Märkligt, jag råkade precis hitta exup.txt på min disk, om den kan hjälpa.
EXUP
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As more and more power is designed into production engines, the smooth
powerband of the street machine disappears and is replaced by the lumpy torque
curve and narrow power of the racing engine. A race engine peaking at about
10,000 rpm will have a pronounced dead spot in the 7000-rpm range, leaving a
powerband only 2500 rpm wide--fine on the track but too fussy for the street.
Yamaha has found a way out of this dilemma so that street riders can have it
all: higher peak power from race-engine tuning and a wide, driveable powerband
from idle to peak. Yamaha calls its new technology EXUP (Exhaust Ultimate
Powervalve), and it works by throttling the exhaust.
As exhaust valves open in any four-stroke engine, residual combustion pressure
in the cylinder rushes into the exhaust header pipe, creating a primary
positive pressure wave moving toward the collector (where the headers join
into a single larger pipe). When this positive wave reaches the collector, it
expands, sending an expansion wave back toward the cylinder. This is the
primary negative wave. The header continues to reverberate alternating
positive and negative. To maximize torque at a given rpm, the header pipe
length is set such that the primary negative wave arrives back in the cylinder
during valve overlap. (Overlap is the period near top center, at the end of
the exhaust stroke and beginning of intake, when both intake and exhaust
valves are slightly open together). This negative or "suction" wave performs
two good works: It pulls residual exhaust gas out of the cylinder, and it
starts the flow of fresh charge through the intake valve.
At lower engine speeds, each revolution of the crank takes longer, but the
exhaust waves move as fast as before. This causes the primary negative wave to arrive too soon, before the overlap period has begun.
At some range, about two-thirds of peak torque rpm, the secondary positive wave arrives during overlap. This causes the dreaded race-engine flat spot, as follows: By arriving during overlap, the secondary positive wave stuffs exhaust gases back into the cylinder, diluting the charge.
?It blows back through the carburetor, delaying intake and causing something
even worse--double carburetion. The carb will carburet no matter which way
gases move through it, and it doesn't care whether those gases are fresh air
or exhaust blowback. Therefore, when the exhaust blows back through the carb,
it picks up fuel. When the intake stroke finally begins, this carbureted
exhaust gas is pulled back through the carb again, picking up fuel a second
time. This doubly rich, barely combustible mixture is now drawn into the
engine, resulting in low torque, blubbering, and high exhaust emissions.
At even lower engine speeds, each crank revolution takes so long that the
secondary negative wave arrives during overlap, producing a secondary torque
peak at about one-half of peak-torque rpm. This is the way long overlap and
strong pipe tuning combine to produce torque bumps and dips. Before EXUP, the
standard methods of flattening these bumps and dips (reduced valve overlap,
use of less resonant exhaust pipes, etc.) also killed part of the power.
Was there a way to damp out the unwanted positive waves that create flat
spots, yet preserve thedesirable negative wave action that produces peak
torque? Yamaha engineers knew that wave action in ducts is destroyed by
throttling; intake waves, responsible for valuable power boosts in two- or
four-stroke engines, are known to disappear when the carburetor throttle is
partly closed. The wave dissipates its energy in passing through the throttle
restriction.
They therefore designed an exhaust system with a throttle between the header
pipes and the collector: The throttle's position is controlled by a
microprocessor, acting through a servo motor. In the high-rpm zone where the
helpful negative wave arrives during overlap, the device instructs the motor
to leave the exhaust throttle fully open so wave action is unimpeded. In the
flat-spot rpm zone, where harmful positive waves arrive during overlap, the
motor closes down the exhaust throttle enough to kill the unwanted pressure
wave. When this positive wave cannot reach the cylinder, double carburetion is
eliminated. Torque rises back to a normal value and driveability is restored.
At idle, the exhaust throttle is open only 10 percent, producing back pressure
that discourages loss of fresh charge through the exhaust. The result is a
steadier idle and reduced emissions.
At a given level of driveability, Yamaha engineers claim that an EXUP-equipped
engine can be tuned to produce 10-20 percent more power than its non-EXUP
counterpart.
