What motor do you have? 48 or 66cc?
To know what Xchamber to get you need to understand how they work and that they need to be tuned to the motor or they don't do squat, at least not in the power band where you may want it to kick in.
Like your ignition, it is all a matter of timing, and timing depends on RPM, hence why you want to be able to tune the pipe to the RPM range where you want the boost.
The object of an X pipe is to shove the wasted
fresh fuel mix that goes out with the exhaust back into the cylinder just before the port closes on the way up.
Too soon and you are shoving
spend fuel exhaust back in, too late and the port is already closed and it does nothing.
I find this animated .gif from SpookyTooth to be worth 1,000 words if you watch it long enough to understand the principle.
That said for the best tuneable performance I like the SIckBikes X chamber.
It has a straight copper piece you can cut to tune it.
For a 48cc with the power at lower RPM leave it full length.
To get the most out of a 66cc from midrange up cut that piece in half.
~$80 delivered but a bit unruly to install as it has 5 pieces, but that is what allows you to 'pipe it' however you want.
One complaint about the SBP pluming is some is steel, some is copper, so you can't weld the 2 together so they include some hose to hold the connections on the outside. Metals that could weld together would be nice ;-}
On my 66cc personal ride I cut the straight tube in half then plumbed it snug up under the motor and above the bottom bracket, then capped it with a foot long 3" glass pack from the local muffler shop for $50.
(cool story about that but I'll spare you hehehe)
That is too much work for my average customers budget so I also found this one piece that does a pretty good job.
The single piece X I use is the one from JNMotors
http://www.jnmotorsbikes.com/product_p/jnm1189.htm
$50 plus a whooping $20 S&H!
Do NOT confuse this pipe with the 'fat boy' from a pocket bike others sell.
They look the same in pictures but they aren't.
I tried a Fat Boy on 5 different bikes and could never get it to work as is. It is shorter than the JN one and always seems to end right up against the Bottom Bracket.
I guess that's why they say it 'may need some bending', Pffftttt on that.
One last thing...
DO NOT use flexible tubing between the cylinder and Xchamber.
It is just to 'bumpy' inside.
For the wave front to pass intact to the back of the X part of the chamber and bounce back to push the wasted fresh fuel back in you want as smooth and gently curved as possible tube.
After that you can do whatever you want, like lowering the pitch of the exhaust closer to the sound of a quite Harley than an a gas powered blower like I did ;-}
Anyway, just passing on what I've learned so far.
I learned a lot from members here when I started, and the tradition continues.
