SickBikeParts Expansion Chamber Mounting, Tuning & Quieting

Just to cover the basics this animated .gif gives a really good idea of what exactly an Expansion Chamber on a 2-stroke motor does.



Both the intake and exhaust are briefly open at the same time, and this causes unburnt fuel to escape out the tailpipe on the up stroke.

An Expansion Chamber is designed to push that unburned fuel back into the cylinder thus getting a denser charge of fuel for the ignition charge, resulting in a more powerful down stroke force.

The SickBikes pipe comes in multiple pieces to allow you to MOUNT it to your bike, and to 'TUNE' it.

MOUNTING:
(See TUNING about pipe length before committing)

The two most common are to go down below the BB like this.



And to go up and around.

I haven't tried that way, something about a hot pipe next to my inner thigh, especially when riding in shorts, doesn't sound good to me.

Down has it's drawbacks to, even if you can tuck it right under the BB.
For one you usually have to remove your kickstand, and for another you have a lower ground clearance. Not fun with tall speed bumps.

I managed to mount mine above the BB which to me has no drawbacks other than the end of the pipe which I will get to.



With the new SickBikeParts.com kit, the order of parts to try this starting at the cylinder is:

SBP Exhaust Manifold.
Angled Copper Pipe.
Straight Copper Pipe.
Short end of Long Curved Black Pipe.
Expansion Chamber.

On my bike, I could just bend the metal mounting tab on the chamber itself to match up perfect to my chainstay for attaching there, and again with the kits strap



If I had planned to keep that I would have painted it and cleaned it, and it works, just use a nut and bolt between the pipe and 90 degree fold in the strapping metal.

For me that is just a secure temp strap until I get to my next step.

TUNING
On the SickBikes Xchamber it comes 'tuned' for a 66cc low end, where most second drive single gear bikes can use it most.

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Tuning can be considered 'timing'.
If you look at the animated example the Xchamber does not show the incoming pipe. That is because some expansion chambers attach with the chamber direct to the cylinder.

With those types there is no adjustment for the power band you want to boost. The chamber itself is tuned to be mounted direct.

With the SickBikes, the chamber is tuned to account for leading pipe.
This pipe holds the majority of the waste fuel you want to re-inject back into the cylinder, the the longer it is, the farther it has to travel, and the longer it takes to all get back there.

The long straight piece of copper is the piece you cut shorter.
As mentioned, the pipes come with a long straight pipe which tunes it tot he low end, and you cut it down to move it up.
About in half (~2") will move it to the high end.
I cut mine to 2 3/8" to keep the power at about 3/4 throttle.

Just try " off the pipe at a time to test.
If you cut too much off, no biggie, 4" of copper tube is cheap.

The the kit now comes with black 'rubber' couplings, hose clamps and springs to keep it together, I am going with just welding everything except the the joint between the front tube and the chamber so I can get it on and off easier.

QUIETING

The kit comes with this silencer on the end.



To understand quieting you need to understand how sound waves travel, because it is NOT the exhaust gasses you quieting, it is the shock wave sound of the piston firing, much like a gun.

Confined in a tube that wave will bonce and even add to itself as it travels.
When that pipe is perforated however the wave does not bounce, and with dampening material like the fiberglass packing in this silencer, the sound wave dissipates, but the exhaust fumes themselves just travel on through unimpeded.

Unimpeded is the key here.
Ideally the exit port on the expansion chamber would be to open air because you don't want any back pressure at all. That would interfere with the chamber itself.

Just for giggle and grins I ran my bike with the silencer off like this.



It sounded like setting off a whole pack of black cat firecracker in a a little tin shed, so yes, the kit silencer helps but I want to quite it down much more and lower the 'pitch' of the sound as well.
Closer to Rhrrrm, Rhrrrum, Rhrrrm than Rat a Tat, Tat if you know what I mean, so I am doing this.



My crude drawing is standard 1 3/4" car exhaust tube that curves in and down to a 12" GlassPak muffler with a matching 1 3/4" input and output.

The muffler will end up right next to the bike rack for mounting there, and not be longer than the back of the back wheel or any wider than the stock pipe already is.

My THEORY of what this should do is again going back to the basics of sound waves.

1. By being 3x longer the new pipe should cut the 'volume' of the sound by a factor of 3 which would really help.

2. By the internal perforated pipe being nearly twice the diameter of the stock pipe, the sound waves need to travel farther before bouncing, therefore they slow down, and a drop in speed results in a drop of pitch.

I certainly don't expect it to sound like a Harley, but it should sure as heck make it not sound like a little dirt bike.

I'll know tomorrow.
I rode over to the muffler shop around the corner and showed the guy what I wanted to do.
He ordered the pipe and he can install it tomorrow for $50 total including welding my other pipe joints.

I'll get some video with sound of the bike with no silencer, the stock one, and my design and share them ;-}