Building an Expansion Chamber

Discussion in 'Performance Mods' started by LRSimons, Jul 26, 2016.

  1. LRSimons

    LRSimons Member

    Hi all, I recently (sort of) finished my tuned pipe. It's taken a couple months of intermittent work, but after I got it fitted and somewhat tuned, I made my long standing goal of breaking the 35 mph speed limit on my testing road (and chased down a harley to boot!).

    To begin, I cut out trapezoidal shapes out of 18 gauge matching up to Jaguar's dimensions the best I could. By just creating a trapezoid, however, the cone does not come out with square ends, as I found out after the fact. The proper way to do it is to cut arcs instead of square edges on the openings of the cone, which requires more math and fun things like ruining bandsaw blades to cut curves through steel. So, I just squared up the edges of the cones on the belt sander. Actually forming the cones was somewhat difficult, as they never seemed to get completely straight. What seemed to work the best was getting one end somewhat lined up, tacking it, then hammering the next inch into shape and repeating. After the cones were finished, they didn't quite line up, so I just welded in some more metal where there needed to be a bridge. At this point, I was just trying to get a pipe made (it would still work to some degree) rather than stick exactly to dimensions.

    Yes, the welds look like garbage. Using whatever machines were available at that moment, I started out with a full size stick welder (terrible), then used the TIG machine (slow), and finally a large MIG machine (which still burned through on the lowest settings but I could do a series of tacks much more quickly and cleaner). The frankenstein part in the middle was a lack of math and planning, as I mentioned above. Feel free to make fun of me now.

    Because I wanted to retain my other exhaust (stock with extension and flattened muffler), I made the header and flange as well. At the shop I was working at for this bit, the tubing roller didn't quite have the right dies and therefore was mostly a tubing kinker, not a roller. To get around this, I stuffed the pipe full of sand with both ends welded shut. The tube bent just fine with minimal kinking. I made the flange at home out of some 3/16 flat stock, after I got some better equipment. I just traced the outline of the original flange for a pattern, drilled the holes and cut to shape. To offset the pipe from the engine a bit, I cut two bits of curved pipe and welded them together to end up with a bit of an offset. It was then a matter of attaching the pipe and tuning the header and stinger. As you can see, after I got the pipe attached there was way too much header. I somewhat stupidly decided to remove an inch or two at a time, test, and repeat. This took all day, and I eventually decided to just cut the pipe to about 8", what was recommended from just about everything I read. That was the ticket, and the top speed rocketed up to 35. However, now that the pipe was actually doing something, the AFR needed to be richened up. I didn't really care on my test ride, knowing that something would break from the heat, but I was too exited to finally have something to show for my couple days of tuning and testing. After I hit the top speed and speed matched that poor guy on the harley, pop, magneto failed.

    Oh well, glad we still have pedals.

    Next step is waiting for a new magneto to arrive, drilling jet (done), and fixing all the stupid stuff I've been putting off in the quest for speed. Now that I can actually reach the speed limit, I'll be able to commute without being run over.

    Input welcome.

    FurryOnTheInside likes this.

  2. LRSimons

    LRSimons Member

    I tried to get some photos attached, not sure if they are on the post but they should be on my profile page.
  3. jaguar

    jaguar Well-Known Member

    is the exhaust port height standard?
    what is your max rpm now?
  4. LRSimons

    LRSimons Member

    The cylinder was ported a while back, but not measured. I was pretty conservative with the file. Today I made a degree wheel and measured the timing for real. It was just a printout mounted to some cardboard, and this was the first time I've measured so the reading may not be completely accurate. Exhaust opened at 118 degrees, give or take a couple. The caliper reading was in the range of 1.14-1.16", from the deck to the top of the exhaust port. For metric friends, about 29 mm.

    As for RPM, I have the standard single speed setup with a 36T rear sprocket and 29" wheels. Not sure if the 29" is with the tires on or off. Anyways, I'm not especially eager to do the calculations myself at this hour, and the mac can't seem to see the rpm calculators on the forum. I will try in the morning, or maybe someone will know.
  5. jaguar

    jaguar Well-Known Member

    My notes show that the 66cc engine has around 142 degrees open port duration so if yours is ported it should be even more than that.
    The hard part of determining port duration is knowing when the piston top edge is equal in height to the top edge of the port. Looking down into the cylinder distorts reality.
    You could retry measuring it by using a thin feeler gauge. Put it on top of the piston with a few millimeters of it inside the transfer port and then turn the crank till the feeler is pressing against the top of the transfer port. Then record the corresponding degree on the wheel.
    Never depend on where you think TDC or BDC is.
    After the first degree mark then bring the piston down and back up to find where on the degree wheel it will be when the feeler again hits the top of the transfer port.
    Then from that range of degrees you'll know the port duration.
  6. LRSimons

    LRSimons Member

    Thanks for the better technique. I measured the exhaust port duration today, with a razor blade as far down on the piston's bevel as I could get it. Duration is right around 145 degrees. Is that fairly conservative? The engine is not a screamer at all, it topped out at 31 mph with the stock pipe (6" extension welded in).

    I could probably raise the port a bit, but am I wrong in thinking that tuning the pipe for higher revs would be better in terms of the low end power left? I know either way the low end will suffer, but having some torque left would be nice. The comparison I'm making between increasing rpm with porting vs pipe is like making power with a car engine- porting would be the equivalent to having high flowing heads and a lumpy cam (making the engine hard to deal with on the street), while tuning the pipe to raise rpm is like adding forced induction- more power without sacrificing much drivability.

    Am I on the right track with this or totally wrong?

    Either way I have to finish balancing the crank first.

  7. jaguar

    jaguar Well-Known Member

    my 60cc engine had a duration of 157 and with the torque pipe still had enough low end power to start off without pedaling
  8. LRSimons

    LRSimons Member

    Wow, I'm surprised that worked. What was the effective range of the pipe, and top rpm?

  9. jaguar

    jaguar Well-Known Member

    3000 rpm is the broadest range available for the best boost from a pipe, all at the top. but my torque pipe doesn't noticeably diminish the power at rpm below that band like other pipes do.
    it rev'd to 9200 rpm.
  10. HeadSmess

    HeadSmess Well-Known Member

    no maths involved for the laying out of pipe templates.

    google and download a lil program called "cone". gives you templates required to print out on paper, glue em to the steel, cut along the dotted line,(rough it out, save the bandsaw blades, use a linisher/belt sander to get it to the line) if you remember to select "include fold lines" it makes it a lot easier as well (something i always forget to do then struggle with rolling cones with a ring roller that never does a "perfect" job).

    this one...lets you specify the angle of top and bottom... very useful.

    ends come out square (or angled as intended for producing bends...)

    just beware that, as freeware. it has some trick of adding 1 second of delay when opened for every day installed... mines reached about 18 minutes now... the developer expects people to pay for it? hmmph!

    then you get something that sort of looks like this...


    label each piece as you print it or its easy to get confused! the more pieces the better. nice gradual smooth curves.
    nasty sharp changes in angles do NOT work well! (ie, this particular pipe was about 30 pieces all up, and was possibly my best one... i made one in about 8 pieces...while it had a nice powerband, it really did not like getting up there, mainly due to turbulence in the gas flow...)

    here we go...worst pipe i ever made for performance :)

    months? takes about a day, even less... when you actually put your mind to it. spend longer figuring out the bends etc than the actual making of said pipe...

    PS... use software that works for designing the pipe or you really are just wasting time and resources ;)
    Last edited: Jul 28, 2016
  11. LRSimons

    LRSimons Member

    Dang, that's a nice looking pipe! Must have been fun doing all the welds. Unfortunately, I found out the software existed just about as I was wrapping up. The project started out as a experiment (throwing crap together whenever I had a few free minutes), but after the time it took to stitch it all up and run nicely I could have made a nicer pipe.

    I'm thinking it could be fun to throw the cone templates in CAD and cut out a pipe in 20 minutes on the plasmacam :). Will have to try for the next one. Heck, one 4X4 sheet of steel could probably make ten pipes.

    I notice for the header section, the diameter increases slowly all the way from the start. Does this have any benefit as opposed to a constant diameter from the exhaust port to the first cone?

    Also, did you roll the header out of sheet from lack of access to tube bending equipment, or the reason above?


  12. LRSimons

    LRSimons Member

    I just walked out to the mailbox and found my fresh magneto there. The bike will be back online and I'll have a chance to do some more testing tonight.

  13. jaguar

    jaguar Well-Known Member

    a tapering header may have some benefit for high reving motocross bikes but not for these engines unless they are modified for high revs
  14. HeadSmess

    HeadSmess Well-Known Member

    it DOES have a straight header! its just that its only about 3omm long!

    the tapered header is a function of the particular software i use... find me one motocross 2stroke with a straight header pipe. or any decently tuned twostroke, for that matter.

    i wont try to explain the pros or cons, but shall simply do some cutting and pasting...

    Diffusers should have an 8-
    degree included taper for maximum energy recovery, and an outlet area 6.25-times that of
    their inlet. Thus, a diffuser to be attached to an inlet having a 1.5-inch diameter should
    have an outlet of 3.75-inch diameter.

    Although these diffuser diameters, tied to the 6.25 constant, remain the same,
    diffuser length may be varied, as there are reasons for using diffuser tapers other than 8-
    degrees. That taper does the best overall job of energy recovery, but it is possible to get a
    stronger inverted wave with diffuser tapers greater than 8-degrees, at the expense of wave
    duration. Conversely, one also may extend the wave duration by accepting some
    diminishing of its amplitude with shallower tapers. A long wave duration spreads an
    engine's power band; a short-duration wave with high amplitude is best for maximum
    power at peak revs. Diffusers having tapers of more than 10-degrees return a wave of
    such brief duration as to be almost useless even for a road racing engine coupled to the
    rear wheel via a multi-speed, ultra-close ratio transmission, and also are rather inefficient
    in terms of energy recovery. For that reason, I do not recommend that you use a diffuser
    taper greater than 9-degrees even when planning an expansion chamber for a road racing
    machine, as you may otherwise find it impossible to keep the engine operating within its
    power band. At the opposite extreme, do not try to use anything below a 5-degree taper
    diffuser in an expansion chamber for an off-road motorcycle. You will find that even a
    5-degree taper results in a diffuser that is almost impossible to accommodate within the
    system's tuned length, and that it returns an inverted wave to the exhaust port too feeble
    in amplitude to be very effective in scavenging a two-stroke engine.

    there you are... from the horses mouth.

    ok... if you have a constant diameter header, you cannot retain a desirable taper to achieve required diameters, whilst keeping the length within the restraints imposed by the desired RPM of operation. the tapered header allows for a wider powerband as the angle of taper is kept within limits...

    it also allows for several increases in taper, that adds to the kick of the band. its hard to tell, but that particular pipe is made up of 3 different tapers.

    wide, abrupt angles...hell of a kick, but restricted rpm range. very restricted.

    straight, almost nonexistent angles... barely any power gain, spread over a wide range.

    use of several tapers... hell of a kick over a wide range :)

    thinking about it, due to the "standard" single speed on the average MB... we want tapered headers! make the taper less than 8 degrees...less overall gain, but over a far more useable RPM range.

    the ideal header is actually a ...ummmm...logarithmic? gradually increasing taper, like a trumpet, but owww... too hard to fabricate.

    (P.S. i did eventually have a go at trying a hydroformed pipe using the pressure washer... initial results were promising, but the welding of the two profiles together poses the hurdle! the edges need to be pre-rolled before welding so the seam isnt being forced open. once the seam splits, its a prick to re-weld the tiny little pin holes. plus i no longer have an operating TIG...)
  15. HeadSmess

    HeadSmess Well-Known Member

    my christmas wish list.... a new plasma cutter, and a new tig welder... yep, you could cut out the profiles in a matter of minutes, make ten pipes a day!
    Frankfort MB's likes this.
  16. LRSimons

    LRSimons Member

    That's fantastic! It makes sense that the powerband is adjustable in location and duration.

    The overall diffuser angle for my pipe is probably leaning towards the useless range. The powerband may be too wide to note a definite point of the pipe kicking in. Wish I had a tach to collect some concrete data. I'll update when I have it running nicely. I enlarged the jet a tad too much and have to dig around for some solder to resize.

    Hydroforming is awesome! Seems like it has the possibility of being faster than rolling cones. Either way, it's a better option than bending 1" tube!

    I wonder if you could turn a couple of old trombones into an expansion chamber...
    It seems like most open too aggressively for our purposes. That would be hilarious though.

  17. zippinaround

    zippinaround Active Member

    I built one of jaguars pipes a while back , just recently added a belly stinger and capped it off , but it seems to be even louder now then without not sure maybe the stinger isnt long enough . I tried upload a pic but it came out sideways haha
  18. jaguar

    jaguar Well-Known Member

    something is wrong because the wave pressure at the belly is next to nothing whereas at the end of the baffle cone it is strong. that pressure equalizing into the atmosphere is what creates the sound and so the higher the pressure the more sound (noise) there will be. A belly stinger is always quieter. Maybe there is a leak. show me a picture or make a drawing using Paint.
  19. zippinaround

    zippinaround Active Member

    this is before i was using a wd40 can muffler didnt last long tho not the best pic but yeah i dont have any fancy equipment i made with a hammer and a few buts of round bar its not exactly to your spec but does significantly increase power compared to the stock garbarge , that was my main concern noise i dont mind once there is power hahah , but my avatar has the exhaust fitted its just sideways haha will try upload something better, this might be better my stinger is only a few inches long but i didnt insert in the belly just welded to the outside flush
    Last edited: Aug 2, 2016
  20. butre

    butre Well-Known Member

    that pipe is looking a little skinny. I couldn't tell you jag's measurements off the top of my head but I'd wager those measurements have nothing to do with his design.