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.

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