Jens
I assume that you the standard black filter box with the trumpet intake on the right hand side (looking forwards) You will get a marginal improvement by using a K&N filter element inside the box but the design is quite restrictive , especially if you have any other engine modifications , and is best replaced by a "cone" type filter which bolts directly onto the air mass sensor.
There are 2 types , the K&N type which uses a fabric filter element , this is cleaned with water , and re-oiled every year or so. I use a JR one , but it did require some minor mods to ensure a good fit.
The other is a foam type element , this is again cleaned with water, I am not sure who supplies these, but all the Racing +8s in the Aero Racing Challenge use them, so ask someone like Bret at BHM who prepares Steve Lockett's car.
