Megaphone length

[stay`tee]

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,,

[Blue]

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.

[stay`tee]

 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,
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 ,  imho,, most tuners put the exhaust on then inadvertently tune to it,, wrong...

[Anvil*]

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.

[Jonny Hotnuts]

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.

[Blue]

Disclaimer: I have no personal interest in Burns Stainless nor am I in contact with them; it was just a recommendation.

[Dynoroom]

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.......... 

[stay`tee]

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.......... 

 That would be collecting data, and opinions would be changed ,,,

[John Burk]

I read that megaphones broaden the tuned  rpm range and lessen the peak effect .

[panic]

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.

[gearheadeh]

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.

[wobblywalrus]

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?

[panic]

I wrote an Excel that predicts a 1 lung meg as I describe, e-mail me for a copy.

[panic]

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.

[stay`tee]

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"),,,