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Thread: Mercs and megaphones--thinking the unthinkable?

  1. #21
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    Having built plenty of successful 4 cly. Mercs over the years. I have found out that expansion chambers are not my cup of tea on deflector Merc`s. Megaphones are still somewhat of mystery, just a ton of variables that can effect performance,porting and other internal mods, how you cool the engine and pipes. I have found that the hotter you can run the engine and pipes the better the performance. I have actually melted an aluminum megaphone off with no damage to the engine. Pipe length and the angle the exhaust exits also has a lot to do with performance. Witness the McAfee sidewinder pipies on their E. Experimenting with cooling can gain you a lot, but can be expensive. The ignition that we run now has an effect.

  2. #22
    Team Member GHMiller's Avatar
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    Quote Originally Posted by rumleyfips View Post
    Harry Brinkman's manual ( google Boatsport.org) has an explanation and 2 diagrammes ; page s 56 - 62.

    Frank Volker has some thoughts on another BRF thread.
    Thank you, I found the thread, interesting read.
    Gardner Miller
    Lone Star Outboard Racing Association

    "Water is for racing. Asphalt is for the parking lot."
    Remember....Freedom isn't...."Free".......

  3. #23
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    Cooling the exhaust causes it to contract, reducing the pressure. Hummm...

    Slow Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Default possible theory of megaphones

    Quote Originally Posted by GHMiller View Post
    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.

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    Default A second theory of megaphones

    Quote Originally Posted by GHMiller View Post
    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....

  6. #26
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    Default the mechanics of the shape

    Quote Originally Posted by GHMiller View Post
    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.

  7. #27
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    Cooling the Megaphone & the exhaust gas, increases the gas density & changes the speed of sound causing the exhaust to appear longer than it is, lowering the powerband revs, just the same as an expansion pipe.

    There are 3 sound "reflection" points, a minor one at the start of the taper & the major one at the end of the pipe & on the return wave at the exhaust port.

  8. #28
    Team Member Master Oil Racing Team's Avatar
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    Here's a couple of experiences not directly related to the subject matter, but maybe in a way that can scramble your brains enough you can "see" a little bit differently.

    A lot of people know this, but probably most don't. You can calibrate a sound making device (don't remember the name) to a certain ID diameter pipe and weight per foot, and when it is open at boat ends, when you shoot the gun, it can tell you very accurately how long that piece of pipe is. Oilfield pipe dealers use such devices to get shipments out to pipe yards quickly when hand measuring takes up so much time. This machine works on linear pipe only, but when the sound spreads out at the opposite in the machine knows how far the sound had to travel to get there and records it at, I guess you might say "the speed of sound".

    Another phenomena that I have witnessed is boiling water at a little over 100 degrees Fahrenheit in a table top distiller. You can see the water boiling at say 103 degrees in 25 inches of vacuum, but you cannot see some of what is going on before boiling begins. This kind of goes back to the old saying "a watched pot never boils". This is what I learned from the chemist that set this glass still up. As the water heats up, it thins some and the hotter water starts rising. As the temperature climbs so does this hot water climb and the water inside starts moving around because of the water being displaced from bottom and pushing up on the upper layers as it rises, then the layers on top start cooling and have to start back down as they are being displaced. So they water is moving around, but the heat is continuing to rise, and still no boiling. Everyone has probably seen how when you try to bring on a full boil under the hottest setting very quickly, once boiling starts you have to turn it down, sometimes four or five points to the desired slow, full boil roll. This happens because as the hot water starts rising, there becomes a tough "skin" on the surface that prevents the water on the bottom where the heat is from evaporating. The weight of the water and the skin on top is not allowing the water to evaporate into steam and air, thus preventing the bubbles from forming. The hotter the water gets, the tougher the skin is until at some point the skin is penetrated by a bubble. At that point lots of bubbles can now form and rise to the top, then not only is their less pressure from the skin, but a whole bunch of smaller bubbles have lightened the water, then the pot erupts into a full boil.

    Got it so far? The vacuum allows boiling at a lower temperature due to atmospheric pressure has been reduced so much, but the skin still forms. Watching this experiment one day, we were bringing up the heat and as the skin was tightening (you can see little surface ripples and a fog forming in the glass bowl) one of the guys accidentally dropped a half inch combination wrench on the concrete floor and the water immediately started boiling. We all looked at each other and were amazed. It had to be the "ding" of the wrench that broke the skin. We cooled the still down a little. You can cool it very quickly by reducing heat and leaving the vacuum on. Then we brought the temp back up to where the skin would be formed and we figured it was getting close to boiling and dropped the wrench from about the same height and once again at the "ding" the water started boiling instantly. So the experiment was repeatable. We never got our project finished because the 1993 economy collapsed and killed our project, but we were going to experiment with harmonics when we got everything working reasonably commercial.

    While these two examples don't directly play into the discussion they do include sound waves, an open pipe, heating and cooling and harmonics. Thought it might help stimulate some thoughts and theories. I just drove boats and took pictures and never studied any of this, but was always curious. Great thread.



  9. #29
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    All of these theory`s make for good reading and I find interesting. But finding a way to put them to practical use when building a 4 cyl. Merc is the trick. Their are so many variables. Even duplicating a really good engine, which you would think would be easy to do is very difficult. So i don`t think there is a hard & fast formula that you can apply to megaphone exhaust. Sometimes in the real world of building engines things just work out. And then you woinder how come that engine is so good. Is it the exhaust? Is the block and internals straight & true? Induction, carbs & reeds better? Did you do some little mod that that gave it that little edge.

  10. #30
    Team Member GHMiller's Avatar
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    Art, I agree there seems to be a lot of variables when building a 44. I think this is what might be playing into what Dixon was getting at in the beginning of this thread. I believe that what works really great with one engine might be "off" a little with another even with duplicated porting. Why is this? I think it's anyone's guess really. But, that is why I asked my questions. I really think that you have to approach each engine as having its own personality and then tune it to what it likes. Not only with a dyno (if you have one) but it really needs to be tuned on the boat in real world conditions with lots of testing. Sure it is nice to say that your motor was tuned on the dyno but I believe the real proof is on the back of the boat. Some of our best snowmobile engines were never run on the dyno and were real hard to beat.
    Gardner Miller
    Lone Star Outboard Racing Association

    "Water is for racing. Asphalt is for the parking lot."
    Remember....Freedom isn't...."Free".......

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