Expansion Chamber Questions

CMA_Decky

New Member
Dec 6, 2008
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North SF Bay, CA
I'm just putting the polishing touches on my bike now (installed a chinese two stroke, 67 cc kit). The problem is that my commute to school (well actually back from it) involves a couple of hills which require full throttle to get enough HP to get up them. My understanding is that this is pretty bad during the break in period. I've been following a couple of the threads on expansion chambers and have a couple questions.

First, would you reccomend an expansion chamber for the middle or upper end torque. Seems like as I'm going up the hill I tend to be in the mid RPM range.

Second, as you answer the first question can you point me towards a cheap source of decent chambers of that type?

Thanks in advance!
 

eDJ

Member
Jul 8, 2008
530
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Wayne National Forest
Hi, welcome to the forum ! :)

I've run several chabers on cycles in my time and want to metion that not only are there low, mid, and high rpm band chambers but there is one called a "torque tube" (which increases the engine's torque all across the rpm band)

Expansion chambers can have a strong abrupt peak spike of power and the best
thing to do with that is go with a lower numeric ratio which will mellow out that
power and stretch it over a wider rpm range.

If you go with the high rpm chamber you'd probably have to gear so you can cruise just below where the tube puts you into the power band. As these motors are one gear ratio that is something to give consideration. But a bolt on
expansion chamber is the simplest way to go to gain performance. Next to that is trying some different gear sizes and do some road testing.

Some of the guys here have found chambers at different pocket bike accessory web sites reasonable or on ebay. The best chamber is one tailored to your engine, although some claim to get performance boost
from something for other engines.
 
Last edited:
Sep 20, 2008
1,668
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Clearwater, FL
web.tampabay.rr.com
eDJ is abslutely right!

Just because a pipe looks better than the stocker does not mean it is offering a correct solution. Off the cuff I would have to say that just about anything is better, (pocket bike pipes etc.), than the stock pipe as far as performance is concerned, but ideally the pipe must be tuned to the engine.

I remember an article by "POPS" Yoshimura back in the 70's in one of the bike rags.

The short story: The volume of the expansion chamber is directly proportional to the amount of air the engine will flow. The shape is not that critical. Obvious by the fact that different motorcycle manufacturers had to tuck them in, in a variety of ways to satisfy the overall bike design.

The distance from the cylinder to the expansion chamber is dictated by port timing. When creating a tuned pipe for a new engne it is important to leave the chamber adjustable, (able to side on the header tube). By altering this distance you will get performance levels at different RPM ranges.

We would modify our MX'ers and then have to cut the expansion chambers off and retune them to go along wth the port work.

Jim
 

CMA_Decky

New Member
Dec 6, 2008
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North SF Bay, CA
Hmmm maybe a mid range. That seems to be the range I'm in as I go up the hill but that's just because I don't have enough torque to go any faster. Hows the torque tube perform? I'm guessing that it's a good at everything, great at nothing type of fix. What should I look for when I'm shopping. I took a quick look at ebay but all the expansion chambers are for specific bikes, how do I tell which one will work?
 

jasonh

New Member
Jun 23, 2008
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Longmont, CO
a larger rear sprocket will help you get up the hills as well. Only problem with that is your rpms are higher at a normal cruising speed, and your top speed will be lower.
 

eDJ

Member
Jul 8, 2008
530
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16
Wayne National Forest
The expansion chabers are generally set to a specific rpm band where the torque tubes increase torque from start to the highest rpm. Thus you just got more torque when you ran one period. No specific "boost effect" at a certain speed.

I don't know if you've looked at this animation of how a expansion chamber works or not but the last illustration shows it working. Makes it much easier to understand.

A two-stroke

This thread had a good run as we talked about chambers, listed free chamber design download software etc. Some of the guys here were into building them after taking all the measurements of their engine and plugging it into the boxes in the software wizard........which spit out the measurements to make, roll up, weld/braze together, and asembel to fit your bike.

http://motorbicycling.com/f13/expansion-chamber-1467.html

It would be nice to build a data base of the most popular engines now of the best ones to post in a thread with the free design software. Then any owner could just download the software and plug in the dimensions to calculate with. Some of the softwares are for sale but don't cost much. I was figuring some guys could go in together on them with somenone who has a shop and can build them. Like partons. Then the holder of the program could build the
chambers for them reasonably enough. With the most popular motors it would
justify turning wooden forms to roll the cones on. (like out of old wooden baseball bats) and have a frame with a dead engine in it to asemble the chambers on to tack weld them before the final welding/brazing. (in which case they could send them tack welded semi finished or fully finished and painted if they wanted) Thus if the ones buying them had a way of finishing the chamber they could do so and save even more. Or the shop could just sell the cones and tubing knocked down to ship.

Whatever. The possibilities abound.
 
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Easy Rider

Santa Cruz Scooter Works
Jan 15, 2008
2,144
4
38
Nor*Cal
Why bother with twisting welding and trying to find the right set up. Your best bet is to take a drive down to San Jose and stop by Pipelyne and pick up a quality tune pipe made for your HT motor. Just bolt it up and ride.
 

RLorange

New Member
Jun 21, 2008
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Australia
I have a similar ride home with a big hill. I have yet to try an expansion chamber muffler. I have the chamber ready but have had trouble finding someone to weld up a header pipe to the engine.

In the mean time the best simple mod I did that gave me at least 20% more power/torque at mid range revs was to lose the gasket joining the cyclinder to the bottom of the engine and use RPV silicone instead.

This does two things: Firstly it boostes compression a bit by shrinking the combustion chamber and secondly and more improtantly it lowers all of the ports thus changing the port timing for the better, my engine trucks up the same hill that it used to struggle on. Highly reccomended.

Also a high load up hills is not bad for a new engine as long as it is not too big a hill or you let the engine struggle to much. In fact it is good because the engine is at WOT with low revs forced upon it by the load this means plenty of richness and pressure to seat your rings.
 

Bikeguy Joe

Godfather of Motorized Bicycles
Jan 8, 2008
11,843
237
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up north now
Why bother with twisting welding and trying to find the right set up. Your best bet is to take a drive down to San Jose and stop by Pipelyne and pick up a quality tune pipe made for your HT motor. Just bolt it up and ride.

Well, there you go.
 

CMA_Decky

New Member
Dec 6, 2008
13
0
0
North SF Bay, CA
Why bother with twisting welding and trying to find the right set up. Your best bet is to take a drive down to San Jose and stop by Pipelyne and pick up a quality tune pipe made for your HT motor. Just bolt it up and ride.
That pipe looks pretty cool but unfortunately is a bit out of my price range. Any chance they're selling just the chamber (and I can do the assembly)?
 

Easy Rider

Santa Cruz Scooter Works
Jan 15, 2008
2,144
4
38
Nor*Cal
Your best bet is to call Pipelyne. Dean has hundreds of different expansion chambers at the shop.
 

stuartracing

New Member
Dec 9, 2008
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Gainesville FL.
I just called up Pipelyne in CA. to order one of there tuned pipes....Nice guys there....Anyone have any experiance with their pipe yet....Let me know what ya think....
Paul...