One of the most significant features on the new FZR1000 is the EXUP exhaust control system. Another Yamaha invention; in principle it is much like the YPVS system which improves 2-stroke engine performance by changing exhaust timing in response to changes in engine rpm.
As more horsepower is designed into production engines, the smooth powerband so desirable for the street is replaced by the "peaky", lumpy power curve of the racing engine. Especially pronounced with high-performance, 4-into-1 exhausts, this results in a flat spot at about two-thirds of peak-torque mm and a rough idle.
Technically speaking, when the exhaust valve opens, residual combustion pressure in the cylinder rushes in to the exhaust pipe, creating a primary "positive" pressure wave moving towards to collector (muffler). Upon reaching the collector, it expands, sending a primary "negative" wave back toward the cylinder. The header continues to reverberate, alternating positive and negative. primary, secondary and tertiary.
Header pipe length is set so that the primary "negative" wave reaches the cylinder at valve overlap (the brief instant when both intake and exhaust valves are slightly open). This negative or "suction" wave does two things. lt pulls residual exhaust gas out of the cylinder, and it starts the flow of fresh fuel/air mixture through the intake valve.
Unfortunately, because these positive and negative pressure waves move through the header pipes at uniform speed regardless of engine rpm, at lower rpm the primary "negative" wave arrives too soon (before overlap), and in its place a primary "positive" wave arrives at valve overlap. This positive wave forces exhaust gasses back into the cylinder, diluting the charge, and it blows back through the carburettor, delaying intake and causing double carburetion (carburetion in the wrong direction). This is what causes the dreaded race-engine flat spot.
Prior to EXUP, the only way to smooth out power delivery was to sacrifice performance (less overlap, use of less resonant exhaust pipes, etc.).
Think of EXUP as an exhaust throttle. By placing a rotary valve driven by a microcomputer-controlled servomotor between the header pipes and the collector, Yamaha engineers were able to control the pressure waves. The Computer senses engine speed from the ignition. By closing this valve progressively as rpm decreases, the harmful positive pressure wave is prevented from reaching the cylinder at valve overlap. Double carburetion is eliminated, torque rises back to a normal level and driveability is restored.
EXUP also reduces exhaust emissions at idle by producing back pressure that reduces loss of fresh charge through the exhaust. The idle is also smoother and steadier. And a new muffler has enlarged capacity to efficiently quiet the increased power.
Equipped with EXUP, the engine produces about 10% more top-end power than an engine without EXUP. Most importantly, driveability and throttle control are greatly improved in that critical upper portion of the power band. There is an astonishing 30 to 40% increase in low- and mid-range torque and smoother acceleration. The idle is much smoother: 30 to 50% less fluctuation at idle rpm. The exhaust note at idle is also quieter. And, hydrocarbon emissions are reduced.