The stock air filter is very large and, unlike some engines, the air filter is NOT a limiting factor on the power the bike can put out.
Many years ago Don Eilenberger ran dynos of a K75 with:
- Stock air filter
- Stock air filter half covered by Saran Wrap
- K&N BM-2605
The results were all the same so, despite the fact that K&N air filters can improve airflow on other vehicles, that's not the case with K bikes.
Also, the intake port on the K75/K100 mass airflow sensor is about 1.7"x1.7" (see attached picture) so an air filter that's the size of a good book is not going to be restrictive. Also consider the size of the opening at the front of the air intake snorkel relative to the size of the air filter.
The BMW recommended service interval for the air filter is 12,000 miles. Unless you live in a VERY dusty environment that really isn't necessary. I change mine every 50,000 miles - whether they need it or not.
