(Shamelessly stolen from R1-Forum - not sure where they stole it from ...
)The Last Great Invention by Mike Armitage
Twenty years ago, Yamaha introduced a landmark technical innovation: the EXUP valve. And nothing has fundamentally moved the game on in quite the same way since.
Engine design is a compromise. Bore and stroke may decide a large part of an engine's character, but the fixed nature of the other components is equally relevant - camshaft profiles work best at specific engine speeds, as do the size and the shape of the inlet tracts, combustion chamber, valves and exhaust. Designing them for high revs and power can mean sacrifices lower in the rev range, and tuning for low rev torque makes things wheezy at the top end. There will also be compromises paid in emissions, economy and response away from the region where intake, cams and exhaust give optimum performance.
The ideal solution would be an engine with variable geometry. Unfortunately the theory is sound but the application is akward, which is why technology like variable valve timing has been tried but mostly rejected. Too much complication and cost, too little reward. Honda's VTEC switches between two and four valves per cylinder based on revs, but isn't really variable as valve lift and duration is unchanged. Yamaha's YCC-I system on the R6 and R1 alters inlet trumpet length but, with two preset positions, isn't exactly flexible. Of the attempts so far, there isn't a truly variable technology that has advanced bike design.
Except that is, for Yamaha's EXUP. A simple throttle valve located in the exhaust pipe, the concept has proved so successful that every major motorcycle manufacturer now fits them.
Tube Talk
Exhausts seemingly play a simple role, ferrying waste gas from the cylinder head to our lungs. But the difference they make to engine character and bike performance is far greater than their basic, inert appearrance suggests.
On a four-stoke engine, the exhaust valve opens and the rising piston pushes hot gas, crammed with residual combustion pressure and noise energy, into the exhaust. This creates a positive pressure wave, travelling down the header pipe to the collector. When it reaches this larger diameter section (or another tailored change of section or shape) the gas expands, slowing down and sending a negative wave back towards the cyclinder at the speed of sound. This reflect back and forth around three or four times (becoming weaker each time).
For a small amount of time when the exhaust valve is open, so is the inlet valve. This is overlap. Its necessary in high revving engines - the valve need to open a sufficient time to let useful amounts of gas past, and so exhaust closing gets later, inlet openings get earlier, and the overlap period increases as the designer targets higher revs. This causes problems. Valve timing wont be ideal at low revs, and gas flow can be compromised - waste gas can get back into the combustion chamber, taking up space and getting in the way of the next combustion cycle. Volumetric efficiency and torque suffer.
An exhausts pipe's pressure waves are useful here. The length and diameter of the header pipes is set so the returning negative wave reaches trhe cyclinder as overlap occures, ensuring everything flows in the correct direction by effectively sucking the waste gas out and starting to flow fresh charge into the combustion chamber. A couple of milliseconds later, just as the exhaust valve is closing, its useful for the pressure waves pinballing around the exhaust to appear in positive form. This pressure wall prevents the fresh intake of mixture short-circuiting directly into the exhaust.
Under Pressure
This is all well and good, except the pressure waves in the exhaust move at uniform speed regardless of revs. Short header pipes might supply the negative/positive double-hit of pressure at the cylinder at the ideal time on a high revving motor, but wont be right at lower speed. An exhaust working well at 10,000 rpm will also work, to a lesser degree at 5,000 rpm, in between - at 7,500 rpm - it'll be wrong. The pressure waves will be out of sync with engine, spoiling efficiency and causing a dip in the torque curve.
For a race bike this doesn't matter, most riders taking a few more horses at high revs in return for a few sacrifices (noise, lumpy midrange, gruff tickover). But its a problem on road bikes, especially when silencing causes further problems - any sudden restriction in the exhaust can reflect high pressure (or 'back pressure'), making the engine work harder to pump waste gas out, sapping power. So manufacturers use cunning techniques to optimize the torque curve. Staggered length headers. Tuned length secondary pipes. Links between the individual headers. Discrete tapers to create reflections over a wide rev range, almost like have variable length.
While these work to a lesser or greater extent, the compromise remains. We need genuinely variable geometry, but an exhaust assembled rather like a slide trombone would hardly be practical. So what we need is EXUP.
Throttled
Yamaha's idea wasnt completely new. Their YPVS (Yamaha Power Valve System) was introduced in the early '80s to tame their peaky RD two-strokes. The height of a two-stroke's exhaust port is the one thing to which there are most sensitive,so Yamaha devised a valve to lower and raise the port roof depending on revs. This allowed an exhaust designed for maximum power with the valve rasised (at high revs), while lowering the valve (altering the timing and time-area) gave better torque and flexibility at lower revs. It made an enormous difference in usability.
Four-strokes aren't as sensitive to a single alteration, but using a computer - basic by today's standards - a team lead by Kiyotaka Yamebe and Hideaki Ueda worked out pressure and flow in an FZR400's exhaust. They discovered a throttle valve located at the end of the collectors could be used to tailor the pressure waves, and the theory was supported by experiments on real bikes. Yamaha realized they could now effectively build a full race system then use the throttle valve, by now tantalizingly called EXUP (EXhaust Ultimate Powervalve), to tidy up any resulting dips or hollows in the shape of the torque curve.
First used on the Japanese market FZR400, Yamaha made big claims when the FZR1000R EXUP arrived in 1989: 10% more peak power than an engine without EXUP; low and midrange torque increased by 30 to 40%; a more stable tickover; and a quieter exhaust. Tests in California showed slightly increased C02 emissions (more fuel being burnt) but significantly reduced hydrocarbons (fuel burnt more efficiently).
Noise reduction were due to to the valve being active a lot of the time. A four-stroke's exhaust tune only really works at one speed (or harmonics of that speed), so EXUP operation wasn't an open and shut case. Literally. At around 3,000 rpm the EXUP opened to around 30%, by 5,000 rpm was open almost fully, but at 7,000 rpm only opened between 40% and 60%. From 8,500 rpm it progressively opened wide. Operation was by a servo-motor controlled by the bike's ECU, with a sensor monitoring pulley position and sending information back.
All round appeal
That was then, this is now. With indisputable benefits and advanced engine management, every major bike builder now employs throttle valves in their exhaust systems, and on all manner of machines - Kawasaki ZX-6R, Yamaha MT-01 and Suzuki M1800R all feature a widget up their chuff.
Designs have evolved. The original EXUP system used one valve running through all four side-by-side pipes at the end of the collector, rotated by a pulley and working like a guillotine. In 2000, Honda introduced the H-TEV (Honda Titanium Exhaust Valve) on the FireBlade, still using one valve but with the four headers arranged in pairs on top of each other, in a square layout. The valve was located in the upper pipes, and when closed it opened a hole through to the lower pair, restricting the total area and making use of the bottom pipes' tuned length, as well as the new length of the upper pipes. The 2002 R1 used a similar arrangement, but with a valve in each pair of pipes.
Today, valve location varies considerably. Kawasaki's ZX-6R and the current FireBlade hide them deep in the silencers, while Triumph's Daytona 675 nestles one in the secondary pipe, between the collector and the end-can, just after the catalytic converter.
'We didn't want the weight and bulk of the valve and its actuator at the back of the bike,' says Triumph's product manager, Simon Warbuton. 'This location is the best place for keeping the bike compact, putting mass where it will have the least impact on handling while still giving us the effect we wanted.'
And the desired effect has changed. With advanced injection and ignition systems, the role of the throttle valve is no longer about filling the midrange or chasing horsepower. 'On the 675 it has nothing to do with emissions or peak power.' continues Simon. 'There's a small effect on torque at lower engine speeds and it can improve driveability in some conditions, but it really helps on noise - a valve in the secondary pipe helps to take the edge off exhaust noise without compromising power.' They're corks bunging up pipes to keep them quiet.
The future
As regulations get tighter, the exhaust valve's popularity can only grow. Its a simple, effective technology for meeting noise restrictions, with the added, if small, benefit of being able to improve an engine's torque curve.
New technologies to achieve targets for both power and emissions can be combined with an exhaust valve for the best results. Influencing intake pressures by playing with the intakes and airbox produces similar effects on the torque curve as exhaust design; it's why Yamaha fit variable length inlets on the R6 and R1.
Tuning the intake to work at the same rpm as the exhaust will give the highest peak output but at the cost of deeper dips in the torque, but a slight mismatch gives a wider spread and helps fill the holes. So using two (or more) different systems on the same bike double the benefits - it's why Honda's exhaust valve has always been linked to a flap in the airbox and Suzuki's system works in conjunction with the ECU-managed secondary inlet throttle butterflies.
But while developments like variable inlets are welcome, Yamaha's original EXUP remains the most effective and most simple innovation. More engine performance, reduced emissions, less noise. Absolute genius.
