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Two stroke engines fascinate me.  They are so simple and fun that I've always enjoyed tinkering with them.  There are many ways to boost the power, one of which is by installing a "tuned" pipe.  The way a two stroke motor works causes them to be fairly noisy and a bit inefficient.  A tuned pipe has a set of cones- Divergent (meaning the cone gets bigger) and Convergent (meaning the cone gets smaller) that are built to cause "echoes" or pressure waves to reflect back, which if done properly can increase the power of the engine.  Check out this link, it will make a little more sense.

Animation of Two Stroke expansion chamber:

http://upload.wikimedia.org/wikipedia/commons/5/52/Arbeitsweise_Zweitakt.gif

As the pressure wave reflects back, it has a similar effect to turbocharging or supercharging a 4 stroke- it rams fuel and air that leaked into the pipe back into the cylinder under higher pressure, causing the motor to have more power (more fuel & air = bigger bang).  That's a very loose explanation, but there you go.

I've always wanted to build an expansion chamber... This is where P.O.J. (pronounced Podge) comes in.  This Piece Of Junk was (and is) my first motorcycle...  found it in a grain bin around '95, back when I wasn't allowed to have bikes.   Somehow, this one snuck home.  I dug it out of storage a few years ago and began this process, so follow along as I make mistakes and learn a thing or two about building your own expansion chamber!

Please note, this is NOT the only way to do this.  I think there is an easier way- done by cutting out two sheets in the correct shape, welding the edges, and the pumping ultra high pressure water in to "expand" the pipe, but I didn't have the tools at the time to do it like that.  This is just how I did it...
 
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Step 1: Equipment, Skills, and Supplies.

Picture of Equipment, Skills, and Supplies.
This is NOT an easy quick project.  It requires a lot of big, expensive tools.  I'm lucky enough to have access to a shop that has everything I needed.  There are alternate ways to make pipes, which I will discuss at the end of this instructable.

Skills you should have:

-Ability to Weld
-Ability to mark & cut metal ACCURATELY
-Understanding of and ability to use Shears, Brakes, and other metalworking tools
-Safe work ethic
-Understanding of 2 stroke engines
-Ability to use expansion chamber design software (OR, be really good at math or want to build 50 different pipes (trial and error))
-lots and lots of TIME

Supplies:

-Cold Rolled sheet metal- I used 18 gauge
-LOTS of welding rod- I used a 110v wire feed welder
-Rubber stoppers for isolation mounts
-Misc. nuts and bolts for mounting
-Duct Tape, shop towels, and band-aids.  You never know! 

Tools:

-Safety equipment- safety glasses, ear plugs, gloves, etc.
-Measuring & Marking tools- marker, scratch awl, steel rule, dividers, etc
-Squaring Shears
-Electric shears or aviation snips
-Hammer (ball pein works great!)
-Stake table with appropriate stakes
-Box and Pan Brake (could just use stake table but...)
-Welder (something capable of welding thin metal!) and related supplies- wire brush, clamps, gloves, helmet, safety gear, etc.
-Hack saw or metal cutting Band Saw
-Drill or Drill Press & drill bits
-Grinder and/or file

Other Usefull Things:

-2 Stroke expansion Chamber Software- I used 2 Stroke Wizard by Build and Click.  Cost $20.00, came with a 4T header design program and a cone printing program that breaks large cones up to fit on 8.5x11 pages you can cut out and tape together.  Great program.  I tried a few free ones, but the first attempt at making a pipe from those didnt work so well...
-The Book Two Stroke Performance Tuning by A. Grahm Bell (apparently, not THAT A.G. Bell...)
-Someone who knows what they are doing, so you can ask questions!  In my case, that was advrider.com





Step 2: A little Groundwork before you Build.

I'm not going to go into great detail, but you can't just build a pipe and throw it on there expecting it to work well.  You have to know a lot of things about your engine- port size and location, port timing, desired application, etc.  You will have to figure out all of this information.

Here is a VERY basic rundown of what you need to do:

Before you can start at all, you have to know your port timing.  Port timing is measured in degrees.  You need a degree wheel, which you can print from here:  http://www.1130cc.com/gallery/files/9/1/Degree360CCW.jpg

Remove the head from you engine.  Remove the engine side cover over the flywheel.

Set the engine at Top Dead Center (TDC)- meaning the piston is all the way to the top of the cylinder as far as it will go.  Attach the Degree Wheel to the flywheel, and align it with something on the engine or use a laser like I did.  it doesn't matter where the wheel is oriented, as long as whatever it is oriented with is pointing at ZERO.  See Picture 1.

Rotate engine slowly in the direction it runs until  you see the top of one of the ports.  Make a note of which port is opening and the degree at which it opens.  Continue rotating, taking note of when the ports open and close.  You will have to do some math to figure out the Duration- the amount of degrees each port is open- and then you are *almost* ready to plug numbers in to your software. 

I did my port timing on Sketchup, which allowed me to draw lines at the degrees I measured, then use the protractor tool to measure the duration.  The results are shown in Picture 3, with RED being the Intake duration, YELLOW the Exhaust duration, and BLUE the transfer port duration. 

***Quick side note about 2 strokes-  you will notice the yellow and blue are open at the same time.  Exhaust starts escaping the cylinder, then fresh fuel and air is being pushed in at the same time which also helps push the exhaust out.  The problem is that some of that fresh fuel and air also escapes out with the exhaust- which is the whole point of a Tuned pipe- the pressure wave is timed so that it will push most of that fresh fuel and air BACK into the cylinder through the exhaust port while the transfer port is closed but the exhaust port is still opened.  Whew.  Clear as mud!?

One more thing we need to measure before we can begin- Port locations and sizes.  This is a little harder.  There are lots of ways to do this, but I'm just going to share with you how I did it on my bike.  It's, well, sorta unique...

Remove the cylinder as shown in Picture 4.  you can see a few of the ports we need to measure.  We need to know the height from the top of the cylinder, their location to each other, and their size.

Line the cylinder with aluminum foil as shown in Picture 5.  Try to keep the foil as un-wrinkled as possible.  Take a balloon or a laytex glove, stick it over an air hose, and set it in the cylinder.  Inflate the balloon, causing it to push on the foil and indent the outline of the ports.  Let the air out and take the balloon out.  As you can see (barely) in Pictures 6 & 7, the ports have been imprinted on the foil.  Unroll the foil, and measure the size & location of the ports.  You can see my results in Pictures 8 & 9.

***Another quick note on 2 strokes- you can change the dynamics and power of a 2 stroke engine a LOT just with simple porting.  I didn't mess around with the porting (yet), but a good port job combined with a tuned pipe work far better than just a tuned pipe by itself.  Again, it depends on what you want the engine designed for- you can't have a high rpm road race engine that will also perform well as a lugging slow revving engine. 

Whew.  Got all the info we need, and we're finally ready to start the design!

Step 3: Designing the Pipe.

Picture of Designing the Pipe.
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Once you have all of your information gathered, you are now ready to design the pipe.  There are two ways to do this- by hand using the equations and information in A. Grahm Bell's book, or by using a computer program.  The first time I attempted to build a pipe I used a free online program.  The pipe came out WAY too small.   It worked, but only at extremely high rpms and it restricted the flow too much.  For this attempt, I used a program called Two Stroke Wizard by Build and Click.

It was pretty simple to use the Build and Click software- fill in the blanks with all of the information you gathered, then click go and wait a minute or two.  It spits out a nice sheet full of information including cone lengths and diameters at each end.  Using that info, you can then make the cone printouts using the cone software that came with the program.  Pretty slick!   See the attached screen captures.

Step 4: Layout & Cutting

Now that you know the dimensions of the pipe you are going to build, we need to make some patterns.  The software I used came with a sweet little add-on that allows you to print cone layouts by entering the length of the cone and the diameters at each end.   It would then generate a flat layout that could be printed on multiple pages, cut out, and taped together.  See Picture 1.

Once you have all of the cones and cylinder patterns, its simply a matter of tracing them on to your metal.  I don't like making lots of cuts, so i tried to consolidate as much as possible by lining up long cuts with each other.  You can see my finalized layout in Picture 2 & 3.

Once it's layed out on the metal, it's time to cut!  This is where the Squaring Shears come in handy- its great for the long straight cuts (see Picture 4).  I used a set of electric shears to cut out the curves, but aviation snips would probably do the job just as well (see Picture 5). 

Once all of the parts are cut out, you are ready to start forming!  As you can see in Pictures 6 & 7, I don't have the "header" or "stinger"- I used the stock head pipe cut to the length given in the design since it was the right diameter, and the "stinger" or last part of the pipe was so small in diameter I found a piece of pipe that was right and just cut it to length.

Ready to bend?!

Step 5: Forming the Cones- Slip Roller

There are two ways to form the metal for cones or cylinders.  The best way for the cylinder is to use a Slip Roller like the one in Picture 1.  It gradually rolls the metal tighter and tighter until you have a nice cylinder, then the top roller lifts and you slip the metal off the end of the roller (Pictures 2-7). 

Finished cylinder, ready to weld! (See Picture 8)

This works really good for straight cylinders, but its a little harder to do with a cone that will have different diameters on each end.  I've been told a trick to this is to pinch the short end of the cone with pliers to hold it back allowing the longer side to get sucked through faster (see Picture 9).  This didn't really work for me because my rollers were to big for the cones I was making anyways.  I think with some practice, though, it would work really good. 

The other option is to "micro-brake"- see the next step!

Step 6: Forming the Cones- Box and Pan Brake

If you don't have a slip roller, don't be too worried because there IS another way!  A sheet metal guy taught me this trick and called it "Micro-braking".  It worked great.  The theory is simple-  You can take a flat piece of metal, put hundreds of small bends in it, and "curve" it around to make cylinders or cones. 

Here's how:

Mark the cone by measuring each curve and then dividing the length by a set amount.  For example, I decided on one of my cones I wanted to bend it every 7mm.  I set my dividers to 7mm and marked that edge every 7mm.  The OTHER end is a little more tricky.  You need the same number of marks on the short end, so the marks are going to be a lot closer together.  In my case, at 7mm I had 45 marks on the big end (see Picture 1).  That means I needed 45 marks on the other side, which worked out to be a mark every 4.8mm.  Its pretty simple math because you have all of the diameters for each side of the cone from the printout, you just have to find the circumference which ends up being the length of the arc you are working with.

***Quick note- on cones that have a big size difference, you may have to cut your number of marks in half or 3rds on the short side- meaning you will use a mark on the short side twice or three times before moving to the next mark.

Okay... So that may be kind of confusing.  Lets put it this way- your markings will end up giving you a pie shaped section on the cone- always going down the center line of the cone.

Now we bend.  Slide the metal under the fingers of the brake all the way until you can line up your first set of marks.  Picture 2 shows a couple of bends done.  ONLY BEND IT A TEENSY BIT!!!  You can see in Picture 3 and 4 that I'm bending it too far.  Not a real big deal, its pretty easy to straighten out.

Once you have the cone roughly bent (see Pictures 5 & 6) take some time with a hammer and the Stake Table to pound the cones into shape.  The split needs to be as near perfect as possible to be welded.  If I had taken more time on this one thing, my welding would have gone a LOT smoother!  Get it as close as you can!  See Picture 7 & 8.  Picture 8 shows how close they should be for welding. 

Finally.  Time to melt some steel!

Step 7: Welding

Something else I've always wanted to be able to do is weld thin metal.  This project was my excuse to give it a try.  I do not claim to be any sort of an expert in the field of welding, so please take my tips and suggestions with a grain of salt... because they could be totally wrong.  This is just what worked for me.

How to weld thin metal

I feel the most important thing for making these welds work is to have the two pieces aligned as perfectly as possible.  Doing butt joints like this means there is a lot of heat right on the edge of the metal, making it very easy to melt or blow holes through it.  Not fun, or pretty.  I spent some time with some scraps getting my welder dialed in correctly (see Picture 1)- take your time, practice, and figure out what works for you!

***Note on welders-  I used a 110v wire feed welder with a thick flux core wire.  Not the best setup for this situation.  Ideally, a wire feed welder with argon gas shielding should work better for thinner stuff, or if you have access and the skill to use it, a properly set up TIG welder would be the best option. My first attempt was done with an Oxy-Acetlyne torch, mainly because I wanted to practice using it.  It can be done, but some machines make life a lot easier! 

Once you've got your welder figured out, the first step is to tack the piece together.  Clamp the ends tightly as shown in Picture 2, then tack weld (just a small spot to hold it together) it about every inch as shown in Picture 3.  Now for the hard part.

I tried a couple of different ways to weld this thin metal together, and the best way I've found so far is to "zappy-zap"...  Kind of a variation on "skip welding".  Skip Welding (see Picture 4) is when you weld a short strip, skip to a different section and weld another short strip, and keep repeating until the whole piece is welded.  It worked okay, but I found that it would build up too much heat and melt holes in my project. 

My solution was to do shorter and shorter skip welds, until my skip welds were about 1/4" long and took about 1 second to do.  Then I started just zapping it for a second, pausing for a second to let it cool slightly, zapping it for a second again (just long enough to get a nice puddle to form to ensure penetration), then letting it cool, and just repeating the whole way down the joint.  This ended up working really well for me. 

Tips on welding thing metal

1.  If at all possible, weld DOWNHILL.  You travel faster and build up less heat, making it less likely to blow holes in your project. 

2.  On the "zappy-zap" welding- if you are still blowing holes in the metal, you can also weld a spot, jump ahead about 1/4" and weld backwards into the already welded spot.  This way, the hottest part of the weld is over top of THICKER metal (because you ended on top of your previous weld) making it less likely you blow holes in it.  Weld backwards, jump ahead 1/4", weld backwards, jump ahead, repeat...

3.  If you do blow holes in your metal, using a filler rod for acetlyne or TIG welding along with your MIG welder can put enough metal in the hole quickly without getting it too hot and making the hole WORSE.  Just hold the rod in your left hand, keeping it over the hole, and weld over top of it with the MIG welder. 

Welding your expansion chamber

Individual parts:  Pick one of the cones or cylinders and start by welding up the joint as explained above.  I started with the belly of the pipe (the center cylinder).  Tack weld it together, then weld it up.  You can do one piece at a time and then weld the pieces together, or you can weld up all of the pieces and then weld the pieces together. 

Joining parts:  Again, I can't stress how important it is that the joints line up as perfectly as possible.  If you took your time during the layout step, they should be pretty accurate, but you will have to do some tweaking to make sure the ends you are joining are both perfect circles and that they line up with each other.  The stake table and grinder can be your friend here- just be sure if you grind anything it IS absolutely necessary.  Once you are sure they are lined up, tack the 2 pieces together as shown in Picture 7, then clean up the burnt flux if you are using a flux core welder, and start welding!  See Picture 8.

***Note on joining pieces-  If things don't line up great, tack it together where it DOES line up, then use your hammer and stake table to "adjust" the fit.  You can get things very close this way if you are patient...

Keep on adding pieces until you are done!  See Pictures 9-13.

So you have a pipe... How the heck is this thing going to fit on the bike!?

Step 8: Making the pipe FIT.

Whatever machine you are building this for, its very unlikely that the port comes out of the engine in a way that will allow you a straight shot out to put your pipe on... which means we have to do a bunch more cutting and welding.  We have to "curve" the pipe around both to avoid obstacles AND to make it shorter- 'cause you really don't want 3 feet of pipe sticking out behind your bike!  There are two ways to do this- the easy way, and the hard way.

The HARD way (Pictures 2-5)

This is the way that was shown in A. Grahm Bell's book... And it's HARD.  Requires a lot of thinking, planning, and way more welding.  Goes like this:

1.  Decide where you want the "bend" in the pipe.
2.  Cut the pipe straight through.
3.  On one side, starting at the widest point of the circle (the diameter) in the direction you want the pipe to bend, cut off a sliver at the angle you want the pipe to go.  See Picture 2.
4.  Weld the sliver back on to the same piece of pipe, just on the opposite side.  See Picture 3.
5.  Weld the other half of the pipe to the first half.  See Picture 4 & 5.
6.  Repeat for every bend...

As you can see, its very hard to get everything to line up nicely.  I did the entire first cone this way, and was not pleased with the results.  I ended up building a new first cone, and used the easy way on it.  I feel it turned out MUCH nicer- and while it is more angular it was so much simpler to do. 

The EASY way (Pictures 6-11)

Lots less cutting and welding.

1.  Decide where pipe is to be bent and at what angle (Picture 7)
2.  Divide angle in half (angle as measured from the centerline of the pipe, see Picture 8), cut pipe in direction you want it bent at half of the desired angle.
3.  Rotate one of the cut halves of pipe 180 degrees, weld together (Pictures 10-12). 


There are programs out there and if you enjoy math, you could sit down before you started building and figure out all of the angles for the cones and have done it in one fell swoop- basically eliminating this whole step.  I don't enjoy math that much, and since this is a one-off project, it was easier to build the pipe, then cut and fit like this. Just keep cutting, welding, and test fitting (Pictures 13-15) to the bike as you go to make sure the pipe is going the direction you want it to go.  My pipe came out better than I had hoped, but I do still think it could have been a lot better as far as fitting to the bike goes.  Hey, its only my 2nd attempt, can't complain too much!

You can see in Pictures 16-29 How the pipe turned out.  After I had experimented with both methods of bending the pipe, I went back and re-did the first cone of the pipe using the "easy" method, and it turned out much better.

Step 9: Mounting the Pipe.

Mounting the pipe can be a little tricky.  Two strokes in general vibrate a lot, at a high frequency.  The pipe is also containing pressure waves- expanding and contracting rapidly.  This can cause parts to shake and metal fatigue, so you want to be sure the muffler you've spent so much time on is well mounted with strong brackets or "Hangers" to keep the pipe from shaking, fatiguing, and eventually breaking.

To help absorb some of these vibrations, I built some "isolation" mounts.  These are not true isolation mounts in that the bolt goes all the way through the rubber and bolts solidly to the frame.  Basically, its just a large rubber washer that will hopefully absorb some of the vibrations.

Building the Isolation Mounts

1.  Drill a hole through a rubber stopper- the hole needs to be big enough for the bolt and the stopper needs to be big enough for your application.   See Picture 1.
2.  Cut the stopper in half- it doesnt need to be that tall!  See Picture 2.

Building the Brackets

Once you have the isolation mounts built, you can start figuring out where to place your pipe hangers.  According to the books, the best way to build the hangers is to use the same metal used for the pipe, but layer it so that it's twice as thick.  This way, you also have two tabs to weld to the pipe- one on each side.  See Pictures 3-4.  

You will need to build your bracket to fit where it needs to go- take your time and lots of measurements!  If you are using isolation mounts, be sure to take into consideration that it's a lot thicker so the bracket will need to be out further.

Attaching the Brackets

To attach the bracket to the pipe, tighten the pipe to the cylinder and make sure the pipe sits where you want it to be.  Tighten the bracket to the frame.  Tack the bracket on to the pipe, then remove the pipe and finish weld the bracket.  See Pictures 6 & 7.If you are bending or forcing the pipe or bracket into position, things aren't going to line up well when you remove the pipe and it can put pressure on the pipe causing it to fatigue and fail.

Place brackets so that no large section of the pipe is left hanging freely- again, thats inviting metal fatigue both from the engine vibrating and hitting bumps while riding.  In these pictures, the entire back half of my pipe is hanging free.  I plan to add another long hanger behind the brake lever, as shown in Picture 7.


Step 10: Testing, Notes, and Finishing

Picture of Testing, Notes, and Finishing
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Now that you think your pipe is "done, it's time to test.  Ideally, you dyno'ed your bike before you did all of this work, or you at least know how much horsepower your engine makes stock.  Check the fuel, oil, and whatever else is important, and fire it up! See Pictures 4 & 5

VERY important Note!- Any time you make changes to the engine, there will be a domino effect- you change one thing, other things are now no longer tuned right.  As was brought to my attention by maxpower49, I forgot to mention the domino of putting on a tuned pipe.  Since a tuned pipe will generally flow more air through it, the jetting in the carb needs to be changed to match.  If more air flows through the engine with the same amount of gas, it will cause a lean condition which can be VERY bad for your engine! 

This can be fixed by installing a LARGER main jet.  With a custom job like this, it's a process of trial and error that involves a lot of swapping of jets and lots of spark plugs.  It's safer to guess rich (too big on the main jet) and then move smaller in little steps than it is to go larger in little steps. 

Here's the quick version of how to jet your engine:  Make an educated guess (and guess on the rich or large size).  Install the jet and a NEW spark plug. Start the engine, let it get good and warm, and then shut it off.  It works best if you can actually ride it for a bit to put the engine under load.  Remove the spark plug and compare to the chart in Picture 6.  You want the tip of the plug to be a nice chocolate brown.  White or grey or totally clean means it's TOO LEAN, you need a BIGGER jet.  Oily and black means its too rich, and you need a SMALLER jet.  This could probably be an entire instructable in itself.  Hmmm......

Testing

While its running, take a moment to inspect for leaks.  One problem with welding it the way I did, if I wasn't paying close attention I would miss a few pinholes.  They are very easy to locate when the bike is running-  the pipe is under pressure so anywhere there is a hole, smoke will jet out.  Mark the holes if any and be sure to weld them up.

If you have access to a dyno, run some tests!  If not, there's always the "seat-of-your-pants" dyno.  I measured performance by speed- my bike would top out at 50 mph in 4th gear with the stock pipe.  With the first pipe I built, it would hit 50 mph in THIRD gear, but the power band was so narrow that as soon as I put it in 4th the bike could barely hold 50 mph.

Once you are satisfied with the location and performance of your expansion chamber, make it look good!  Not just because you spent so much time building it, but because its metal, and it WILL rust.  Thoroughly clean the outside of any oil, dust, or welding debris.  A wire wheel on a grinder works great!  Use a HIGH HEAT paint and apply a few good coats to prevent it from rusting.

 I haven't quite made it to this step yet, I'm still building the rearsets, kickstand, and custom brake lever for my bike.  

There you go!  I hope you have enjoyed this instructable on hand forming an expansion chamber.  Please check back for finished pics and the seat-of-my-pants report on how the pipe works.  My goal is to be done by JUNE!  The pipe is done, I just have to finish up a few other odds and ends.  


Step 11: Resources

Picture of Resources
Here is a list of things I found helpful during my build.  I will probably be adding to this as I finish up my project and remember things I've forgotten...

Software

2 Stroke Wizard by Build and Click- http://buildandclick.com/html/all_products.html

Books

Two Stroke Performance Tuning by A. Grahm Bell
Two Stroke Tuner's Handbook by Gordon Jennings
(pretty sure you can find BOTH of these for free as .pdf's if you look around...)

Links

My original build thread with LOTS more info:  http://www.advrider.com/forums/showthread.php?t=655547
Welding Tips and Tricks!  http://www.weldingtipsandtricks.com/
Wiki on 2 strokes: http://en.wikipedia.org/wiki/Expansion_chamber
Misc

Degree wheel- http://www.1130cc.com/gallery/files/9/1/Degree360CCW.jpg
T
he OTHER way to build an expansion chamber:


More on Hydroforming exhaust:  http://www.eurospares.com/frame8.htm

Expect an instructable from me sometime (not in the near future...) on how to hydroform an expansion chamber!
dave.bolger211 months ago

Dont bother buying 2 Stroke Wizard. NO CUSTOMER BACKUP. I HAD IT AND IT STOPPED WORKING. Sent them many emails and got automated reply's but nothing from them. Dont waste your money. Plenty of free programs out there that work just as well if not better. Theires is all smoke screens.

00buckshot1 year ago
where can i purchase
2 Stroke Wizard by Build and Click.
azza19801 year ago
great work on the sheet roleing and math but you realy should have tig welded it and used a gas perge line inside the pipe with something to plug the ends then have a small hole to let the used argon excape .not bagging you by any means we don't all have acces to a tig I am sure it still worked awsome
StretchCT2 years ago
good writeup. i enjoyed all the practical tips. thanks
rchanter2 years ago
If it helps the engine visually looks like a YB100 which is rated as a 97cc I did have a go at cross referencing some of the engine parts but could only get some numbers for your bike and none for mine a main dealer might have better resources.
rchanter2 years ago
Hi I'm a member of www.yammy100.com a UK forum and I can see your bike is a 100cc yamaha but not sure of the model there are a few of us that would like to make a tuned pipe, can you update the build on how well it works?

Cheers Rich.
dorkpunch (author)  rchanter2 years ago
The bike is a '67 Yamaha YL2C, also known as the Trailmaster. I have not run it much since I finished the pipe- still working on the rearsets and am trying to locate a part for the oil pump. I will definitely post back with test results when I've had a chance to ride it!
jaguar572 years ago
When you let the pipe gas exit from the belly and not from the end cone then the noise is automatically greatly reduced. No power loss either. Actually you gain some low rpm power. Then you won't be banging your head with frustration at not being able to find a truly quiet silencer that doesn't restrict the flow (and reduce the power).
jaguar572 years ago
Nice writeup but it is based on outdated info. For the latest free info on how expansion chambers work and how to design them please go to dragonfly75.com/motorbike/ECtheory.html
The info there exposes the flaws in the former ways of thinking about pipes and how to design them. If you are serious about pipes and want the best results at zero cost then check it out!
rsmith192 years ago
Very impressive!

You have managed to break down a fairly complex operation, combine it with the relevant theory and still make it readable.

rimar20003 years ago
Very interesting!

I am a stick welder apprentice, and to weld thin sheet I use a method that I invented: use an additional black iron wire, as in autogenous (Oxy-Acetlyne) welding. It is not easy, but allows some welding tasks otherwise are very difficult.

A question: the "tuned" exhaust seems like a resonant circuit. Does it enhance the motor only in a certain speed or range of speeds? Forgive me if you said that, I don't speak English and it is difficult for me to read and undestand all the text.
dorkpunch (author)  rimar20003 years ago
Yes and no... Two strokes have what is referred to as a "power band"- the rpm range where the pipe is MOST effective- just like what you are saying. You can change the rpm range by changing the shape of the cones and length of the "belly" of the pipe- creating a "tuned" pipe.

My pipe was designed to be a little less effective but cover a larger rpm range. Some pipes can cause the engine to be very sluggish unless its in a very narrow rpm range (my first pipe, for example). That type of pipe has a VERY notice-able powerband- when it hits the right rpm range, the engine really takes off.
Thanks for both responses. What about the noise?


dorkpunch (author)  rimar20003 years ago
Believe it or not, this pipe was more quite than the stock pipe the bike came with... It is possible to build your own silencer too. When I get the rest of the bike done to the point it's ride-able, I will do some more testing- a silencer may be in the works.
Years ago I bought a simple silencer, tube type. I worked very well at 100 Km/h or over, but under 90 Km/h it was very loud. The grief was that almost always the car was driven at less than 90 km/h...!
dorkpunch (author)  rimar20003 years ago
Oh, and I like your welding tip. I was basically doing the same thing, but using a MIG welder. The extra rod helps cool the puddle down so the base metal doesn't get too hot, melt, and fall out. Never tried it with a stick welder though.

I believe TIG welders *dip* the rod in the puddle when welding aluminum for the same purpose- to cool the base metal.
TSC3 years ago
Nice job good infomation!
Furf3 years ago
Sorry If i somehow skimmed past it when reading your instructable, was just curious as to what make of bike yours was? It has a great cafe look.
dorkpunch (author)  Furf3 years ago
I never did mention that... sorry!

It's a 1967 Yamaha YL100C- also known as a Trailmaster. Yamaha also built a "street" version called the Twinjet, which was a 100cc 2 cylinder two stroke. Mine was meant to be a dual sport, but I'm in the process of converting it to a cafe bike.
Interesting bike--I've got a '70 L5T-A 100cc Trailmaster, enduro (what they used to call dual sports). The bike in your picture looks very similar to mine.

Nice article, BTW :-)
hoomzi3 years ago
i did few pipes for my tomos targa, some with oxy-acetilene, some with mag, and the last one that wasnt tested yet with TIG and i'd say TIG is def. the way to go:) few more things i discovered, silencers dont affect power too drastically, so if you want your bike to be silent do a bigger silencer without doubts:) buuuut, bent stinger pipe is bad for preformance, so dont do "scooter" style exhaust with bent stinger and silencer facing to the front of the bike....
http://www.youtube.com/watch?v=S_bpV11N0zg
here's a vid of my pipe its on a 50cc 2 speed automatic bike:)

http://shrani.si/f/2L/4E/4mCdnXas/3.jpg
and heres the last pipe i made, that still needs to be tested
henway3 years ago
20 gauge is a LOT easier to work with and oxy-acetylene is definitely the way to go. Also remember to clean EVERY weld on the inside as you assemble it. Especially with your 18 gauge, it would be easier to take all the pieces with DXF file to a metal shop and have it cut for you. Make sure to coat the metal with anti-rust and then clean the edges before welding.
Y-Geo3 years ago
Very interesting, I have been building pipes for 2 strokes for nearly 35 years .and this was well written, one thing you should stress is that when making the "sliver" to shape the pipe the thin edge ends on the MAX diameter of the tube/cone. That is the only way to keep the pipe to its designed tuned length.
One book for the average rider you left out of your list is the late great Performance Bikes Tech author John Robinson's "2 stroke tuning" the other book for the "scientists" is Prof. Blair's research paper

Keep up the good work.
menahunie3 years ago
Your probably that guy that goes by my house at 3 AM sounding like a horny weed eater on steroids..
BlackSmoke3 years ago
Nice instructable! When I was researching venturis and critical flow for work I came across this concept, but it's cool that you brought it down to earth.
WEDGIE3 years ago
read Gordon Purves Blair of Queen's University in Belfast he published the formula's for building tuned pipes with rev range and and port timings.
so you can get the most ride-ability out of the bike
C'mon, lets hear it going like snot(on the pipe for the layman)! Video with sound, please!
dorkpunch (author)  luckylulujoe133 years ago
I know, eh?! My goal is to have it done by June. I will for sure have pics and vid of it running by then! I still need to rebuild the oil pump before I can run the engine, and I still have to finish the rearsets, rear brake assembly, and put the new front tire and tube on. Getting closer...
bishopdante3 years ago
Another way to make metal tubes:

http://www.youtube.com/watch?feature=player_embedded&v=DGszBVy3cgU
maxpower493 years ago
Great instructable but I was thinking, I have been working on my yamaha banshee and I added new exhaust (wish I saw this first but it oh well lol) any way if you increace the airflow of the exhaust don't you have to rejet the carb I know my banshee ran extremely lean after the new pipes.
dorkpunch (author)  maxpower493 years ago
Yes, that is very true. I will make a note of it in the testing section, thanks!
No prolblem I love two strokes too they are so simple but so powerful, most of my toys are two strokes and I am going to have to try this out on something, and thank you for a great instructable
great instuctable, funny name dork punch! I also love playing around with 2 strokes the simplicity is genius. playing with expansion chambers can be addictive, Ive built a few, it great to go on that first ride with your new pipe.
The other thing that is cool to play with on a piston port 2 stroke is putting a spacer under the barrel it changes the port timing, and can make a sedate putt putt scooter into an animal. Try making a base gasket out of 2 or 3mm aluminium, you will be surprised. it works even better if you take 2 or 3 mm off the top of the barrel, to keep the compression ratio the same.