Instructables
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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efrem.hug9 days ago

What are some options then?

dave.bolger23 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.

00buckshot11 months 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
StretchCT1 year ago
good writeup. i enjoyed all the practical tips. thanks
rchanter1 year 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.
rchanter1 year 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)  rchanter1 year 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.

rimar20002 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)  rimar20002 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)  rimar20002 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)  rimar20002 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.
TSC2 years ago
Nice job good infomation!
Furf2 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)  Furf2 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 :-)
hoomzi2 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
henway2 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-Geo2 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.
menahunie2 years ago
Your probably that guy that goes by my house at 3 AM sounding like a horny weed eater on steroids..
BlackSmoke2 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.
WEDGIE2 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)  luckylulujoe132 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...
bishopdante2 years ago
Another way to make metal tubes:

http://www.youtube.com/watch?feature=player_embedded&v=DGszBVy3cgU
maxpower492 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)  maxpower492 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.