I'd like to know which aftermarket air filter will make the most performance gain on my 99 Scrambler 400. I realize there will be a minimal performance gain with an aftermarket air filter vs. a stock filter but I would still like to know which one makes the most difference.

 

K&N ... but don't just get the filter no matter which one you get... Take your jet out, and take it to the local dealership.. they should have an assortment of different size jets... buy a few different ones, and go do some "test and tune".. Good Luck..