I purchased a fmf q4 for my daughters bike and was very happy so I put one my 07 crf 450x and it fit perfectly.Performance gains were better than I expected with a nice increase in the low range and mid .Sound is quite a bit louder than stock but not offensive .Overall this product is excellent.
Louder than stock. But, not as loud as by friends stock YZ450 and KX450F. Hard to say how much power it gave as I did a re-jet and airbox mod at the same time. Needless to say the bike RIPS NOW !!!!! I would recommend it.