well i have a rsr gt2 exhaust straight down pipe and i need a silencer and i don't know what will fit in it? can anyone help. all i know is that is 4"(100mm)

 

even if you get a good silencer it will prly whistle. ive been wrong before but man every one ive tried and seen the exhuast blows out the sides and whistles bad... sounds cheap IMO

 

As opposed to finding the right silencer, a muffler shop could put a resonator on for you if you don't like how loud it is.

It will be a bit quieter and have a smoother, deeper tone.

Finding JUST the silenecer will be difficult unless you go straight to the manufacturer of your exhaust.