How to make a reverse cone exhaust quieter?

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.

@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?

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.

This is the insert in my gsxr leo vince slip on. For an example. Set screw goes in at the bottom. This came from the manufacturer.

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.