I'm sorry if this has been asked before but is the Exup valve realy that important, I mean is it really as good as people say it is. If I replace my exhaust with an aftermarket pipe and get rid of the Exup valve will I see a performance increase/decrease? From my understanding the Exup helps with low end power however some people even go as to say that this isn't true and that it's only a gimmick to pass some emission standards. Opinions, suggestions?
deffinitly not a gimmick, just have a look and you will see that it indeed works and indeed asists bottom end power. The exup valve creates back pressure in the bottom end to assist power and then opens fully by the time it hits the top end to allow for easy escape of exhaust gases. Removing the exup and getting a totally new exhaust system will give you minimal performance for the money, it will allow for more top end power, but will kill your bottom end power, so you'll have to take off everytime by revving the guts out of it. Just get a muffler if your desperate to change the exhaust sound. my 2c.
As the EXUP valve is part of the 4/1 join in the pipe there is no easy way to replace it other than an early model (86-87)header. There most likely be no after market pipes that fit. I will vouch for EXUP helping ridability. Best left working.
IF you wanted to remove the EXUP valve then you would at the very least need to adjust the jetting of the carbs. If you wanted to do it properly (seriously properly!) you'd need to get new cams ground up for you, aswell as the jetting. The reason the EXUP valve is needed is because of the valve timing overlap between the cams. The backpressure created by the EXUP valve at low revs ensures that it restricts the flow of gases at low rpm meaning the bike can still run with a moderate amount of power. The valve overlap means the top end power is also better than without the overlap & EXUP valve. IF you remove the EXUP, get new cams made up, and re-jet the bike... you'd end up with more bottom end power (marginally), more mid-range power... and about the same amount of power at the top end. The reason for this is that the new cams would have to work effectively through the entire 19500rpm rev range, which is practically impossible without a vario-cam or 3d cam profile; therefore sacrifices have to be made. In short... leave the EXUP valve there. It's a nightmare to get it running perfectly without it, whilst producing anywhere near the same amount of power.
