I ran one of those reusable K&N air filters a few years back. I only noticed a difference in the last 300rpm, it certainly breathed better, but at the sort of rpms I rarely approach. That was on a car. On motorcycles/atv's, you'll notice a little loss in bottom end (well, if you removed the airbox...much better to leave these things stock unless you race).

If you do get one, be careful not to oversaturate it. Just about everybody does.

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'64 230SL