Exhaust pipe diameters
- Thread starter acotrel
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I suggest that the rate of flow down an exhaust pipe is not relevant to motor performance. What would be the rate of flow down an expansion chamber ? I suggest it is sonic and the high pressure points in the wave are far higher than 90 PSI. With two strokes you don't hear a note, you only hear the crack, especially if the port is horizontal at the top.
acotrel said:
So you are saying the pipes don't make any difference !!??
Its not noise that is being tuned here, its big pulses of pressure and reversion waves.
And timing them to be useful, rather than detrimental.
The noise just goes along for the ride.
You seem to be just making the science up.... ?
lcrken
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And I would suggest that it is obvious that rate of exhaust flow is relevant to engine performance. If you don't have a high enough flow rate to get rid of the exhaust gases, you won't be able to fill the combustion chamber with intake charge. Once you have enough flow rate, you can play with tuning the pipe length to get a little more top end, or to get a wider power band and a little less top end, or whatever sort of torque curve you need. As Prof. Blair used to say, you tune the intake to get the major intake ramming peak near the required engine speed for maximum power, and then you tune the exhaust length to eliminate the intake ramming trough. That's pretty much the basic formula, unless you're building a landspeed record bike, in which case you tune both to a maximum horsepower peak, but get a very narrow power band.
For anyone who is really interested in all this (that includes you, Alan), I'd suggest reading section 6.4, "Empiricism for the Optimization of Exhaust System Tuning", in Dr. Blair's book, "Design and Simulation of Four-Stroke Engines. ISBN 0-7680-0440-3. Another good read on the subject, even if it is 50+ years old, is "The Scientific Design of Exhaust and Intake Systems", by Smith and Morrison. ISBN 0-8376-0309-9. Also lots of really good SAE papers out there, available through most public libraries, on the subject. People like Dr. Blair have devoted large parts of their lives to removing the mystery from engine design, and it's just silly to ignore it and try to reinvent everything by speculation.
Wow, I feel better now. I don't know why I let myself get so involved in these posts. It's clearly a personal flaw of some sort. My wife says not to worry, it's just normal age-related loss of brain function.
Ken
For anyone who is really interested in all this (that includes you, Alan), I'd suggest reading section 6.4, "Empiricism for the Optimization of Exhaust System Tuning", in Dr. Blair's book, "Design and Simulation of Four-Stroke Engines. ISBN 0-7680-0440-3. Another good read on the subject, even if it is 50+ years old, is "The Scientific Design of Exhaust and Intake Systems", by Smith and Morrison. ISBN 0-8376-0309-9. Also lots of really good SAE papers out there, available through most public libraries, on the subject. People like Dr. Blair have devoted large parts of their lives to removing the mystery from engine design, and it's just silly to ignore it and try to reinvent everything by speculation.
Wow, I feel better now. I don't know why I let myself get so involved in these posts. It's clearly a personal flaw of some sort. My wife says not to worry, it's just normal age-related loss of brain function.
Ken
acotrel said:
I would like to note that I was not in any way talking about fitting an expansion chamber for a four stroke engine, as for “fitted with a megaphone is more difficult to ride well” – surprise, surprise, ever since the tightening up of the decibel noise at the tracks here in the UK the Manx Norton’s etc fitted with the ACU designed exhaust have run faster :!:
As for the electrically adjustable carb inlets, I can remember some time ago that in the WSB Ducati fitted adjustable-muti diameter inlet at the inlet end, I do not know if it was fuel injection or carbs, but it was some sort of muti diameter inlet tract that enlarged the inlet or closed it, that is the hole clearly got larger as the revs rose, I saw it on a wsb tv program once and thought it was amazing :!:
does anybody know what it was called :?:
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Piston-port 2-strokes develop BMEP (btake mean effective pressure), which is a dynamic measure of compression, in relationship to the height (timing) of the exhaust port. As rpm rises, so does BMEP. Varying the height of the exhaust port by using a powervalve is just like being able to vary cam timing, yielding a broader spread of power rather than tuning for best power over a certain narrow range.
Think about a 4-stroke engine with large overlap- at lower rpm, the charge is nearly going straight out the pipe from the intake. It's only at higher rpm, when flow inertia slows the charge relative to valve opening speed, that the charge becomes trapped in the combustion chamber and produces power. Higher rpm allows the cam to catch up to the rate of flow and make power. Variable cam timing engines change the amount of overlap according to rpm. Kawasaki and Ducati both have versions of variable cam timing.
Think about a 4-stroke engine with large overlap- at lower rpm, the charge is nearly going straight out the pipe from the intake. It's only at higher rpm, when flow inertia slows the charge relative to valve opening speed, that the charge becomes trapped in the combustion chamber and produces power. Higher rpm allows the cam to catch up to the rate of flow and make power. Variable cam timing engines change the amount of overlap according to rpm. Kawasaki and Ducati both have versions of variable cam timing.
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When I first started racing in 1967, I had a Triton based on a 650cc Triumph motor which was short stroked to 63mm to give 500cc. It came fitted with 4 inch megaphones. On a big circuit it was scary, however on a short tight circuit it was impossible. The motor had a power band from 5,500 to 10,500 RPM. At 5,500 RPM if you had dropped below the cam spot in a slow corner, when you slipped the clutch to get power, the bike would go instantly sideways. Before I started racing, I was a half decent rider - the Triton turned me into an instant DUD. I simply could not get decent lap times. I then fitted a 2 into 1 pipe to it - lost 1000 RPM off the top and was able to ride it extremely well. With megaphones, it was just bloody dangerous particularly when combined with a drum front brake, friction steering damper and T1 Triangular Dunlop tyres. I learned how to race by crashing a lot.
I was at the Broadford Bonanza a couple of years ago. A guy started up a hot commando fitted with megaphones. As he revved it you could hear the great big cam spot. If it comes on while you are cranked over ..,.. ? Mid-range torque is much more important than top end power, you simply gear to suit it.
When you fit race cams to an old British road bike, the first thing you notice is the cam spot. Overall there is usually an increase in power even below the cam spot, - with megaphones everything about the power band is intensified.
I was at the Broadford Bonanza a couple of years ago. A guy started up a hot commando fitted with megaphones. As he revved it you could hear the great big cam spot. If it comes on while you are cranked over ..,.. ? Mid-range torque is much more important than top end power, you simply gear to suit it.
When you fit race cams to an old British road bike, the first thing you notice is the cam spot. Overall there is usually an increase in power even below the cam spot, - with megaphones everything about the power band is intensified.
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One thing I would say about the 2 into 1 pipe on my Seeley, is that it is difficult not to over-rev the motor as you come up through the gears. In the 70s there was a mod which was done to TZ250 Yamahas in Europe. It consisted of re-shaping the rear cones of the expansion chambers. Effectively what happened was the power delivery would cut off earlier - about 11,000 RPM from memory. There was a friend of mine who returned from coming 2nd in MotoGP 250s and raced on a borrowed TZ250 at Sandown in the mid 70s. He did that mod, fitted the good tyres and carbs and was the length of the front straight ahead of the A grade field during his races.
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acotrel said:
acotrel said:
So according to acotrel, it is in part about mass transfer yet a component of rate of flow is....mass transfer (mass per unit time = FLOW "rate")
From:
http://www.accessnorton.com/sir-eddy-rocket-t14978-195.html
comnoz said:
In one respect, for a given rpm, various pipe diameters should have identical average flow rates but different instantaneous flow rate profiles and certainly different velocity profiles over crank angle time. So I would expect a different back pressure versus crank angle-time for the different pipe diameters. From Jim's comments the pipe diameter has an impact on performance. So I can understand the perspective that flow rate is not what it is all about but it factors in. Most every text I read on exhaust length tuning referred to the speed of sound which is more or less a rough constant to work to. Yes, we know that the speed of sound changes with medium, density, temperature etc.... but it is a somewhat bounding condition that most hobbyists can work to and fine tune from.
Paraphrasing what Ken so eloquently stated above:................go read a book!
or better, follow comnoz lead and go build an engine and test it and experiment and test it and experiment test it and on and on and then I am sure the inquisitive mind will see.
I knew Prof. Gordon Blair and he assisted us on a build. His text really does not offer anything new with regards to the physics and phenomena of gas flow and tuning. Where his text hit home (in my opinion) is the integrative approach he presented by first disintegrating the various 4 stroke 4 cycle phenomena in discrete events, collecting data for each discrete event, modelling and integrating it into the whole phenomena. As an example: he presented the concept of mapping the flow coefficients (lift versus flow coefficient) of each of the valves (intake and exhaust) both with a forward flow and reverse flow. The valves really do flow both ways so it is important. I am reasonably sure others have considered this in the past but Prof . Gordon' Blair's text was the most integrated presentation of the material.
For those struggling with the term "standing Wave" do a Youtube search with the term: "standing waves physics"
or if you are not up on a Youtube lecture on "standing wave physics", the following is illustrative:
[video]https://www.youtube.com/watch?v=QN67-FahC40[/video]
Off the soap box for a bit now.
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I suggest that every time a firing occurs on one cylinder, there is a sharp pressure pulse into the exhaust pipe which first starts it resonating and keeps it resonating. The wave works in two directions because there is a reflection from the open end of the pipe. The situation is not normal flow because of the high pressure pulses - the wave amplitude affects the rate at which gases are transferred. That is why a two stroke expansion chamber can have such a small outlet (stinger), and the noise is a very loud 'crack'. At each firing the high pressure pulse probably moves down the pipe one wavelength and the end one moves out. Then the one remaining in the pipe moves back into the cylinder as the pipe resonates. The velocities involved are sonic, so any irregularities inside the pipes cause unexpected problems which a low pressure flow bench will not detect.
What you would probably need to test the pipe would be a transducer at one end and a very sensitive sound meter at the other, then compare input and output sound energies.
It would be interesting to calculate the gas flow through a two stroke engine then measure how much gas the stinger on the chamber will fow at 90 psi.
What you would probably need to test the pipe would be a transducer at one end and a very sensitive sound meter at the other, then compare input and output sound energies.
It would be interesting to calculate the gas flow through a two stroke engine then measure how much gas the stinger on the chamber will fow at 90 psi.
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What happens in a two stroke exhaust is a similar deal. The Kadency effect also applies to four strokes albeit mostly in a minimal way. If you think the function of the exhaust on a four-stroke is only to carry the gases away as quickly as possible, you are kidding yourself. This is not about flow but resonance. How do think a megaphone works ? - an expansion chamber is only a megaphone with an end on it. You will notice that on most four-stroke race bikes the exhaust valve closes well after TDC while the inlet valve is open. On piston ported two-strokes the timings are symmetrical. The reason rotary disc valves are used is that asymmetric timings (as on a four-stroke) work better with the chambers to deliver more power. It is the whole system - inlets and exhausts - that need to be optimised.
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Thanks Ken, Exup is very interesting and I think I could fit it to the Seeley 850:
'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. '
'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. '
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acotrel said:
Good analogy. Furthermore, when you consider most megaphones are used with a reverse cone your analogy above seems even more valid.
From the article cited earlier:
"Discrete tapers to create reflections over a wide rev range, almost like have variable length."
This may be a decent explanation of the function of a megaphone on the exhaust.
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You guys might have a better idea than I have about how Exup works. My feeling is that the butterfly in the collector might have a stifling effect which makes the bike more ride-able. If it actually changes the wavelength of the standing waves in the header pipes, that says to me that the when the butterfly is open, then the whole system resonates to suit high revs and the header pipe lengths suit low revs when the tail pipe is partially isolated. If Exup actually fattens the midrange, it might be good on a high-geared commando.
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