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Is a cone(megaphone) the best exhaust for a big single

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Hi there

I have a fundero engine in rs125 frame and am trying to decide on a silencer am led to believe a big single requires a cone in the exhaust system so i thought iwould stick a megaphone on it but most off the shelf meggys have a straight perforated pipe inside them which defeats the purpose of the cone,i can make one myself with a perforated cone inside it but also wondered if it might be a bit loud even if i make the reverse cone exit 38mm. it has 35mm downpipes into the collector box and a45mm pipe  coming out, engine std apart from aftermarket air filter, any advice welcome.

                                                                                                                       thanks in advance  dougie.

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Depends on the tune of the engine and the riding situation.  I had a BSA 500MX and it had a large head pipe and a small reverse cone megaphone; looked like a small expansion chamber without a stinger. The bike had awesome bottom and mid range with a sudden drop off on the top, almost like a rev limiter, but that provided a good shift indicator.   Also very very loud.  I replaced it with a Supertrap which smoothed out the power, provided more on the top and added at least 500-1000 rpm on the top. 

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well its a road bike and am looking for linear power right through the rev range if possible rather than low down grunt

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With stock exhaust, on grass, in 3rd gear,  I could out drag Maico 400s to the first turn, with the front wheel in the air. I also had a Husky CR400 at the time and it was faster in a drag race on pavement but not on dirt. According to dyno reports all three engines were within about 1 hp of each other but the BSA power delivery to the ground was different than the 2Ts.

Probably not the best for street as I preferred the Supertrap for trail and Enduro riding.

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You want the muffler after the cone.

Design the header length for about 1000rpm shy of the rev limiter, figure out your gear change rpm figure out the tuned length for 500+ rpm lower than your gear change rpm, that's the length of your megaphone, now you want the megaphones large end to be 6.25x the area of the exhaust port, so the angle is determined the by the breadth of power band you are looking for.  Longer cones broader power band, steeper cones concentrate the power.   I made oversized reverse cones, then a hole pile of endcaps in .1" increments like a 2.200" endcap, 2.300", 2.4" that I could rivet onto the oversized reverse cone.

Once you figure that out you can add a muffler to the end of the megaphone.

People who do this

Jemco, thunder alley, d&d crf450 low pipe, myself.

This one is well raced so forgive it's appearance

WIN_20151202_205052_zpsojy1de1j.jpg

Muffler pics on a uas kart

IMAG2689_zpsdgqhwjaf.jpg

IMAG2694_zpsxxr0r8as.jpg

P3281111_zpshwprnc42.jpgP3281097_zpsrb0su9lx.jpg

 

Something really trick(y)

Hydroforming

IMAG1364_zpsprlfxrku.jpg

That pipe has 27 angles.... starting shallow and ending steep....

Resulted in a constant area gain instead of a falling area gain. (Not sure the Jawa gave a single F... about that)

Real pita to build....

 

 

Side note, 

It's funny that pro circuit built a special pipe for the Honda Baja team (johnny Campbell and the rest) that was a megaphone, hydro formed like I did, but didn't offer it to the public.

140.jpg

They make more money pedaling straight tube junk.....

  • Like 2

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You want the muffler after the cone.

Design the header length for about 1000rpm shy of the rev limiter, figure out your gear change rpm figure out the tuned length for 500+ rpm lower than your gear change rpm, that's the length of your megaphone, now you want the megaphones large end to be 6.25x the area of the exhaust port, so the angle is determined the by the breadth of power band you are looking for.  Longer cones broader power band, steeper cones concentrate the power.   I made oversized reverse cones, then a hole pile of endcaps in .1" increments like a 2.200" endcap, 2.300", 2.4" that I could rivet onto the oversized reverse cone.

Once you figure that out you can add a muffler to the end of the megaphone.

People who do this

Jemco, thunder alley, d&d crf450 low pipe, myself.

This one is well raced so forgive it's appearance

WIN_20151202_205052_zpsojy1de1j.jpg

Muffler pics on a uas kart

IMAG2689_zpsdgqhwjaf.jpg

IMAG2694_zpsxxr0r8as.jpg

P3281111_zpshwprnc42.jpgP3281097_zpsrb0su9lx.jpg

 

Something really trick(y)

Hydroforming

IMAG1364_zpsprlfxrku.jpg

That pipe has 27 angles.... starting shallow and ending steep....

Resulted in a constant area gain instead of a falling area gain. (Not sure the Jawa gave a single F... about that)

Real pita to build....

 

 

Side note, 

It's funny that pro circuit built a special pipe for the Honda Baja team (johnny Campbell and the rest) that was a megaphone, hydro formed like I did, but didn't offer it to the public.1X20-20Exhaust1.jpg

 

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29 minutes ago, EnglertRacing said:

You want the muffler after the cone.

Design the header length for about 1000rpm shy of the rev limiter, figure out your gear change rpm figure out the tuned length for 500+ rpm lower than your gear change rpm, that's the length of your megaphone, now you want the megaphones large end to be 6.25x the area of the exhaust port, so the angle is determined the by the breadth of power band you are looking for.  Longer cones broader power band, steeper cones concentrate the power.   I made oversized reverse cones, then a hole pile of endcaps in .1" increments like a 2.200" endcap, 2.300", 2.4" that I could rivet onto the oversized reverse cone.

Once you figure that out you can add a muffler to the end of the megaphone.

People who do this

Jemco, thunder alley , d&d crf450 low pipe, myself.

This one is well raced so forgive it's appearance

WIN_20151202_205052_zpsojy1de1j.jpg

Muffler pics on a uas kart

IMAG2689_zpsdgqhwjaf.jpg

IMAG2694_zpsxxr0r8as.jpg

P3281111_zpshwprnc42.jpgP3281097_zpsrb0su9lx.jpg

 

Something really trick(y)

Hydroforming

IMAG1364_zpsprlfxrku.jpg

That pipe has 27 angles.... starting shallow and ending steep....

Resulted in a constant area gain instead of a falling area gain. (Not sure the Jawa gave a single F... about that)

Real pita to build....

 

 

Side note, 

It's funny that pro circuit built a special pipe for the Honda Baja team (johnny Campbell and the rest) that was a megaphone, hydro formed like I did, but didn't offer it to the public.

140.jpg

They make more money pedaling straight tube junk.....

Thanks!!  It sounds Ironic to say this was a breath of fresh air to someone like me.  

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On 4/6/2017 at 6:21 PM, EnglertRacing said:

You want the muffler after the cone.

Design the header length for about 1000rpm shy of the rev limiter, figure out your gear change rpm figure out the tuned length for 500+ rpm lower than your gear change rpm, that's the length of your megaphone, now you want the megaphones large end to be 6.25x the area of the exhaust port, so the angle is determined the by the breadth of power band you are looking for.  Longer cones broader power band, steeper cones concentrate the power.   I made oversized reverse cones, then a hole pile of endcaps in .1" increments like a 2.200" endcap, 2.300", 2.4" that I could rivet onto the oversized reverse cone.

Once you figure that out you can add a muffler to the end of the megaphone.

People who do this

Jemco, thunder alley , d&d crf450 low pipe, myself.

This one is well raced so forgive it's appearance

WIN_20151202_205052_zpsojy1de1j.jpg

Muffler pics on a uas kart

IMAG2689_zpsdgqhwjaf.jpg

IMAG2694_zpsxxr0r8as.jpg

P3281111_zpshwprnc42.jpgP3281097_zpsrb0su9lx.jpg

 

Something really trick(y)

Hydroforming

IMAG1364_zpsprlfxrku.jpg

That pipe has 27 angles.... starting shallow and ending steep....

Resulted in a constant area gain instead of a falling area gain. (Not sure the Jawa gave a single F... about that)

Real pita to build....

 

 

Side note, 

It's funny that pro circuit built a special pipe for the Honda Baja team (johnny Campbell and the rest) that was a megaphone, hydro formed like I did, but didn't offer it to the public.

140.jpg

They make more money pedaling straight tube junk.....

 

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Thanks for the replies,I take it its 6.25x i port theres two exhaust ports and am running a two into one exhaust.

                                   cheers dougie.

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exhaust port 1 Pi Radius squared + exhaust port 2 Pi Radius squared x 6.25 = area of cone end as per professor Blair, he determined through testing that most all useful energy was expended by this point so going larger was not useful  .

dual exhaust ports is really not the best thing in the world

by the area

you have the equivelent of 49.5mm (1.95") at the port(s) then 45mm (1.77") at the collector (much more reasonable size for an engine like this) infact if the port was small enough to allow it i would run 1.625" for a short distance distance

it ends up wanting a 123mm od cone, which is quite um large.....

100 to 111mm would be far more reasonable and more what you would find if this engine had a single exhaust port.

for the end cap i would expect it to end up at minimum 50mm but probably closer to 60mm maybe even 70mm

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My experience with reverse cone megaphone was they have slightly higher peak power at the expense of a broader spread from a standard pipe design. So for racing disciplines where you can keep the bike in the peak they can be fast. Probably not the best for an enduro bike. They also tend to be loud.

If the lengths/taper are out of sync with your cam they don't work at all. Had one on a Harley that worked well with stock cams, but would not work at all with longer duration cams.

 

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I have seen enough 1D modeling of exhaust systems to suggest the following statements are most likely true. 1. Reverse cone megaphone systems are a complete crap shoot without 1D modeling to refine to a specific machine. 2. A Muffler added after the reverse cone section is designed stands a better than excellent chance of screwing the whole thing up. 3. In general, if you insist on a RC system, stick to shallow angles, (1-3 degree included for the divergent cone.) they appear to have the least negatives. 4. Much of what was written by Bell and Jennings on this subject is wrong most of the time. 5. Blair's optimum effective area as suggested in SAE 9830466 is true but does not apply directly to your divergent cone because Blair's equation is about an immediate expansion of area. That does not happen in the divergent cone. 6. Straight, stepped pipe, and very shallow expanding pipes systems offer greater predictability, will allow you to integrate a muffler with less fuss and can be made to appear an indeterminate length to the motor so they don't have the negative of an anti tuned RPM. 

Edited by pcnsd

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