Does the K&N Filters really work?
 

Any info in the k&N ?

I replaced the factory air filter by a K&N on my X5 2 months ago which resulted in a slight improvment in gas milage (average +1.5 mpg). I did not notice any significant improvment in performance though

I would look into GreenFilters instead of K&N. The GreenFilters are much better made. If you can find photo's take a look at the edges of the filters and you will see what I am talking about.

The K&N's will potentially allow the engine to breathe better and take in more air, but as a result it will cause your vehicle to consume more gas because the engine management computer will need to enrich the mixture so it doesn't lean out. You will, of course, get more power as a result, but with increased fuel consumption as the byproduct of this (nothing is free). To be able to achieve this increase, however, it may be necessary to improve the backend, i.e. upgrade the exhaust system to allow for increased throughflow. If not, you will have a bottleneck at the back end that will prevent any substantial increase in air flow, and you will end up with little or no increase in performance.

On some vehicles a K&N either does nothing or reduces the HP. Performance mods should be looked at on a by vehicle basis and not just believe the hype. I'm sure that a K&N can increase power by 25HP, but that also means the vehicle manufacturer didn't do their homework. Do you think BMW would put so much effort in to their vehicles and then at the last minute through a filter in? The Big Three are guilty of that; parts bin shopping.

Keep in mind that many manufacturers, including BMW, recommend against using a K&N filter because the oil in that filter that provides great filtration often ends up damaging Mass Airflow Sensor. Something to keep in mind.

No, they do not work. Also, there is a chance of oil getting on your MAF sensor and destroying it.

This is exactly what my Dinan guy told me when I asked him about it... :yikes:

Ive just looked under the hood to have a nosey, where abouts does the airfilter go? I know the filters on these are about 20 '' long (or similar) but cant see where it would actually fit.

Goes in the airbox.