Experimenting with exhuast and Expansion Chamber s...

Springfieldscooter

Springfieldscooter

Member
After breaking in my little china girl, I decided to begin experimenting with different exhaust in hopes of getting a little more speed.
I started with a Garmin GPS speedometer so that any and all changes could be recorded. Its a very handy tool for this type of work, and I highly recommend getting one. Trying to calculate changes without one is a waste of time! Without a speedometer, it seemed like louder equates to faster, which is not true!

So here is the bike Im working with prior to the experimentation....

Here is a link to my build thread:
http://motorbicycling.com/showthread.php?t=59263
 

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The first thing I changed was the stock muffler end cap. I drilled 3 holes in the end, like so many do. Sounded louder, but very little increase in speed.

Then I removed the end cap and baffle all together....Very loud, and actually lost a little speed.
 
I thought the bannana chamber (fat cat) would surely be the ticket! I liked its design where as it had a short header length, and increased in diameter right from the beginning. I installed it without any modifications. I saw an increase of aprox 1 mph. I was surprised as to how quiet it was. I was expectiong a loud ring-ding sound, but it didn't happed. I concluded that this chamber must have a baffle inside. So I removed the chamber and cut it open....It not only had 1 baffle, it has 3 baffles!
 

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Not only does it have 3 baffles, but the intake was too small and I felt it could be improved. So I fabricated a new and larger opening....
 

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This improved top speed by another 1 mph aprox. So I fabricated a longer and larger diameter stinger on the tail and re-installed...
 

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I increased top speed by 10% by going from a modified stock muffler to a custom chamber.

And Im still not done!

I have a few more ideas Im currently working on, so more to come soon!
 
Very nice work. I'm interested in changing my exhaust too. I see your chain is rubbing the rear tire just like mine did. Keep an eye on that. I have a trail tech speedo that works good, I also use a speedo app to confirm my speedo is right.
 
Mota- Yes , the new chain is still stretching. The stretching is slowing down, but still stretching!

Patchy- Being able to adjust the header length "on the fly" would be great! A carb that features an external main jet adjustment would be great also! Like a Tillotson.

Leo- The baffles are laying in my scrap metal bin. You want them ?
 
no, i was curious because i have that same exhaust on the way.
did removing them improve performance? how about noise?
 
The Fat Cat or Bananna is quiet as is, in stock form.
Removing the 3 baffles increased noise, but decreased top speed.
The intake didn't match up well with the cylinder.
The stinger diameter was very small.

I fabricated a better intake and larger diameter stinger. I also increased the length of the stinger.

Its now a little louder that it was originally, but still a lot quieter than I expected.

I thought it would be my best option for a top speed pipe, but it isn't!

Ive made 2 more expansion chambers and Im still experimenting with them....

Ill post more soon!
 
Expansion chambers need to be tuned with header length. Having an expansion chamber right off the exhaust won't add any effect other than a loud exhaust.
 
I agree with YILDS here... The pipe has to be tuned to match the engine's powerband or it's just going to make a lot of nois and go no where fast... If you can fab a new flange and weld up a banana pipe you can get a KX65 or CR80 pipe and make it fit your bike... then Hold on... The 60 to 80cc dirtbike pipes are tuned to about the same rpms a souped up CG would run so they tend to work really well without doing a lot of mods, and what's also nice about them is the slip on silencers when you need to be quiet but don't want to give up the performance...
Used dirtbike pipes are a dime a dozen on ebay and you can usually get a good one for under $50, just don't buy any if they're dented.
Last time I was looking at pipes I snagged up 2 KX65 pipes for $40, one had a few dents so it didn't get used, but the other was nice, then I got a used Pro Circuit pipe off a CR80 for $50 that's in excellent shape other than a little surface rust.
 
Back in the days of 2stroke superbike racing, one of the top tuners was asked how big an expansion chamber should be for best performance.

He replied, "when it no longer fits on the bike. "

I've found that to be basically true.

I never had any success with pipes "made" for china girls. Most of them are modified scooter and goped pipes.

I built one from a ktm 80cc pipe that was 6" diameter in the largest spot. Took a ton of tuning to get it right, and unfortunately, I can't describe it all, and wouldn't work for anyone unless you had the same set up as I did.

And dents won't affect it, unless they're so bad it's crushed. I had to dent mine in various spots to clear the frame, and it worked fine. The waves will travel over the dents and bends, but they don't like right angles.
 
It depends where the dents are... my last pipe was just plain ugly and I basically smashed almost the entire belly section down so it would fit between my frame and pedal cranks, it still worked as I intended it to. Now if there's a dent in the baffle cone area it can effect performance but not drastically enough for most people to notice... a few pea size dents on a baffle cone on a dirt bike type pipe would go un noticed since the taper is long and smooth, but a spoon size dent on a street racer with a really narrow power band it'll be more noticeable, but anyone running a pipe like that for general street riding would probably hate it to begin with since there's not a lot of power below or above where the pipe is tuned to, then there's a ton of power that seemingly comes from out of nowhere, but only lasts for a few hundred rpm then it falls off as quick as it comes on if reed past this tuned rpm... these style pipes would never work with single speed bikes anyway.
Here's my last pipe... there's nothing pretty about it but it pushed a heavy mountain bike with 225 pounds of me on it 43mph thru a stock nt carb and with a 44T rear sprocket.... this pipe is crushed and dented everywhere but the baffle cone...
v96BbZ.jpg


the reason I said not to buy the dented ones is because there are plenty of dent free ones to choose from, people can smash and dent the pipes where they need to be for clearance and all once they get their pipe, and if appearance counts, ya definitely want to start with a good dent free one, it's just a lot easier to start with one that's straight and dent free than try to pull them out of a dented pipe... and the difference in price is almost negligeable... my other bike looks a lot better so this pipe wouldn't be acceptable on that bike, and I'm also rebuilding the silver bike with a new frame so it's also getting a new pipe and I'm not going to be routing it low because of the hassle of welding, cutting, more welding and cutting, then smashing the whole thing to fit... it still worked but damn it's ugly...
 
Well, yeah, if you're gonna buy a used pipe you don't want one that's all banged up. Unless it's a coupla bucks at a swap meet.

And instead of "dented," I shoulda clarified that.

I used an old seatpost and a hammer to make a smooth roundy indentation to conform with the frame.
 
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