a brief explaination about the EXUP??

it's part of the exhaust system.

In brief- at lower revs the exup valve remains closed (on the fzr 250 this is up to 8000 revs). This means that there increased backpressure within the system, and less flow through the exhaust, allowing all the fuel to be properly burnt (rather than being ejected before being 'exhausted'). Overall, there is an increase in power at lower revs because of this system. It smooths out the power curve effectively.

I have also read an article explaining something about 'waves' (I think fzr dude pointed me to this article). The exup system prevents waves from entering the exhaust at lower revs backwards (or some such process). I read it as : at lower revs there is not enough flow through the exhaust, and as a result waves of air could enter and flow through the exhaust (via pressure because in the engine would have lower pressure / faster moving air than externally).

Regardless the result is the same - the exup valve maintains a suitable backpressure within the whole system allowing a proper amount of fuel / air flow increasing power at low revs. Between 8000-12000 revs on the fzr250 the exup gradually opens until it is fully opened at 12000. This also coincides with the greatest power from the fzr250 (between 12000-14000).

With all that said many people have found the exup to be more trouble than what it is worth on the fzr250. The 250 doesn't make much power to begin with, so the slight increase is sometimes imperceptible. Without the exup the bike will also sound a little 'throatier' at idle and low revs, which is an aesthetic reason not to use th exup.

The exup is still being used on today's yamaha's (i.e. r1) so it obviously was and still is a good technology. Other manufacturers have since developed their version of the exup valve (different enough so they don't infringe patent, but same principles apply).

(please correct me if some things I said are not entirely accurate - it is early, and my workplace ran out of coffee beans!)

 

it's brief compared to other articles that I have read on it! As I said, one article was explaining waves etc. and going into details about the physics.