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Thread: Mercs and megaphones--thinking the unthinkable?

  1. #11
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    In expansion chambers, a narrow cone produces a wider power band than a a narrow one. Not sure if this is also true with megaphones, but...

    Jeff

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    Quote Originally Posted by GHMiller View Post
    This thread is pretty interesting. I would like to ask a question about megaphones. What would be the change in performance, if any, if you had a narrow cone vs. a wider cone? For example, an 8 degree cone vs. a 12 degree cone? Or a longer cone vs. a shorter cone? Again for example, an 8" cone vs. a 14" cone? And then any combination of said examples what would be the performance advantage? Thanks for any input, just trying to learn.
    I'm sure there are others here that would be better qualified to answer that one on megaphones. From dyno experience I have found that with expansion chambers , the convergent cone, at the far end from the engine, controls the power band width. A very sharp angled cone sends the return pulse back to the exhaust port for a very short duration for very peaky power band. The highest hp numbers should be seen this way but are just about useless except maybe on a snowmobile where the clutch can be set to keep the engine within a couple of hundred rpm's. A very slow taper gives a much broader peak, but sends a weaker pulse back, so less actual hp, but still better in most situations.
    Last edited by Dixon263; 05-05-2014 at 06:03 PM. Reason: Divergent wrong-should have been convergent

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    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,
    Gardner Miller
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    Team Member Lil' Blue Rude's Avatar
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    GHmiller, if your interested in a good read that might help explain the effect of different tuner lengths you should look up Gordon Jennings two stroke tuner handbook. Should be able to find it through google search in PDF form. It has a chapter about the expansion chambers and how they work and what changes affect what and why. Definitely worth a look though.

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    Team Member GHMiller's Avatar
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    Quote Originally Posted by Lil' Blue Rude View Post
    GHmiller, if your interested in a good read that might help explain the effect of different tuner lengths you should look up Gordon Jennings two stroke tuner handbook. Should be able to find it through google search in PDF form. It has a chapter about the expansion chambers and how they work and what changes affect what and why. Definitely worth a look though.
    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.
    Gardner Miller
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    Remember....Freedom isn't...."Free".......

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    Jennings is a very good first book to read on this subject, but there are several more that offer a slightly different viewpoint & are more up to date.

    Try, Two-stroke Performance Tuning by A Graham Bell, next

    Then, Two-stroke High Performance Engine Design & tuning by Cesare Bossaglia

    Then, The High-Performance Two-Stroke Engine by Dr John C Dixon

    Then, Design and Simulation of Two-stroke Engines by Gordon P. Blair if you can do the maths.

    The Two Stroke Engine it's Design & Tuning by K.G. Draper, Two Stroke Power Units by P.E. Irvin & Motorcycle Tuning two stroke by John Robinson, are also worth a read.

    The only one of the above that deals well with megaphones & mini pipes is John Dixon, if I remember right.

    I'd be interested to know how much the powerband moves if you spray water on the pipes, during the dyno run.

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    Team Member zul8tr's Avatar
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    Quote Originally Posted by Seagull 170 View Post
    Jennings is a very good first book to read on this subject, but there are several more that offer a slightly different viewpoint & are more up to date.

    Try, Two-stroke Performance Tuning by A Graham Bell, next

    Then, Two-stroke High Performance Engine Design & tuning by Cesare Bossaglia

    Then, The High-Performance Two-Stroke Engine by Dr John C Dixon

    Then, Design and Simulation of Two-stroke Engines by Gordon P. Blair if you can do the maths.

    The Two Stroke Engine it's Design & Tuning by K.G. Draper, Two Stroke Power Units by P.E. Irvin & Motorcycle Tuning two stroke by John Robinson, are also worth a read.

    The only one of the above that deals well with megaphones & mini pipes is John Dixon, if I remember right.

    I'd be interested to know how much the powerband moves if you spray water on the pipes, during the dyno run.
    Here is the Graham Bell book in pdf. Chapter 4 on exhaust but nothing on magaphones:

    http://iheartstella.com/resources/ma...nce-Tuning.pdf

    Here is something more specific on megaphone design for Victory cycles (not 2 strokes but informative):

    http://victorylibrary.com/brit/mega-c.htm

    A bit more on megaphones:

    http://www.motorcycle.com/how-to/how...care-3423.html

    Expansion chamber exhaust has more design flexibility than megaphones.
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    Team Member GHMiller's Avatar
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    Mr. Zul8tr,
    Thanks for the links. I realize that expansion chambers are better in most cases, however, we typically use megaphones with the Mercury 44 in the modified classes. I would like to experiment a little, to see what might improve or get worse, but I need to know where my starting point is and why. I'm trying to educate myself a little and I realize that using a dyno would be ideal but I don't have access to one. I will have to do this with trial and error and seat of your pants feel.
    Gardner Miller
    Lone Star Outboard Racing Association

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

  9. #19
    Team Member GHMiller's Avatar
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    The article at the Victory library is the most useful one yet that I've read. Thanks again for the link Zul8tr.
    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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    Default Megaphone

    Quote Originally Posted by GHMiller View Post
    The article at the Victory library is the most useful one yet that I've read. Thanks again for the link Zul8tr.
    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.

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