CAI is probably better known in the tuningworld as 'Cold Air Intake'.

When you place a open air filter on the intake of the engine, it will probably inhale a lot of hot air from under the bonnet. It can get pretty hot under there.

Hot air can seriously decrease an engines efficiency, what can eventually cost you power.

Hot air contains less oxygen than cold air. You want as much air (and thus oxygen) in the cylinders as possible, so you want it to be as cold as possible aswell.

This increases the combustion and thus the efficiency of the engine.

To keep that hot air out of our intake, some people create a cold air intake to the air filter. There are multiple ways to do this. You can direct cold air from outside the car through some pipes to the filter.

Another way to do it is to move the airfilter out of the engine compartment entirely. You can also try to shield the filter from the hot air.

There are a lot of complete kits for sale for a lot of cars already, complete with piping and filter to create a CAI.

But generally, a trip to your local hardware store and some creativity is enough to save yourself some money on those expensive kits and make something yourself that can work just as well.

The big tric with making your own CAI is to get as much air and as cold as possible into the filter, without to much tight bends along the route. To much tight bends in the piping could interfere with the airflow, reducing the effect of the CAI.

The results people get from their CAI is often a point of discussion and very much dependent on how well it is made. I think you shouldn't expect wonders of it and big gains in the power outpunt of the engine.

But in theory, it should work and it is very easy and not to expensive to make. So why not be sure and try it.