andygt wrote:If changing the mufflers tip would it have any effect on performance? making it faster or slower? Or it just making different sound?
No. No. Slightly different sound. Very slightly.
changing just the muffler = no more warranty?
Unless they can prove changing the muffler was the direct cause of whatever failed, warranty still alive.
Are you simply wanting to change the tips for appearance or were you after noise/performance as well? If simply for appearance, most muffler shops should be able to custom make a twin tip for you. Alternatively you could get something like this

It offers a twin tip appearance but still retain the stock muffler and its single outlet, without the need to change to a new dual outlet muffler. Otherwise if you're after more noise/bit of performance, plenty of JDM mufflers out there, or the cheaper alternative is your local muffler shop who can custom make you something using universal fit mufflers.
I've always been a fan of Remus with their square-ish twin tips and awesome note. I wouldn't mind fitting a universal fit one if only the tips would fit in the rear bumper.
