Can we fabricate our own expansion chamber?

Discussion in 'Performance Mods' started by gone_fishin, Feb 10, 2007.

  1. gone_fishin

    gone_fishin Guest

    for real. i see that a few of you know the theory behind expansion chambers. i can understand it when it's layed out for me. i saw it on here somewhere...anyway, maybe it's possible to make a decent one on our own?

    IF all i had to work with was my current down-pipe & muffler, would it be possible to construct a primitive series of nozzels/cones/whatever that could be placed in the muffler? could someone do the guesswork needed to make the project worth trying?

    better-than-nothing technology is what i'm after here.

  2. jaker

    jaker Guest

  3. gone_fishin

    gone_fishin Guest

    i got it jaker, thanks, that's an interesting site :)

    it looks great, but i'm still lost...maybe someone who's already hip to pipe theory could have a look and give us the basic values needed to get a calculation?

    see, my 70 has about 350 miles on it, new plug, 36:1 pre-mix, full breather intake, 3rd notch on needle. at topend, which is strong, i can tell it could do a bit more if i had the "pulling" effect of an expansion pipe.

    anyway, i'd love to see this project take flight...i'd really appreciate someone piping in (pun intended) 8)
  4. Steve

    Steve Guest


    I obviously have no experience with tuned exhaust systems on these engine, but I do have experince with them on other 2 cycle engine, all R/C model engines. What I have learned about them in general, is they provide more power, however it is at the one RPM range where they are tuned, and always at the expense of torque at lower RPM ranges. They can be tuned for a more broad RPM range, but then the gains are less.

    I guess what I am trying to say is, unless you want all out power in one RPM range for racing and such, IMHO a tuned exhaust on one of these bikes is not a good deal. I would think a good free flowing exhaust is what we really need.

    Just my opinion on the matter.
  5. chieflets

    chieflets Guest

    you would want something with mid to top end unless ur doing allot of hill climbing u would want bottom to mid range torque i guess

  6. gone_fishin

    gone_fishin Guest

    Steve: yup, that's what i thought i remembered...thanks for that...time to go for unrestricted exhaust, i've already cut the inner pipe all the way off, maybe i should widen the outlet too?

    i have to take a 5 mile ride in just a few minutes, i'll take the bottom of the muffler off & see what happens.

    however...i fully intend to enter into the realm of MotoredBike racing, this topic will stay alive.
  7. srdavo

    srdavo Active Member

    your neighbors will be
  8. Barret

    Barret Guest

    About that...I actually don't have much of what you would call a muffler any longer, as mine rattled loose while ridding one day, and it suddenly gave me a stronger push and a bit more power. I only have the "inner muffler" on, and not the outer shell [pic below]. After riding like this for a while, I checked my spark plug for too rich/too lean mixture, and it's never been either direction. I've never looked back, and no one has complained about the noise level yet. I don't ride it after 9 p.m. or at the butt-crack of dawn, and there are other kids in the neighborhood with loud toys of their own (mini-bikes, homemade scooters, etc.). I love the way it rides now.
  9. pianoman8t8

    pianoman8t8 Guest

    what exactly does an expansion chamber do? i have no idea about what these do...

  10. etheric

    etheric Guest

    To bring some people up to speed on the expansion chamber (and a good review for myself and others)
    How Stuff Works
  11. gone_fishin

    gone_fishin Guest


    i took the bottom of the muffler off. louder but not too loud, i asked around. it sounds great, imo.

    but OMG, this bad baby has a strong "model-airplane" buzz all the way to top-end, then it only cuts out because it's rolling as fast as it's gonna...well, for now, that is.

    next: bigger jet :evil:
  12. jaker

    jaker Guest

    well all this expansion chamber talk is getting me stoked to go to the exhaust shop my pal owns and build one i am familiar with the program and with expansion chamber design.
    the free flowing exhaust thing is not what we need btw
    theres a lot of misconceptions on this subject it seems..

    the expansion chamber is tuned to the engines exhaust pressure waves. some provide one powerband some provide up to 3 or more depending on the design...

    The way it works to create more power is simple to explain but hard to create perfectly..
    The pressure waves bounce back at exhaust port keeping unburned fuel from escaping as eaisly into the exhaust and raising cyl filling .. backpressure this is not.

    put simply it is a reflected pulse from the engine..the design on the chamber itself decides where this effect is most efficient. the reason a tuned pipe can create more than one powerband is rather complex as the reflected pressure waves can be timed right at more than one rpm giving the effect of multiple powerbands.

    with proper design a tuned pipe can be made to increase power over a broad range of rpm and is therefore not just a racing thing as it can be very street handy. also u can put a muffler on the end of a tuned pipe and not change the preformance of it much and still be reasonably quiet.

    so plz plz do not just remove those mufflers.. it may help a little but theres better ways to make more power than pissing people off with the sound of our bikes ...

    i for one know that when too many noise complaints are made about such an unusual vehicle, laws tend to change in the complainers behalf.
  13. gone_fishin

    gone_fishin Guest

    well, jaker, i enjoy a fairly remote location so i also enjoy a bit of auditory freedom :p your point is a valid one tho 8)

    now, back to the OP: do you think there's a "better than nothing" configuration that would fit in the same space as the muffler? i'm guessing the wrong configuration could in fact hinder overall performance?
  14. chieflets

    chieflets Guest

    Just quick note,

    grubee have an expansion chamber allready made up if people have actually taken the time to view there webpage

    but on the other hand people might want to experiment too

  15. gone_fishin

    gone_fishin Guest

    i lived on the grubee website when i first started shopping for an engine, never could actually find that pipe available anywhere. dennis foster has plans to make it available this year but he can't say when. he also said it requires a lot of extra space in front and hits some tires on some bikes.

    good advice tho...i agree that everyone should read an entire site before getting involved or criticizing the admin.
  16. jaker

    jaker Guest

  17. mrsaxman99

    mrsaxman99 Guest

    Yeah I've just now done a good bit of reading on tuned pipes, very interesting and make a whole lot of sense. I totally understand why opening up my exhaust has now caused my engine to cut out at the higher rpms. I'm just pissed I don't have the resources to create my own exhaust(s), and try them all out to see the differences in power.
  18. Lukass

    Lukass Guest

    Putting the data into that pipe calculator is giving me tuned lengths of up near 180cm, it's the relatively low rpm of the motor that is the biggest problem.

    I am working on a small chamber with a proper packed silencer, pics and report to come.
  19. Making the chamber specifically for a particular engine is the optimal solution
    for making power but I must admit that I tossed in the towel on that one
    (design is fairly easy but actual Fabbing and bending the metal proved too much for me)....Instead I opted to find a factory chamber / silencer and custom fabbed the header to tune the assembly....Not 100% optimal BUT Really Really Good!!....Maybe this and sprockets! Hmmmm

    Attached Files:

  20. Ghost0

    Ghost0 Guest

    I think this pipe is the best for the typical 50cc-70cc unit for off the shelf, inexpensive and requiring the lease amount of modification.
    Some mods will be required but it will be a huge improvement over stock. It has a 1" diameter tube so it will mate perfectly to the current exhaust header portion. Because it was not made specifically for the motor/carb combo (70cc China unit) some tuning will be required. Depending on many variables, gear ratio especially, you will either need to increase or decrease the distance from the exhaust port to the end of the pipe. The shorter the pipe, the more top end and the longer the pipe, the more bottom end. If I had to guess, since the pipe was made for a 49cc engine, I would start with it longer and shorten it an inch at a time until you reach your desired results. I would start by cutting a stock China pipe about 4" from the head and attach the new pipe. You can use fiber reinforced silicone tube with hose clamps for your tests so you don't have to weld it every time. You can usually get the tubing from most auto parts stores or performance shops. It is typically blue or red. I know I will be buying one as soon as I get my moto-bike complete.
    Last edited by a moderator: Nov 5, 2007