Induction kits  
The basics
When looking under the bonnet of modern cars it is clear that the air-flow is being restricted by all sort of curves and bends. Air flows the best in a straight line, not slowed down by any sort of filtermaterial. An induction kit delivers the air straight to the engine with as less resistance as possible. Low restriction air filters can show improved engine response with power and torque increase and possible improvements in the fuel economy depending on your drive style. Cold air is denser and provides a positive gain in power over other filter kits that draw hot air from the engine bay. 

  Learning the jargon
Directs air more efficiently towards the throttle. Vector cones use inward pointing cone end plates to direct air down.
Vortex An acceleration of air flow due to a change from high to low pressure. Ram pipes create vortexes, increasing air speed and the volume of air in the cylinders.
Mandrel bent Smooth and even diameter bends within ram pipes and additional induction pipework. Offer a more even passage for intake air, by reducing internal air flow restructions.

  A typical kit explained

Filter Material
Most induction kits are constructed from either a cotton gauze fabric or from foam. Both materials are excellent for air filtration. It's worth bearing in mind that some filter are supplied 'dry' and need to be treated with special dirt-retention additive (normaly included in kit). This additive is also re-applied when the filter is washed to clean it.
Breather vents
A lot kits will have a neck on them to connect to a cam (or crankcase) breather. This is normally through a 20mm hose (either supplied with kit or original one re-used). Some kits get round this by having the breather on a separate mini filter. While this looks cool it isn't strictly legal. Oil vapour fumes should not be vented to atmosphere. A jobsworth MOT tester may fail a car with this set up.
Induction trunk
Some kits are supplied with these and some kits fit straight on the factory one. The length and shape of this trunk can alter how the engine performs. If the kit is supplied with a different trunk you will also need to check that the correct sized hose clips are supplied. You also need to check that this is a tight fit and there are no air leaks.

Cold air feed pipes
Included with some kits, these flexible alloy ducts can be set up to feed the filter with cold air (which is denser than warm air which helps the engine produce more power). These are normally run from the filter down to the front of the car. They can be bought separately through most car spare shops.

The result
Fitting these induction kits doesn't have to be done by a mechanic, it's pretty easy when following the instructions and is therefore an easy and simple way to improve the performance of your car. Not only you get more power (claims are +/- 4%) you also get the added bonus of monster induction noise. For a full listing of well-known companies visit our links section.