If I modify my bike will it void my manufacturers warranty?

Probably not. In America we have something called the Magnusson-Moss warranty act which says, in essence, your factory warranty can not be voided just because you have added options or modified the machine including intake and exhaust systems. Now here's the catch (and the reason for the probably). Your warranty 'can' be voided or a dealer can refuse to honor the warranty 'if' your modification can be shown to have caused a problem. I.E. You modify the intake but don't take steps to make sure the air/fuel mixture is reset. If you burn a hole in a piston or your rings disintegrate because you're running lean then the dealer can say 'sorry, no warranty'. However, if you modify your intake and a wheel bearing goes bad. Well, obviously there is no connection between the two so the warranty stands.

You can do your own research on Magnusson-Moss (which many dealers don't even seem to know about) at the following link: http://en.wikipedia.org/wiki/Magnuson-Moss_Warranty_Act