i have a 400ex it has the k&n filter witht the pre charger i herd its ok to run it with out your airlid and it will give u more hp

 

you heard correct.. but more Hp your looking at maybe 1-2 and it still varies depending on your jetting!!

 

I have the predator and had a k&n filter on mine. it was in the open also. Depending on what you ride on (sand, dirt, water,) you can do more harm than good. I got a lot of sand in my carb and had to clean it out, and with no air bok dont take it though the water. For grass drags its good but not for trail riding i wouldnt recommend it and for the couple hp gain its not worth it.