Quote:
Originally Posted by Lotus7
The problems with K & N's are that when freshly cleaned and lightly oiled (when they flow well) they admit small dust particles that will increase cylinder bore and ring wear. They really don't become effective filters until they load up with some dust and dirt, and then they don't flow as well as a good paper filter.
I actually like K & N's for the RIGHT application. I use them on (2) normally aspirated race cars, and clean them every couple of months. But, I have no problems with rebuilding those (2) engines every 5 to 7K miles. Also the "warrantee" is my wallet. I don't have to argue with a manufacturer. If the rings wear faster, I'm the one paying for it. I consider the K & N's as "gravel strainers" that keep rocks out of the throttle bodies. 
If you plan on trading in or returning the car on lease with only 40 or 50K miles on it, and you don't clean the filter too often, a K & N is a possible choice. Otherwise, for long term engine health, it's hard to beat a paper filter that's replaced every 20K miles.
The extra flow when they're very clean (the K & N's) may, or may not, add 1 or 2% more power at max. revs (again on a normally aspirated engine only). On turbocharged engines with normal boost regulation, they add absolutely no power, since the turbo easily compensates for even moderate input flow restriction changes. |
= stick to BMW original filters and change them every 20K miles depending on your driving conditions
