possible theory of megaphones
Quote:
Originally Posted by
GHMiller
Thanks for the reply's. But....I'm looking for a bit more on the technical side. Like the how's and why's of the megaphone. What I mean is, I've always just run whatever length and degree of cone that came from whoever made them. And accepted it as the best possible combination. Is there a better combination? I don't know and thus my questions. I've never tried to tune them and want to know how and why they were made or came to the final dimensions of the finished product. And how those final dimensions affect the performance of the motor. Please keep this discussion going and I think it will tie into the original questions that started this thread. Mr. Dixon263, I hope you don't mind, if you do I will move this to a thread of it's own.
Thanks,
One thing I used to think about what really happens inside a megaphone is......
As the sound waves move outward as the cone diameter increases, the waves divide thinner and thinner, more and more. It is like if you drop a pebble in water, you will notice an initial wave made that spreads out around and divides into multiple ripples as it goes outward from the disturbance. This effect basically creates a barrier of waves that act like an expansion chamber's belly, deflector and stinger pipe. Now, revisiting the "pebble dropped in water theory", the next point is that as the pebble enters the water, air follows in with it naturally. But as the pebble is encased by water, the water then collapses around the trail of submerged air and resonates it back into the atmosphere. Basically, imagining all of this, the wave ripple theory explains how the sound waves can create a situation of a density like the water is to air in the fore-mentioned figurative scenario. The pebble represents the sudden shot of powerful exhaust gasses piercing this accumulation of waves and hot gas. The excess fuel that is wasted out behind the exhaust is like the air above the dropped pebble that enters behind it and the air being pushed back is basically like the collapse of the waves /gases resonating the fuel back into the cylinder. This can probably take place as the initial exhaust gases are going to be strong exiting whereas the fuel/air following will be weaker and easy to drive back. Maybe this could be a bit off the wall though.
A second theory of megaphones
Quote:
Originally Posted by
GHMiller
Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.
Somewhat relative to my first theory, I have a second possible idea of the true nature of these tuned pipes....
Now, in this possible scenario, as the sound waves multiply into higher-frequency/thinner waves they move slower exiting, thus the exhaust barrier is created. As the shot of exhaust collides into these waves from behind, a springing action occurs. Basically, the inner waves are pushed against waves toward the open end of the funnel therefor being compressed together. As this happens, heat between the waves causes a return resistance/expansion that drives the excess air/fuel that was scavenged behind the exhaust to be resonated back into the cylinder. Think of it like a spring....
the mechanics of the shape
Quote:
Originally Posted by
GHMiller
Thanks for the heads up. I've read it and understand expansion chambers but it doesn't go into megaphones and how to tune them. Does the same theory apply? It's probably close but not the same as there are I'm sure some different variables to address. This is what I'm trying to find out.
I have been thinking of a clear way to finally address the reason of the cone's general design, as to how it's various characteristics affect the physics involved.....so here it goes.
generally, the whole idea of a "cone" design is what I wrote of earlier. The cone affects the division of the wave into more waves. For example, If you have a straight pipe the exhaust will blast through in a wave like a shunt. However, if the pipe increases in diameter from beginning to end, the wave will break and divide (one into two ect) as it tracks along the wall of the pipe naturally. That is the purpose of the angle of the cone. As you increase the relative angle, the wave multiplies more actively in shorter distance. The only drawback is that the angle can become less affective in excess. Basically, if the angle increases too much the sound waves will not track along the inner wall and will affectively break loose. At this point, it will be no better than a vintage open exhaust. The diameter ( at the start) of the pipe is relative to the area of the exhaust port opening. This also affects length particularly taking "duration of relative time open", into account. The cone angle ultimately affects your power band. If you create a barrier of mega-waves closer to the port, your resonation of scavenged fuel will take place quickly, good for high RPM / lower port duration engines like the modified class. This can be accomplished with a higher angle cone. The needed length of the cone is due to duration of the port to ensure that the full desired function takes place before the waves exit or desipate in the pipe thus loosing the barrier, particularly due to the limit of RPMs the engine can handle or what the carburator can efficiently supply due to CFM for effective power affecting the exhaust strength.