about fitting one on my 996....can i just bolt it straight on or are there any other mods i need to make,also,apart from the extra noise...what advantages do they hold?
yes just bolt on. simple. not sure if they have a cooling effect too, probably. the only things to consider are how strong it is in the event of an off and also be careful when cleaning that you dont pour loads of water in there.
It is simply a cover for the clutch to protect the moving parts from damage, so no mods are really required although many replace the springs and bolts within the clutch for stainless type to prevent rusting, I would recommend doing this anyway.
The vented clutch cover releases both the dust build up and that wonderful dry clutch sound (in other words the volume of the rattly clutch will be increased).
I would, just remember that they don't offer the same protection from the elements than sealed versions and so I'd be more inclined to limit their use to dry rides.