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Discussion Starter #1
Hi all...I recently installed a reverse cone exhaust on my 2017 Yamaha SR400. When I fired it up for the first time I immediately realized that it's too loud. I'm quite sure my neighbors are going to hate me for this mod. My question is, how can I make this exhaust more quiet? I've seen some videos of people adding baffles to their exhaust but in these examples the person had a (more or less) straight pipe and they were able to drill a hole in the pipe to secure the baffle. When I look at my reverse cone, I don't really understand how I can do that. I'd also like to add that I'm a newbie at making modifications and I don't have any fabrication skills. So, if you have a suggestion for me, it'd be best if I could buy something that's semi-easy to install. Here is an image of my exhaust...

103800


Thanks in advance.
 

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That's a shorty pipe.
Not sure how to break this to you, but you bought the wrong exhaust system for street use. That one is perfect for the race track IF it passes the sound check.

Your bike came fitted with an oxygen sensor, does or did the bike originally have a catalytic converter? We have strict laws up here that make it illegal to tamper with the emission control equipment on a motor vehicle this new. & yes, an irate neighbour might help you discover the existence of those laws.
 

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Are those pop rivets just in front of the reverse cone or just reflections on the photo?
If they are pop rivets, drill them out and that will gain you access to what's inside there now.
 

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Discussion Starter #4
Hi @TrialsRider ... thanks so much for your response and the heads up regarding the emission controls. I'll look into my state's laws requiring catalytic converters. I'm still learning so I have a few questions for you:

1. If a catalytic converter is required (by most localities I assume), what do most people do in regards to modifying their exhaust? My stock exhaust is too dorky looking and I'm trying to go for a more cafe racer look.
2. If I bought the wrong exhaust, what should I have purchased that would still allow me to have the reverse cone look?
3. If I can successfully remove the rivets, are suggesting that I might be able to re-pack the inside with more fiberglass (or whatever the material is)?

Thanks again.
 

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" I'll look into my state's laws requiring catalytic converters." It's a federal law and it won't state you require a cat, it will say you can't mess with your pollution and noise control equipment. It will also compel the original manufacturer to put a warranty on those parts that exceeds the warranty term on the rest of the motorcycle.

If the aftermarket exhaust you buy does not specifically say it meets the rules for street use it won't! If it says for track use only it means it's absolutely not street legal.

Woven fibreglass cloth has and can be used, but mufflers dampen sound by making the exhaust pulse reflect off things and change direction before exiting the hole, that is the baffle part. If you can't change the direction of that exhaust pulse before it reaches the opening, it's too late, we already heard you.
 

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:unsure: Does anybody here buy a brand new car and immediately remove the exhaust system to fit it with loud pipes ?
Why do people think that is ok to do on a motorcycle.

Too many are living in the wrong era, the days when you would dump your oil changes down the storm drain in front of your house are gone people! We are suppose to be beyond that point and in this century the general public has been made fully aware of the problems it creates.
 

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Anything that you can stick in there that reduces the outlet size will reduce the sound.
Usually baffles are a smaller diameter perforated tube with end caps that fit inside the exhaust and mount with a set screw from the outside.

Here's one as an example without the perforated core


This is likely something like what you saw.

The set screw is there so it can be removable. If you don't mind it being permanently installed, a couple spot welds could be placed inside the exhaust to hold it in place.
You could always grind out the spot welds if you needed to remove it.


I would get a very accurate measurement of the ID of the muffler, give the dimensions to someone that could weld up an insert like that. test fit it to see if you like the sound and spot weld it.
 

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This is even something if you are interested in and have some extra $$$ you could buy a welder and learn to do yourself if so inclined...
 

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Discussion Starter #9
@jcw - thanks for the useful information. Am I right in thinking that your comment "and mount with a set screw from the outside" is not easily possible since I have a cone? Or, do I disassemble my exhaust (e.g. remove rivets) and replace the core? Do baffles exist that have a different mounting mechanism other than an external screw? Or, is welding it in my only solution?

@TrialsRider - thanks for the information, I certainly learned more about exhaust from your few comments and, in a way, wished I had known this info prior to purchasing my shorty reverse cone. I guess I was just excited to get moving on my mods. But, with that said, what is one to do if they want to modify the exhaust on their motorcycle? After all, isn't that the reason why some of us have joined this forum? Heck, even the picture at the top of this page has three motorcycles with custom exhausts? Do the exhausts on these three bikes have catalytic converters.

Can anyone recommend a "cafe racer" style exhaust that meets emissions laws?

103801
 

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@jcw - thanks for the useful information. Am I right in thinking that your comment "and mount with a set screw from the outside" is not easily possible since I have a cone? Or, do I disassemble my exhaust (e.g. remove rivets) and replace the core? Do baffles exist that have a different mounting mechanism other than an external screw? Or, is welding it in my only solution?

@TrialsRider - thanks for the information, I certainly learned more about exhaust from your few comments and, in a way, wished I had known this info prior to purchasing my shorty reverse cone. I guess I was just excited to get moving on my mods. But, with that said, what is one to do if they want to modify the exhaust on their motorcycle? After all, isn't that the reason why some of us have joined this forum? Heck, even the picture at the top of this page has three motorcycles with custom exhausts? Do the exhausts on these three bikes have catalytic converters.

Can anyone recommend a "cafe racer" style exhaust that meets emissions laws?

View attachment 103801
First off, those are really really old motorcycles, they never had pollution control equipment to remove.

and yes, there are a very few aftermarket companies that are making aftermarket performance exhaust systems for specific model motorcycles, and they have gone to the extreme effort of having those systems tested to meet with current environmental law. The purchase price will reflect that.
 

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A large percentage of the regulars on this site race their bikes, there are people on here that are into breaking land speed records, people that scratch build high quality frames or restore near priceless antique bikes, you can find people on here to help you make your bike into a track only bike real easy. Great place to ask a technical question about a weird bike.

Not too many here are into firestone whitewall tires and freshly stolen USD front forks grafted onto a bobber CB350, you might want to ask about those things somewhere else.
 

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Discussion Starter #13
@TrialsRider - it's my first bike so I have nothing to compare it to. I bought it because it seemed like I could modify it into a cafe racer given my limited skillset. It was also relatively new so it would be more reliable versus a more classic bike from the 70s or 80's. I'm not much of a speedster either so the single cylinder is fine for now. I could see myself bumping up to ~650cc in the future but I want to successfully convert this bike first.
 

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Swapping out pipes is quite common even on new cars but unlike our bikes, most US states - maybe all- don't test bike emissions and since we don't have OBD2 ports, they can't check the ECU for codes. That's not 100% true in as much as newer bikes with fuel injection usually have a diagnostic port that requires an adapter and specific hardware & software to read the codes.
So we tend to get a free ride. Some states are particular about noise, but again, many are really lax.

Cars can't get away with our level of exhaust modification because they are emissions tested and really need to keep the catalytic converter, so they offer Cat back systems and downpipes with replacement cats and so on.

But back to the noise issue. There are two basic ways to reduce noise in a muffler. One is with baffles - bits of tin designed to change gas flow and more importantly to interfere with sound waves. So a typical muffler consists to an outer shell and inside, the pipe from the motor sits alongside the pipe connected to the outlet. The gas and sound waves have to go around the long way. That's not exactly an engineering description but should be enough to grasp.

There are books on the subject, so let's try for the Cliff Notes version.

The second technique is what's known as an absorbtion silencer. They are also called "straight through". That's what your megaphone is most likely to be. The outer shell should contain a smaller diameter perforated tube surrounded by fiberglass mat so that sound waves are absorbed by the glass as exhaust gas carries on down the pipe.

They tend to be louder and the larger the inner core diameter, the more of that sound that escapes out the end.

There are two possible way to attenuate the sound. One is to fit a smaller diameter core and the other is to fit a mechanical end baffle. They are readily available on ebay as "db killers".

If you really like the pipe you bought but want less sound, remove the "muffler" and drill out the rivets to show us what's inside. You can most likely fit a db killer that JCW showed inside the core and then with any luck, just rivet the end cap back on. But before you take a drill to it, can you see if it's completely hollow or has a core and if you can see straight through it. If so, try to measure the core diameter and post some pics and we'll try to point you in the right direction.

Check out coneeng.com for more reading material.
 

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@jcw - thanks for the useful information. Am I right in thinking that your comment "and mount with a set screw from the outside" is not easily possible since I have a cone? Or, do I disassemble my exhaust (e.g. remove rivets) and replace the core? Do baffles exist that have a different mounting mechanism other than an external screw? Or, is welding it in my only solution?
First thing is maybe check with the company you bought the exhaust from. Sometimes they will sell an insert to reduce db.
If this was an internet special, then u are stuck measuring and searching for your own insert.
You've pretty much listed your options in your post.
#1 find or fab an insert and figure a way to secure it in place
#2 try to modify your muffler
#3 go back to stock.

I would try 1. If you can't find an insert, I might be willing to help fabbing one up...
Strictly for offroad use...
 

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The pipe you have in hand is workable, I won't say it's street legal, but I bet with some rework you can make it awesome. Don't send it back take it apart imho. take pictures best you can of what you have to work with.

I think the SR is a really nice bike, always did. You chose wisely. :cool:

and hang the original on the garage wall once you get this one working right, those parts should go with the bike for the next owner. Your grand son or whatever
 

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You're going to need some expanded metal screen that won't rust, to go with that fibreglass cloth.
and it needs to take intense heat, but you don't need tons of it,
you need enough to make a cone, pretty darn close to the same size as the long cone, you need it to keep the glass in place.

... if you line that long cone with the sound absorbing material you will have something far less tinny for the reflecting pulse that you are going to be creating with the short reverse cone. In a perfect world :/ that pulse would arrive back at the exhaust valve at a magic calculated time, & the math required for that to happen makes my head hurt.

Have you ever watched the movie Worlds Fastest Indian
you should (y)
 

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As someone who made a living designing OEM autmotive exhausts for about 20 years, I've got a few misc comments.

First off, singles are a bitch to quiet down for a couple reasons. One of those is that you have one piston firing every other revolution and not spinning all that fast which means the sound has a lot of low frequency content. Lower frequencies are harder to quiet down than higher frequencies. They call them Thumpers for a reason.

In general, the lower the frequency the bigger the silencer should be. With a shorty megaphone there isn't much volume to work with. The rule of thumb is if you want a quiet exhaust without excessive restriction, you want a muffler ten times the size of your engine. So you want two 2-liter bottles as a muffler. No one does that (other than OEMs) but that's fine cuz no one wants a TRULY quiet aftermarket motorcycle exhaust. So don't worry about not having 10x but keep in mind you need volume to quiet things down.

Ok, so let's forget about reflective tuning with helmholtz chamber(s) and other OEM stuff. Ain't gonna happen in that space, ain't gonna happen without knowing a shitload of info you don't have.

Realistically you're looking at the same thing 99% of all aftermarket mufflers use, basically a glass pack. A perf tube wrapped in fiberglass stuffed into a shell. This type of muffler can be good at attenuating high frequencies. This is good because most people find high frequencies more annoying and low frequncies more pleasing. Think of a jet ski compared to an old Chris Craft. This is bad because at lower frequencies the fiberglass has zero effect on sound, the muffler acts like it's empty. Empty volumes need to be larger to attenuate lower frequencies. Shorty megaphones aren't large.

Ok, so now you want to core the megaphone and slide a perf tube in there with fiberglass around it. That won't work all that great either. In order for the glasspack to work well the space between the two tubes should be at least about 3/4". So the outer tube should be at least 1.5" bigger in diameter than the inner pipe. That doesn't happen over much of the length of the megaphone. And length matters too. Longer is quieter.

Perf tube sizing - the larger the tube the less it attenuates low frequencies. Don't use a tube any larger than you need, and that's likely smaller than what you think it is. Looks like about 1.75" on your current pipe? That seems pretty big for a 400cc engine that you want to be quiet. A smaller tube attenuates lower frequencies. More space between the inner and outer tubes attenuates higher frequencies better. Win/win.

Round holes work just fine in perf tubes, as long as they're wrapped. They can cause whistles if they don't have something covering them. Louvers are often used in cars if there is nothing backing them because they don't whistle. The louvers do NOT have to reach into the flow to "scoop" up the sound. Sound doesn't work that way.

To sum it up, if you want a quieter muffler, ditch the megaphone, find something cylindrical with a smaller tube on the inside and stuffed with fiberglass. Stainless steel is occassional touted as being better because it doesn't blow out. It's true it doesn't blow out but it's also true it doesn't attenuate nearly as well as fiberglass. So stainless makes a louder muffler that lasts a long time.

All this is over simplified but you get the gist. Anything you do to stop the exhaust from having a straight path through the pipe and out the end will help too. FWIW catalysts quiet things down too, not sure where it is in an SR400 but for some bikes (like my V85 Guzzi) a "slip on" system is like a cat back system on a car. You can buy dB killers that are essentially a substrate with no precious metals in it, basically an untreated catalyst. They fit right in the pipes and probably work pretty well but I have no idea where to buy them. Helpful, I know.
 
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