04-27-2014, 11:05 PM
(04-27-2014, 06:03 AM)Red Mist_imp Wrote: I wish my bike had this. All the Yamahas I've had were fitted with EXUP and they are able to retain excellent, flat torque curves, whilst meeting emissions regulations, by using this feature. My GSX-R 750 has this also and has very good low end and midrange.
There is all kinds of science involved with this kind of valve; it costs more money to include but it is a performance-enhancing feature. It's not there to cut noise or anything else.
My FZ8 doesn't have this and it exhibits the typical "flat spot" on the dyno we get with an inline four. A lot of I4's with a race pipe, which is always tuned for top end, really "come on the pipe" with a kick which makes the bike feel quite a bit faster than it really is. We don't need top end for street riding, low-end and mids are what it's all about.
The flat spot is caused by the Heimholtz resonance, which is basically two pressure waves fighting each other in the midrange. By varying the exhaust back pressure, this is eliminated and greatly helps airflow through the engine. This lets us maintain top-end power whilst preserving driveability. Racebikes don't use this because they rarely get that low in the powerband.
[url=http://en.wikipedia.org/wiki/EXUP]Power Valves explained
A little footnote to this, many of the guys/gals on one of the other forums I visit, Gixxer.com, are busy tearing out the power valve and yanking cables and heaving parts into the trash bin. They might just as well have bought a GSX-R 600 because they've just removed most of what makes the 750 great: its low end and mid-range power.
Do the Gixxers also have the secondary butterflies that are on the DL (VStrom) bikes?
I never ceased to be amazed at the endless threads on Stromtrooper and other sites from guys who removed the secondary throttle system "for more power" and then spent endless hours and money trying to get the power delivery back to something close to stock. All that for perhaps 3-4 HP, and that only at WOT?