The performance kind:

An air filter is used (as the name suggests) to purify the air that passes through. A standard automotive air filter does this much like a sieve, effectively straining out any unwanted matter and allowing only pure air through to the throttle body. The standard filter can be replaced with a performance part designed to allow greater flow into the engine (thus producing slightly more horsepower).

There are two main types of performance air filter - 'In box' replacement elements and 'Pod' or open element filters. As a general replacement filter (or for cars with sensors in the airbox assembly) the In box type filter is required while for high performance and modified cars the pod style is preffered. A basic performance guide is included below - note that the percentage is a measure of resistance, not flow. This means that 100% represents the lowest flow(bad) while 37.5% shows the highest(good).

  1. stock box w/ filter 100 %
  2. stock box w/ Pod 100 %
  3. stock box w/o filter 100 %
  4. modified airbox (trimmed) w/filter 62.5%
  5. modified w/Pod 56 %
  6. individual filters 44 %
  7. manifold only 37.5%
Test results provided by

In box: (filter)
This is the easiest and simplest form of replacement filter. It will look much like the standard filter element in your car and does much the same job. To install it you will simply need to remove the top of the airbox (usually a big plastic thing attached to a thick rubber hose) by undoing the clips/screws. You should then be able to lift the old filter out and drop the new one in its place. Replace the top of the box and you're done. As you can see in the table, the performance gain is likely to be negligible with this type of filter. This is because althought the filter may be capable of higher flow, there is still the resistance of the airbox limiting the intake. If you plan to use this type of filter in a performance application it is recommended that you also modify the airbox (eg. Cut holes in it) to allow the filter to breath.

Pod filter:
If you look in any contemporary modified car magazine you will notice that most (if not all) vehicles therein sport a vaguely cone shaped object on the end of the intake duct (the big hose). This is called a pod filter due to its appearance. When used properly a pod filter can increase an engines horsepower output with a pretty good power/cost ratio. Before you go out and whack one onto your engine however, take note of the following: If the filter does not get cold airflow it will most likely decrease the power of the engine. This is because in stock (standard) form the airbox provides a degree of heat shielding for the filter. Obviously the pod filter does not have this could well end up sucking hot air from behind the radiator (for example). This hot air is less dense than cooler air around it, and the more air you can pack in the better. To negate this most people add some ducting from lower down in the engine bay (a hole in the front bumper is the best) to the mouth of the filter. The majority of the air will then come from the cooler part of the bay (the hot air of course, rises to the top).

Installing a pod filter is slightly more involved as you need to remove the entire airbox, not just the top. Once this is gone you can simply clamp (most pod filters have one) the filter onto the hose. Several companies offer cold air kits which are basically filters and extra ducting in one. With a cold air/pod setup in place you can reasonably expect a gain of up 5hp (Naturally aspirated) or up to 12hp for a turbocharged or blown engine. Along with the added power you can expect a gruntier induction noise when you hit the high revs.

No filter:
Just don't do it. Engine internals aren't supposed to deal with dirt/grit/pebbles or whatever and with enough exposure (usually not much) they won't.

Zoom zoom

It used to be that whenever I had my oil changed there would be a discussion about my air filter. My owner's manual suggests replacing it every 15,000 miles and I had my choice of paying a premium to have someone else change it or taking time out to run to the auto parts store for a filter that I would have to install. Our local Advanced Auto Parts store has always treated me well. I rarely feel as if they look down on me for being an out of place female who knows nothing about autos and the advice and service they have delivered has been well above average.

I can't remember what I went to the store for initially however as long as I was there I thought I could prepare for my next oil change by purchasing an air filter ahead of time. I enjoy a great relationship with the guys who change my oil. Mobil One Express is the only place I go as the service is outstanding while the prices are extremely reasonable. The Firestone down the block takes care of my tires and going to the same places allows me to establish relationships that reduce my chances of getting ripped off.

While I was at Advanced Auto Parts I found out that you could purchase standard air filters or I could spring for one that had a million mile warranty. After discussing the advantages with the guy behind the counter I decided to go ahead with the purchase even though I would have to wait for a filter to get shipped in from another location. The packaging claimed that this was "The World's Best Air Filter" so I thought I would give it a try. My K&N washable and reusable air filter came with a special sticker that lets whoever services my car know that this air filter should not be discarded.

My new and improved air filter was $54.64 after tax and according to the manufacturer it should be cleaned every 50,000 miles. Cleaning instructions are on the K&N website. The process is slightly more complicated than the service tech made it out to be however the cost savings, waste reduction and improved air flow make this purchase worth it in my opinion. No other air filter I've seen comes with a warranty that is this nice, a million miles may not be a theoretical lifetime however you have zero warranty with disposable filters that do not last nearly as long or perform as well.

Even if you do not own an exotic, luxury, or performance vehicle I would urge you to consider investing in a good air filter. The K&N box reminds users that cars with clogged filters are like people trying to breathe through straws and fuel efficiency can diminish with a dirty filter. K&N filters increase horsepower, K&N is also a 5 time winner of the Manufacturer of the Year award which is presented by the Performance Warehouse Association.

If you purchase this filter I would recommend taping your receipt to the box, prominently displaying your sticker so service people don't have to hunt for it and I would also write down your odometer reading at the time of install because if your memory is anything like mine it is good yet short. My air filter was purchased on March 17, 2011, Advanced Auto Parts did not charge me freight for my filter and so far I have been pleased with its performance since I have saved money by not having to buy another filter.

For most people a car is a significant purchase so taking care of it makes sense. It is much cheaper to maintain an auto than it is to pay for repairs and if your filter increases efficiency there will be a fuel savings as well. Small economies add up to larger ones, cleaner air and less environmental waste create winning situations for people and the earth so please consider giving your motor vehicle the simple gift of The World's Best Air Filter.

To learn more about K&N products, services and statistics visit their website at

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