.

Tests for HP and torque do not specify RPM. At low RPM the difference will be much smaller.
They speculate in their Apr. '08 article that: "It wasn't so much the air filter as the air tube that makes the power in these kits. The air filter itself can flow more air than the Jeep can use."

In Jim Allen's jeep book "Jeep 4x4 Performance Handbook" he says he witnessed a dyno test that showed a 15 HP gain @ 3,600 RPM using cool air (88°) compared to hot (155°) air.

The stock filter on my 2001 Wrangler was a combination foam paper filter, but when I went to my dealer for a Mopar replacement ($37) it was paper only.
Hastings ($25) and Wix ($13) have combination foam/paper replacement filters.
The owners manual says to change the filter every 30,000 mi. or more frequently (Sched. B) for dusty conditions.

One forum says to put AC Delco or Wix/Carquest paper air filter in factory intake box.

A snorkel provides the same benefits (cool air and better flow) for about the same price.

Jeep Air Filter at jeep.us41.org/
A review at parksoffroad.com/johnsjeep/ compares AEM and K&N and reported a 1-3 MPG gas mileage increase on a 4L YJ.

Pre-filters (bags of nylon like material over the filter) will keep the filter from getting wet or clogged with mud, but won't help much with dust. It only filters particles down to .005" (128 microns) while fine dust is 1-80 microns.

See: Jeep Air Filter Upgrades - Air Filters Blowout Which Brand Filters Dirt Best And Helps Make The Most Power?
Outerwears Pre-Filter and Volant

Major Brands:

K&N High Performance Air Filters & Cold Air Intakes ...

Note: Dyno tests are not comparable because they are on different vehicles.

AIRAID Filter Company

Airaid claims "At 2 microns, our filters capture almost twice the percentage of particulates that our competitors do, and more than 6 times the amount at 0.5 microns."

They say their Absolute Efficiency Ea Air Filters (EaA) with nanofibers remove 2.5 times more dust than the average cellulose (paper) filter and 50 times more dust than the average wet gauze media.

AEM Brute Force
Power improvement.

AEM Brute Force says they are capable of filtering down to 1 micron (.00004 in.)

The DRYFLOW Synthetic filter's unique non-woven polyester filter media is independently tested at over 98% initial filtration efficiency (filters out 98.6% of airborne dust) (99.4% cumulative efficiency) based on SAE J276 JUN93 Fine Dust Air Cleaner Test Code procedures. Oiled filter initial efficiency is typically 90% to 95% efficient. It filters down to ONE MICRON of particulate!

A Review at parksoffroad.com/johnsjeep/ shows AEM vs K&N with dust let through the K&N.

AEM results and Southwest Research tests sponsored by AEM (Efficiency and Dust Capacity of several filters (pdf), AEM test (pdf) -Independent Lab using ISO 12103-1,A2 Fine Test Dust ):
Element ID Pleats Material Efficiency % ** Dust
Capacity g ****
Initial final 2005 2007 i 2007 4
AEM 21-3059-
51 Polyester 99.5 99.3 129 249 163
AIRAID 700-470 62 cotton + SynthaFLow 98.0 97.6 126 117 77
K&N RE0870 60 Oiled cotton 95.6 94.7 167 141 122
AFE 72-40035   synthetic 98.5 97.9 167 116 71
Note: None indicated any dust leakage.

Initial efficiency indicates a clean filter, and cumulative efficiency indicates a filter at the end of the dust test cycle.

**** Dust retained by element after a pressure drop increase of 10 inches of water, at 240 scfm.
2007 i - Initial test, 2007 4 - After 4th washing

Another set of tests was done in 2007 showing higher dust capacity for AEM.
The 2005 test showed a much lower dust capacity (74 g) on initial use prior to testing and cleaning.

Results: (Test by Southwest Research sponsored by AEM)
K&N and Airaid require some dust to be trapped to gain maximum efficiency.

In initial tests (before washing) AEM had the highest restriction.

Test Dust:
ISO 12103-1,A1 Ultrafine Test Dust is nominal 0-10 micron size.
it is used as a test contaminant for fuel system components, water filter performance evaluation, and other custom applications.

ISO 12103-1,A2 Fine Test Dust is nominal 0-80 micron size. ISO Fine Dust is identical to SAE Fine Test Dust

ISO 12103-1,A3 Medium Test Dust is nominal 0-80 micron size with a lower 0-5 micron content than ISO 12103-1,A2
ISO 12103-1,A4 Coarse Test Dust is nominal 0-180 micron size.
See: ISO STANDARD 12103-1 at PowderTechnologyInc.com, GlobalSpec.com

Filters in Home and Garden
"Gas-Saving" Products: Fact or Fuelishness? at the FTC
Donaldson Air Filters
How well do K&N air filters work? at AutoBLog.com
True Flow High Performance Air Filters Facts
Facts You Should Know About Air Filters at Quadratec
Efficiency and Dust Capacity of several filters (pdf) - Southwest Research tests sponsored by AEM
AMSOIL Nanofiber filter material
Can a better air filter improve fuel economy? at cars.about.com
The Fiber Society 2007 Spring Conference

last updated 1 Apr 2008