I remeber on the old KX500 site a guy had one and said it increased top in quite a bit,but it's a trade off for low end.He wanted to trade it for a stock system because he was putting the engine in a quad,said it was reliable and made lots of power(just remember he was trying to trade it) for what ever thats worth,www.pentonimports.com should have them.If I remember right the guy who wanted to trade one does the big bore kits for KX500's into 640's or something like that,Im sure Paul knows who Im talking about and has his phone# or Email.