Modern Vespa

I know this is Honda but wanted to share the info on what's inside this muffler !!!
Have seen Vespa muffler and info is good !

Ever wonder what's inside the HELIX muffler ? What's rattling ?

This is my 1986 OEM muffler that started rattling. I googled everything but never could find a definitive answer on what was inside.....answers from ceramic to catalytic converter to broken chambers etc. never pictures !
A few years ago I cut a small across and found the rattle and removed it and welded it back up but removing a broken pipe just messes up exhaust flow ....
Anyway I decided to fully understand the muffler so here it is exposed once and for all !!

I painted it for easier understanding.

First of all the rattle is a longish pipe that breaks clean off from the entrance manifold to the pipe in the first bulkhead and just annoyingly rattles!!

I made a plastic pipe to simulate the proper connection !

The muffler is double walled with fiberglass between. The inner is perforated mesh.

There are 2 sealed all around baffles that all piping is welded to, effectively making 3 sealed chambers ...

Exhaust starts down through Orange pipe passing through (NO OPENINGS) the Bulkheads and dumping out in chamber 3.
Then it goes through the slightly hidden White tube and dumps out in chamber 1.
Now it goes through the Peach pipe and dumps in chamber 2.
Finally it enters the Lime green pipe to hook around and exit the muffler.The large bulbous end on the orange pipe looks like a resonator of sorts. has a perforated inside pipe and the exhaust just goes down the center and exits in chamber 3.
nothing in the walls.. just a pre expansion area ?

The bulbous end on the Lime green before the exit has a plug at the beginning and a perforated interior pipe.
The exhaust hits the plug and goes around the perforated interior pipe and goes through all the holes and down the middle out the end exit.

It is quite the muffler. never took one apart before. Quite a long length inside with extra expansion chambers also.

A muffler is not a simple device! If you view it as a musical instrument with primary outcome of stopping sound, it's easier to appreciate.
The outrush of gasses from the engine have a very wide range of frequencies and amplitudes that are generally just called "noise". Frequencies are "grouped" by frequency and amplitude passed through resonating chambers and/or absorbing materials to damp and create destructive waves countering incoming sound waves. This is done numerous times in adjoining chambers with tuned-length/volume/diameter delivery tubes and chambers. And it's all packed into the smallest, least expensive to build assembly.
Not simple, but quite interesting. Many people work their entire professional lives designing these things!

Yes I agree on the science of the exhaust. I wish there was more info and then I would consider playing more. The only scooter muffler inside picture I have found is a vespa and yes it is packed in there also.

Why I am interested is I am playing with fuel injection for the helix. It will be a long learning process and the muffler and its design plays into it also.
There is much info out there on fuel injection so I am being kept busy.
Have 2 helix so I will not lose my ride playing !

You've picked a big project and it should be interesting.
Yes, back-pressure from the muffler will play into fueling. The important thing with the muffler is to be consistent. Don't change the muffler in the midst of developing your fueling map!

yes.. I understand that !
everything is dependant on everything else, but it is that way with a carb also.

now I am up to installing test bungs on the original muffler so the first step is just monitor o2 behavior. Get a proper reading wideband sensor !!
not that easy as slappng it a couple of inches down on a too small exhaust pipe because that is all you have, does not appear best..

that is why the interest in the interior of the muffler as I plan testing the spot where my Orange pipe dumps in chamber 3.
A chamber can be a "large" pipe.
OK it may respond a bit slower and being an electrically heated sensor anyway I am sure it will stay hot. that puts the sensor 31 inches from the exhaust valve.
suggestions go around 40 inches !!! Muffler gets mighty hot and I bet all will be fine.

As close to engine exhaust as possible. I use a wideband lambda sensor on my shifter kart and the response can be slow if you're not close to the engine. I would also suggest an exhaust temperature sensor near the lambda sensor.

I thought that would be too close. and hot !
I want to test 25 inches from the manifold right after the Orange pipe where it dumps into chamber 3.
I think your cart has a longer pipe where the HELIX is crammed with clamps etc with possible leaks ?
I will try other places too.....first is to get a reliable reading on a stock system.
never thought of temp monitoring exhaust..
I will keep it in mind though.

Heat is not an issue for the lambda sensor. It IS an issue for the leadwires!

Did not know that.....! I keep reading about so many inches from exhaust valve and I equated it to hot gas issues.
I still do not know how that sensor will fit in a small pipe, but I do have my cut muffler to play with.
That is how I will start breaking the project into pieces.

A good illustration of an exhaust design where a broad power and torque band are needed while maintaining low sound level.

First point is the longest possible effective downpipe length. Then a high volume expansion chamber, then a further long path before the exit pipe with more direction reversal to remove energy from the pressure pulses and smooth the flow. Of course its all very heavy because some tough steel is needed to withstand the constant heat and pressure reversals.

That would be my uneducated design goals without math.
there is kind of a long down pipe on this muffler but most is inside the muffler then the expansion's then the longish exit with direction changes !!!
the effective down pipe length isn25 inches before the first expansion chamber , and heavy as most is inside the double wall muffler !!!

The over all length of the header is key to performance, the rest just makes it quiet. The shorter the header the higher the RPM where it gives a power increase. Most aftermarket pipes are too short here.

The holes inside the wide section of the orange section form a side resonant silencer (I see one hole in the photo, are there more?). That would be tuned to absorb the loudest frequency produced in the unmuffled exhaust.

the three chambers form diffusion silencers. These filter out a wide range of frequencies but let some frequencies through. They are typically setup so the gap in one is covered by another.

Oh yes.....Lots of holes !!
The exhaust goes straight down the middle and expands through the holes and hit the outer wall..... Just an expansion chamber.....

I consider it having a long down pipe except it is mostly inside the muffler....!

Your explanation of the musical instrument analogy is a perfect description. Then when you add to the process of trying to find and save certain sounds to make a pleasing sound experience out of it makes the goal of the designer espically easy to respect.

I remember when i had my piaggio hexagon 250, which has the same engine, my muffler was quite rotted out when i bought it and i couldnt explain why there was constantly a popping sound when i closed the throttle,

traced it back to being related to the exhaust and a lot of people online told me the popping could not be fixed with this model unless the muffler was replaced with a stock one, this was stupidly expensive so i opted for a replacement for a chinese cn250 clone which was the only other option at that time.

unfortunately this did not even fix the issue, which i then found was typical of the chinese clone exhaust, it was a great scooter other than this one issue.

Great to finally see the inside of one and why it was such a difficult thing to replace the muffler on.