Landracing Forum
Tech Information => Technical Discussion => Topic started by: stay`tee on November 13, 2010, 12:45:57 AM
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Have been building my own exhaust systems 2&4 stroke since i started racing some forty+ years ago,,, the current 4into1 thats on my Kawasaki ZX12 seems to be doing the job,,,, i was told a long time ago to trim the megaphone opening back to where the soot witness mark change`s colour within the meggie,,
"Is this the right thing to do in regards to achieving more horsepower??."
The pipes are built to a formula utilizing cam timing figures,, interestingly, the current soot witness mark conincides with the length that the "Engine Aanalizer" programme recomends the meggie to be,,
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Go to www.burnsstainless.com and submit your info on their X Design form, their analysis very good and they are the best for the merge collector/venturi/megaphone design.
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Have used the Burnes analysis page in the past to check, compare various exhaust designes against each other,, unfortunatly almost all come back with diffirent dimenshioning figures, which says to me, what a "black art" exhaust design is,, however the question that i have put forward, is based on old skool here-say, and i am interested to know if anybody out there has maybe tested this theory ,,,
heres something to think about,, had a bike on the dyno tuning in the fuel system, the particular exhaust system on it had a short meggie,, at the end of the day, just to see what would happen we put a meggie that was much longer over the existing one, there was a 5hp increase at the wheel, :roll:
Unfortunatly in this day and age almost everyone(motorcycles) goes out and purchases an aftermarket exhaust system, easy,, these systems have been designed for the market, the reality is that dollars spent for "true" horsepower gained is ridiculous :wink:, imho,, most tuners put the exhaust on then inadvertently tune to it,, wrong...
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The soot mark can show the reversion point but the engine speed and area where a bike tends to run rich varies. More often it shows where flow separates from the inner wall and the megaphone stops being a clean fixed length (the tuning tends to go off). Unless I measure port pressure at the valve I prefer to trust the math. If I see early flow seperation, I think more where to change the megaphone to a lower angle to get the desired length without flow separation (this point also moves with exhaust exit conditions).
Still, it's good to test the short bit before welding on the new section. You're tweeking a system and the extra data point is handy to help understand how it responds.
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Also one must consider the air flowing on the outside of the megaphone. The air passing over and past the megaphone will change how the exhaust is expelled so optimum results on a dyno will be different with 220 mph air passing over it.
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Disclaimer: I have no personal interest in Burns Stainless nor am I in contact with them; it was just a recommendation.
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You might find that if you were to cut the pipe at the "soot" line that a new soot line would soon appear. Cut again, then another.
Not that I know, just saying.......... 8-)
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You might find that if you were to cut the pipe at the "soot" line that a new soot line would soon appear. Cut again, then another.
Not that I know, just saying.......... 8-)
That would be collecting data, and opinions would be changed :-D ,,,
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I read that megaphones broaden the tuned rpm range and lessen the peak effect .
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AFAIK that's true.
The instant explosive release of gas (straight pipe cut off @ 90°, exposing the entire circumference simultaneously) provides the strongest wave.
The megaphone's diffuser with its very shallow taper provides a much lower pressure change, but it takes place continuously along the length of the taper.
Obviously, the time element in a straight pipe is instantaneous, and no energy is preserved.
In a tapered cone, energy is still available to be reflected as the gas progresses to the next section of the cone until the expansion reaches 625% of the primary area, or primary ID × 2.5 (quoting Jennings, Naitoh & Namura, and Blair). The useful length of the meg is limited by this final ID (at which wave strength is gone), and of course by the included angle.
Unfortunately, the obvious "middle ground" (a very high taper angle, to shorten the gas expansion time and maximize wave strength) causes high energy loss due to flow separation.
IIRC the highest included angle that produces reliable results is about 12°. This has the strongest wave energy, but the affected RPM range varies with the time in seconds for the wave to reach the end of the taper, so a high angle has a narrower engine speed range. IMHO the point where the primary enters the cone must be as geometrically perfect as possible to prevent any seam or joint irregularity from "tripping" the gas and causing separation.
The reverse condition (a shallow taper) produces an even weaker wave, but will have greater length to reach its dead-end 625% expansion area. About 5° still provides enough wave strength to be useful.
If the taper is shallow enough, the length will become impractical to hide.
My idea (but limited to a single primary):
The primary length for peak power (measured from valve to diffuser opening) is determined by the usual Bell etc. formulae including gas speed, temp, exhaust valve opening point, and RPM.
The diffuser taper (primary end to 625% area end-point) is determined by the taper angle.
The angle is a compromise how much RPM range you need (upshift RPM loss, etc.), an how much wave strength you can get before the range is too narrow.
The minimum RPM that will receive benefit is the combined primary + diffuser lengths, deconstructed from the bell method.
How the diffuser area is determined from a collector system is yet a puzzle, since each primary loses a great deal merely by entering the collector, suggesting that the maximum cone size is much smaller than the formula.
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AFAIK that's true.
The instant explosive release of gas (straight pipe cut off @ 90°, exposing the entire circumference simultaneously) provides the strongest wave.
The megaphone's diffuser with its very shallow taper provides a much lower pressure change, but it takes place continuously along the length of the taper.
Obviously, the time element in a straight pipe is instantaneous, and no energy is preserved.
In a tapered cone, energy is still available to be reflected as the gas progresses to the next section of the cone until the expansion reaches 625% of the primary area, or primary ID × 2.5 (quoting Jennings, Naitoh & Namura, and Blair). The useful length of the meg is limited by this final ID (at which wave strength is gone), and of course by the included angle.
Unfortunately, the obvious "middle ground" (a very high taper angle, to shorten the gas expansion time and maximize wave strength) causes high energy loss due to flow separation.
IIRC the highest included angle that produces reliable results is about 12°. This has the strongest wave energy, but the affected RPM range varies with the time in seconds for the wave to reach the end of the taper, so a high angle has a narrower engine speed range. IMHO the point where the primary enters the cone must be as geometrically perfect as possible to prevent any seam or joint irregularity from "tripping" the gas and causing separation.
The reverse condition (a shallow taper) produces an even weaker wave, but will have greater length to reach its dead-end 625% expansion area. About 5° still provides enough wave strength to be useful.
If the taper is shallow enough, the length will become impractical to hide.
My idea (but limited to a single primary):
The primary length for peak power (measured from valve to diffuser opening) is determined by the usual Bell etc. formulae including gas speed, temp, exhaust valve opening point, and RPM.
The diffuser taper (primary end to 625% area end-point) is determined by the taper angle.
The angle is a compromise how much RPM range you need (upshift RPM loss, etc.), an how much wave strength you can get before the range is too narrow.
The minimum RPM that will receive benefit is the combined primary + diffuser lengths, deconstructed from the bell method.
How the diffuser area is determined from a collector system is yet a puzzle, since each primary loses a great deal merely by entering the collector, suggesting that the maximum cone size is much smaller than the formula.
Man this is great stuff, Thanks.
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The Burns Stainless program is for a collector system. Is there a nice program for designing a megaphone for a single pipe system without a collector? It does not need to be free. Some cost is OK.
Also, the Triumph has a port in the exhaust tract that is very close to the exhaust valve. A pollution control air injector was there at one time. Is this a good location for a pressure sensor?
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I wrote an Excel that predicts a 1 lung meg as I describe, e-mail me for a copy.
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A comment about RPM range: on paper, the calculated reflection from any pipe will occur at a specific engine speed, which suggests a spike with a drop above & below.
The exhaust opening point (the "bang" release) is used to time the interval to TDC, when it's presumed that the overlap triangle is at or near its maximum lift for both valves, and the greatest window for gas exchange occurs.
I suspect that the reason the pipes work better in practice (have a wider range than predicted) is that you get some pressure delta in the chamber (initiates intake flow before vacuum occurs by piston motion) if the wave returns at almost any points where both valves are open by at least some minimum. My guess: at least .020", perhaps .050", but this may be pretty wide and since it changes the arrival time (both shorter and longer) it expands the power range from a wave return quite a bit.
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Panic writes,, how the diffuser area is determined from a collector system is yet a puzzle etc,etc,,,, well said, and to back up that comment ,, in "every" multicylinder exhaust system that i have constructed the soot mark has been short of the recomended diffuser length,,, in the example given in an earlier post about the short megaphone, i was testing this theory (as quoted, "something to think about"),,,
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stay ' tee please keep pondering because this is stimulating stuff--- and at most of our ages we love stimulation :-D especially if we think it might help us go faster
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This is how I understand exhaust tuning . There are 2 main rules . A pressure wave going out returns as a vacuum wave or viceversa . A returning wave bounces off the ex. valve and maintains its phase . A pipe tuned for the 1st cycle needs to be about 6 ft long . The 2nd cycle goes out neg. and comes back as a pressure wave so it's no good . The 3rd cycle comes back as a vacuum and is a good length so that's what headers usually are . What you need is the good 3rd cycle in your power band , the bad 4th cycle below it and the bad 2nd cycle above it . If you know the temperature everywhere along the pipe , the speed of sound at those temps. and factor in the gas speed , tuned pipe length is simple math .
Trombone exhaust with constant 3rd cycle would be perfect .
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You Sir, have a wicked, wicked sense of humor! :-)
One pipes not too bad to understand but as I read it, we have those nasty little waves coming and going all over the place. Say we use a merge collector. If originating at an arbitrary 88 degrees and timed to return at TDC (268 degrees later). A wave will return to all cylinders at that time. One will aid in cylinder filling and the rest will be reflected back out, only to return as a pressure wave 268 degrees later, not to mention all those bounces in between that land lord only knows where in the cycle of things. As 268 degrees doesn't fall back on the original timing it just keeps on. Every cylinder that fires contributes to this cacophony. This may take more than one napkin and a beer to figure out!
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Practical lessons learned years ago while trying to come up with the perfect megaphone for a DOHC 4 cylinder dragbike engine.
For the example below the header had 1-5/8 primary tubes with a merged collector.
After multiple hours on the dyno with various diameter and length megaphones, a hacksaw and a welder..
Results.....
Short length, large exit diameter (24"X 4.5") yeilded higher torque lower in the RPM range and less HP in the higher RPM range,
Longer length. smaller exit diameter (30"X 3.125) yeilder lower torque throughout the RPM range and higher HP to all the way to redline.
More important.....header primary tube diameter, and design, made a much bigger difference in high RPM HP than any changes made to the megaphone.....
Interesting topic.....
Buzz 1513B
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"...lower torque throughout the RPM range and higher HP all the way to redline."
Dakzila: Please explain how this is possible.
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I don't have the intellect to explain the physics. Just the observed results of many, many hours on the dyno.
Buzz1513B
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The physics is simple: Hp = Torque x rpm
If the torque is reduced across the rpm range, so is the horsepower.
It is unclear what your observations were, but they cannot be as stated in that sentence.
(Maybe too many hours breathing fumes at the dyno.)
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Ok, someone pick apart this concept:
In the interests of aiding a weak exhaust port to the maximum possible extent, how 'bout putting the cone or 'megaphone' right at the exhaust flange, ala a 2-stroke style header?
Wouldn't the cone then positively affect the sound waves that push out from the port immediately, thus helping scavenging?
Two-strokes have a reverse cone further down the tube, past the expansion chamber that supposedly sends back a weaker, differently timed wave to help with keeping the intake charge from squeaking past the exhaust port, but if you don't build in that feature, wouldn't the concept help a normal 4-stroke port too?
Or would the benefits be outweighed by a normal tube-header design?
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The physics is simple: Hp = Torque x rpm
If the torque is reduced across the rpm range, so is the horsepower.
Yes, the physics is simple on paper and in quotations, (most hide behind this) :-D,, real world application brings conjecture and also facts
(some step up) :cheers:
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putting the cone or 'megaphone' right at the exhaust flange
That dates back 50+ years to the "blooey" pipe. Became a dead issue when Walter Kaaden of MZ figured out most of what has been used since - which begins with a primary before the diffuser.
There's been some improvement, but Jenning's math ("2-stroke Tuner's Handbook") is still very effective as a starting point.
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As we are talking about megaphones and exhaust systems, I need to ask. Are the gasses traveling down the megaphone subject to the Combined Gas Laws? If so, the wave slows down with the corresponding drop in temperature. Do the positive and negative waves interact with one another?
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Do the positive and negative waves interact with one another
Always, Blair refers to this as superpositions.
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"Do the positive and negative waves interact with one another?"
:evil:"And that Dr. Watson is the elementery Question!" :evil:
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It seems most shoot for the 3rd order harmonic. Anyone know the formula for the 1st order?
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Bell gives the formulas for primary, secondary and collector (including megaphone) lengths in "Four Stroke Performance Tuning". As far as wave interaction, Dr. Jon C. Morrison of The Institute of Mechanical Engineers, concluded wave interaction within collectors was insignificant. When you think about it, in 2-cycles, the reflected positive wave has to pass through itself to push the over scavenged charge back into the port. I ask about full length tuning as I am building a set for Ray and I's next round and a Jimmy has a lot of distance between the first exhaust port and the last. At a 7,500 RPM length, there isn't much room to bring them down and into a collector pointing back under the car. I think there is much to understand concerning the overall resonance within the system and where the returning waves ( positive and negative) fall in the other portions of the exhaust cycle.
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In both Bell and Morrison's formulas there is a term for the number of degrees that the exhaust valve is open before TDC. At what lift is the valve considered open enough to allow the exhaust gases to propagate the initial positive wave?
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Put the bike on the dyno a couple of weeks ago, didnt have enough time for serious testing, only time for "what if", :wink:,, had trimmed the megaphone back to the soot witness mark, no other changes, straight up five extra hp at the wheel, adjusted timing and fuel, picked up seven in total :-),, put a small reverse cone on and seen no extra in peak, however almost eight percent increase in average :cheers:,,,
:evil: "I guess Mister Watson, all jokes aside, that is the answer to the threads topic". :evil:
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The method Stay-Tee mentions in the previous post seems promising. We tune at elevation 150 feet and this is a lot lower than the salt flats. Is there an adjustment we can calculate for the cone length determined by low altitude dyno work to make it work better at higher altitude?