Oscaryu1, I hope you don't mind if I write a bit about expansion pipes?
Okay, to start off, I just want want to kill the common misconception that back pressure makes 2 strokes go faster, it doesn't, in fact back pressure with give you less power (but slightly better economy), however, back pressure can help an expansion chamber work a bit better but back pressure by itself is a no no. Expansion chambers work by rebounding the pressure from an exhaust pulse back to the exhaust port which (at the right rpm) will push the air fuel mixture that has leaked out the exhaust port back in thus "supercharging" the engine. (Well, its a lot more complicated than that but hey, I'm trying to make it a bit simple here)
This gif shows this well.
A break down of the components of an expansion chamber.
Header.
This is the first part of the chamber that connects to the exhaust port. Usually a strait pipe or slightly conical (2 or 3 degree taper being larger where it connects to the diffuser). The headers cross sectional diameter can be anywhere can be anywhere from the same to 150% that of the exhaust port. About 115% is best for max power at max rpm. Its length should be 6-8 times its diameter for max power at max revs, 11 or 12 for a broader but less powerful power band.
Diffuser or Divergent cone
Probably the most important part, this part creates a vacuum pulse that sucks the air fuel mixture through the transfer ports which helps massively with power. This part also focuses the rebounding pressure wave to push the the air fuel back into the chamber. The angle of the cone is usually between 7 and 11 degrees. A steeper angle will make for a shorter but harder hitting powerband and vice versa for a shallower angle. The area of the circle at the mid section should be 6.25 times that of that at the header end. There are also 2 and 3 stage diffusers which are made up of several cones with short, stepper angled cones at the mid section end and shallower angles ones at the header end.
Mid section or belly
Quite simply, a shorter belly will give you a narrower power band with more power at high rpm. Longer will give you a longer but less powerful power band. (Seeing a theme yet?)
Baffle or convergent cone
Obviously has the role of rebounding the pressure wave. The angles are usually between 14 and 21 degrees, steeper angle equals more power at higher rpm on a short power band, the opposite is also true.
Stinger
The end piece, it's diameter should be 0.58-0.62 times the diameter of the header, the smaller diameter will hold more pressure in the pipe and because waves will travel better in a pressurised environment this makes the pipe work better. How ever, this holds more heat in the pipe and engine, thus making the power band occur at higher rpm.
Basically, a longer expansion chamber with lass aggressive angles will give you a lower, longer and softer hitting power band and a short pipe with aggressive angles will give you short, hard hitting power band at high rpms.
Heat
The heat contained in the pipe affects where the power band hits and how long for greatly. Quite simply colder pipe will move the power band lower and make it occur for longer. Hotter will move it up and also extend it a little. The later 2 stroke GP bikes had water or water and alcohol injection into the expansion chamber to cool it down, this could extend the power band by as much as 2000rpm downwards from its normally occurring rpm, a huge boost in usable power obviously. Expansion chambers can be heat wrapped to extend the power band upwards slightly.
To properly design an expansion chamber you need to use super complex mathematics which I have not included here for reasons of simplicity (this is the "
Basic 2 Stroke Performance Enhancements" thread after all). You could build a chamber under the dimension limitations mentioned above and it would most likely provide an increase in power but there are many other things to take into consideration to make a pipe work the best it can. I would suggest downloading a computer program that designs a pipe for you, even though they all seem to miss out on many major variables they will provide the dimensions of a pipe for your engine that will almost certainly work very well.
A professionally designed and constructed expansion chamber can give as much as a 50%+ power gian.
(Sorry for my crap grammar
)
Thanks
Hayden
) I basically just took it as far as I could with out upsetting the decompresser, never fitted it to an engine though. Speaking of the decomp(the UFO shaped thing on the head), there's no point in removing it unless it's broken, in which case a spark plug will fit in its place but have fun trying to start the bastard, you will break pull starter cords or your arm. I found my Victa was happier when I lapped the decomp valve which might be worth doing if you're taking it apart. 
