Welcome to the site !!
Basically , what it will do is slightly increase the noise output (depending on how much length is removed , obviously the shorter the muffler , the louder the bike will be) , there may or may not be a difference in power output , again related to how much is actually removed from the overall length , if you cut it in half , it would most likely be significantly louder and change the power delivery , 1" - 4" not so much , example: , I cut 4" off my 02 CRF450R OEM muffler , for the very reason you want to , i also have a Pro Moto Billet Endcap on it so I have the option of a Spark Arrestor which I always use , and a "Silent Insert" which is a restrictor that helps keep the noise down , it does slightly change the performance , but if I am using it , I am most likely on a trail somewhere , so I want the bike to be quieter anyway and don't really care if there is a slight loss of power because of the restrictive insert , I did notice a slight increase in noise (very very slight) , but it was far from obnoxious ,
I did at the time also repack the muffler , so that way I was starting off with a fresh setup , and in the end , I did not notice any difference in performance , if there was a difference for better or worse , it was not noticeable , but I did not have that huge muffler that the 02 is famous for , sticking back almost to the end of the fender , it is currently (why i chose the length to cut) the same length as the 05 CRF450R muffler , so i stuck within what the manufacturer uses , so keep that in mind when deciding how much to cut off
What i would do is measure one that you know is correct and use that as your measurement , that way you are within manufacturers spec , and it should give you the visual appeal that your looking for ..... , with that said , many of the aftermarket mufflers actually offer various lengths of certain model mufflers , (FMF for example) depending on needs or use , but for the most part , almost all aftermarket mufflers are noticeably louder than their stock counterpart , so I would personally stick with stock specs as much as possible , as certain groups of people are always looking for a way to take away riding areas and tracks , noise is the biggest complaint and usually the reason for losing a riding location , so keep that in mind when making your cutting choice
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