How to make a reverse cone exhaust quieter?

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.

Rather than edit that rambling I'll add more here... Another reference point or two. My 851 is more or less (ok, more more than less) two of your engines in one. Each cylinder is roughly the same size as yours. To quiet that bike down it's got two mufflers, each 16" long, with 1 7/8" cores. But the outer diameter is about 2" bigger than the inner so it's got roughly 1" of fiberglass packing over 16" of two mufflers to quiet it down. I say two mufflers because there's a crossover between the two sides - each cylinder breathes through both mufflers. That helps quiet things down too, along with the flow dynamics of two cylinders working with each other. But I digress...

As for frequencies... you might think that it would be twice the frequency of yours but it's not because it's a 90 degree V-twin (save me the "L-twin" marketing bullshit Ducati fans, mathematically it's a 90 degree V). The 90 degree offest with the single crank pin makes the fundemental frequency an odd order which basically introduces fractions into the mix. That adds it's own complications when using reflective tuning but it's moot since it's now got glasspacks on it just like every other '90s Ducati.

ANYWHOS... long story short, an 850cc V-twin with mufflers larger than yours will still set off car alarms when you let off the throttle, so you've got your work cut out for you making the shorty megaphone quiet.

These mufflers are actually small in modern terms.


Here's a more modern V-twin (Edit: But still nearing 20 years old). Each cylinder is about 500cc, glasspacks again. This bike has two of these on it to quiet it down. Ya want quiet with low restriction, ya gotta go big.


If I were you, and I wanted to use a small megaphone, I'd search on dB killers and find something that looks like honeycomb foil in a tube. Install that upstream of the muffler and see how it sounds. Then go to the trouble if rebuilding the muffler if it's still obnoxiously loud.

And/or bite the bullet and get a bigger muffler to start with.