First off welcome to my (very) rough guide to performance air filters,basically there are three styles of air filters
1. Bolt on filter,these are used on older cars that use carburetters (sometimes known as pancake filters due to their general shape) the thing to remember when adding these is to get the fuel/air mixture checked immediately afterwards (ideally on a rolling road to get the best results),this is because unlike modern engines they can't automatically adjust themselves to cope with the extra volume of air entering the engine.In most cases the car will run 'lean',which means that there is too much air and not enough fuel entering the engine.Usually the jets inside the carburettor are changed for larger ones to compensate for the extra air going into the engine.
2. Cone filter/induction kit,these are generally used on modern fuel injection engines,in most cases where the car runs an air flow meter and lambda sensor the engine will automatically adjust the fuelling to cope with the extra volume of air entering the engine,on some older injection engines especially the turbo cars e.g RS Turbo/Cosworth it is critical the fuelling is checked and adjusted,because on these the wrong mixture can kill an engine in no time at all,especially on high boosting tuned engines.Fitting an open style filter will produce a roar under acceleration,which personally i like,but some people don't,which leads me onto....
3. Panel Filters,these are for the person who wants that bit of extra power without the associated induction noise of an induction kit,also when you look under the bonnet the engine still looks standard.These are extremely easy to fit as they simply replace your standard air filter element plus especially for turbo cars with high under bonnet temperatures and cars that spend a lot of time sat in traffic in town,less hot air is 'soaked' into the filter itself,which can cause a loss of performance. A fair bit more expensive than a standard filter but can be cleaned out and reused so eventually will pay for itself if you keep your car a long time.
The most important thing with all air filters is a good supply of cold air.Usually the best thing to do is route some ducting from behind the front grill or bumper grill upto the air filter. Another thing you can do is take out the factory fog lights in the bumper and run the ducting from there,this is also a good way of running ducting to the front brakes to cool them if you do some hard driving/track days etc.
What i have done with my present car is to get rid of the restrictive factory cold air feed and run some larger diameter ducting from the front of the car upto the standard airbox and filter,this has made a huge difference to the way the car goes,especially when upto speed.Only cost me a couple of quid (literally) to do.All you should need are some tie wraps and some air ducting available at most car spares shops.
Thank you for taking time to read my very first guide,as far as i'm aware what i've wrote makes sense,just some random stuff that i've picked up from working on my cars over the years.