Home Made Expansion Chamber

Motoized Bike Guy ... You mentioned making a home made exp. chamber. I have one on my Cranbrook build, and am making another one for my Hiawatha MaB. The first one turned out OK, but makes the 66 cc engine run rich when cold, and improves to only slightly rich when warmed up after a couple of miles riding. The second one will be slightly shorter in hopes of running better. I'd be interested in seeing pics of your's and maybe dimensions and performance characteristics. The two bikes I've put together so far don't allow pedaling with the stock muffler, so I've never run a MaB with a stock muffler (Kings black cat catalyst muffler ). The first exp chamber bike goes 25 mph cruise, and 32 mph top with 37 mph down hill. The diffuser is 10.73 in. and the baffle cone is approx. 4.96 in. with a 2 in. straight section in between. The second epx. chamber will be 8.10 diffuser x 3.90 baffle cone and no straight section. I'm going to try a smaller jet size on the first one to see if performance improves.

I'll be watchin' you guys!

I'm all for an abt 20" header pipe, before diffuser cone, with a longer chamber than mentioned. ...pretty much with bottom end torque in mind... or actually, to say, a wider torque band. Bottom end is pretty much a function of compression ratio, and the port timing... ????

PL's #2 will be real pipey.
just kinda difficult to ride, IMO...
all the best power it will produce will be at the top of the rpm range, at least as compared with the first dimensions he listed.

Get a second opinion.
I'm certainly not an expert, but I won't tell how long I've been making mistakes!
(Probably cuz I can't remember back that far anymore)

Best
rc

Hi Rustycase ... I appreciate your input as I'm just experimenting with exp. chambers. I found a web site for making cones from someone on this site, and also read the 2 stroke handbook also found on this site that had the formulas for exp. chamber design. The first one I made was kind of an average of all the variables. The second one was the shorter version, and I'm making a third one that is the longer version. I'm adding threaded connections so I can switch the 2 nd and 3 rd exp. chambers to test their performance. You mentioned that the second one might be pipey and difficult to ride. Could you explain that a little more? My lead-in pipe will be approx. 16 in. to 18 in.

Hey PL,

ur threaded sections are pretty fancy. don't know that I've seen that before.

16-18" for the header pipe should be dandy.

Pipey... that's when the engine will start and run, well, perhaps, and move on down the road... when the rpm eventually gets up to 7 or 8k it comes alive and ZINGS up to 12k !!! all the power is developed at way-top rpm.
Basically, NOT very useful, but will definitely make lots of noticeable power.

Good for all out racing using 6 speed transmissions, and stuff like that.

People racing for all-out top speed sometimes have the stinger on the Xchamber adjustable. It will go in and out, by a lever control, to fine tune the output when they are actually on their speed run. It works, probably cuz a chamber will kill low rpm power and make the motor darn near un-runnable and they must make some sort of adjustment as they come up to speed for their run.

I'd like to learn about the effect of a center tap on the Xchamber, so it does not have to be soo long. the moped guys use them. They got the same lack of space issues that we do...

Best
rc

rustycase ... Thanks for the explanation on what performance I might expect with the smaller exp. chamber. I have it made already, so will give it a try to see what it does just for my own knowledge of exp. chambers. Here's the dimensions : diffuser cone: inlet dia. = .75 in. outlet dia. = 1.90 in. length = 8.10 in. ---- baffle cone : inlet dia. = 1.90 in. outlet dia. = .50 in. lenght = 3.90 in. I'm adding copper threaded connectors to each end so that it can be changed out to another size if performance isn't good. I've read other posts that mention putting the stinger/ tail pipe in positions other than straight out the back and it would be interesting to know more about it. For my first test of this chamber, i'm going to run it directly into a black cat muffler, since I'm more interested in quiet exhausts.

Here's the dimensions of the first exp. chamber already installed on my Cranbrook: Diffuser cone : inlet dia. = 1 in. outlet dia. = 2.5 in. length = 10.75. Center section : dia. = 2.5 in. lenght= 2 in. Baffle cone : inlet dia. = 2.5 in. outlet dia. = .75 in. length = 4.95 in. The lead-in tubing from the engine is .75 in X 12 in. connected to more lead-in pipe of 1 in. X 12 in. The stinger/ tail pipe is a 3/4 in Briggs muffler.

Here's some comparison pics of the second exp chamber that will go on my Hiawatha bike. Both bikes have the Kings Motors 66 cc Skyhawk engines on them. As you can see the second one is smaller in length and diameter and I'm going to try using the black cat muffler instead of the Briggs type. Diffuser : inlet dia. = .75 in outlet dia. = 1.90 in. length = 8.10 in. No center section. Baffle cone : inlet dia = 1.90 in. outlet dia. = .50 in. length = 3.90 in. The lead-in pipe from the engine will be approx. 16 in. to 18 in. X .75 in.

Really GREAT, what you are doing, PL !!!

The comparison test will be very valuable to many people.

Have you got one of those 'bicycle computers' to mount on your handlebars for speed tests?
Or do you have a back-road section you could mark off for timing?

It would be valuable information to have.

Best
rc

I've got a cheap bicycle computer from China that gets scrambled up once in a while from either bouncing around, or I've heard the electronics on the bike itself will cause it. New bike will have the CDI moved away from front of bike to seat post tube. I have a tach that I can use for rpm's as well. I got the exp. chamber mounted up yesterday, now have to take it back off, paint it and re-install it. Also need to test fit the gas tank, finish painting it, then install. It looks like we'll have our first 50 degree day next week, so I might be able to get it running for the first time. I'm glad you guys are interested in this, and I hope others will send any helpful info that they have. There's only a few different Exp. chambers that I've seen on this site, and the majority are purchased ones. For me, the purchased ones might be the easiest way to go, but they just don't look right for the old Hiawatha that I'm making.

MB Guy ... how's your exp. chamber coming along? I would like to see pictures of it. I'm sure there are many ways to design an exp. chamber, and various sizes that give different results. The one I'm doing might be worthless, but at least it will help as far as eliminating ones that don't work good enough for the 66 cc engine.

There shouldn't be too much difference between an Xchamber built for a 66cc and a 48cc motor, where the rubber hits the road in actual use.
Quite possibly a tad different stinger, either in diameter or length might resolve the difference for all practical purposes other than the ultimate-high-speed-run.

Great thread!
I was giving it more thought...
Fabricating the header tube with a 2 degree increase is a difficult task, especially when the bends required are factored in. Most will opt to use straight wall tube.
However, once a suitable chamber is determined, for the performance desired, it might be a good idea to experiment a bit with length of the header, before it enters the chamber.
This will most likely make more difference in the bottom end power, and width of the power band, chamber being kept the same...
Good luck!
rc

Great!

I'll be watching for pics!

I just reviewed the DAX thread abt the new motor he's offering... and looked at the pipe he made up for testing.
...might be gud to ask him about diameter... see what his experience is.

...the dirtbikes here in the yard all have huge pipes on them. I suspect at least some of that applies to these tiny motors.

Best
rc

The Hiawatha bike I built, is now running and has its first two miles on it. I don't know what to think of my home made exp. chamber yet, since I had to stop riding because of a fuel leak. The leak was at the intake tube for the new style CNS carb. The tube that hooks up to the fuel line was loose in the carb housing, so I've JB welded it and now waiting for that to harden. I'm really happy with how quiet the exhaust was, but I'm not sure what kind of performance it will give. I pulled the spark plug after those 2 miles, and it was wet with no carbon build up. Its way to soon to tell. Tomorrow is another 70 + degree day so I'll be out on it again. Here's a couple pictures of it.

OKAY !

Very nice looking motorized bicycle, PL !

...and now I'll have a chance for engineering criticism! lol

and a red flag. 8-:

It would seem to me, that the header to your ex system is too long for optimal power development. ..from what I have heard, somewhere around 20" is optimal.

...and the actual size of the chamber does not seem to correspond with what I have seen on other examples...

Possibly best example pics may be found in moped racing pics... the euro guys take 'ped tuning' VERY seriously and have all the nicest stuff !

I DO like the look of your pipe, it goes with the bike well, yet I don't think it is going to bring the performance increase you could get with a different configuration.

Basically, it will bring some increase in power, but not near what you could easily obtain with a better configuration, for the same amount of work.

RED FLAG !!!

I would strongly advise you fabricate some superior fender struts for the rear-most portion of both fenders, especially the front one.
...when they cut loose, it is an INSTANT 'Nightmare on Elm Street' !

if that front fender should wobble or warble, dropping down just a tad, it will grab ahold of the tire and crumple itself up against the front fork with disastrous results. In a BIG hurry!

I REALLY DO like the full-fendered LOOK, but there is a very good reason why racers forego the front fender!

Perhaps not quite so bad with the rear fender, as you can skid to a stop and retain a bit of control when the wheel locks up... I know, I've locked up my back wheel 3 times to date. (all because of shabby mechanical work, on my own part!)

Well, while I'm on a roll.... lol

Pls do urself a favor and dump the CNS!
they are junk. garbage. just will not EVER run as well as u would like. Period!

Perhaps u might try the dellorto clone that Dax sells, or a Mikuni VM 16 that Ratchetbirds is running a thread on.

It's nice work u have done, building a great looking motorized bicycle, and definitely worthy of a quality carb!

and the rag joint! lol
Before doing anything but resolving the fender issue, I would strongly suggest you make the investment in converting to a suitable rear hub and sprocket adapter from one of the vendors here.

Gosh, I hope you are not offended by criticism.
I am among those who feel it would be a dis-service to enthusiastically slap everyone on the back and give 'em 'attaboys' when improvement could be made that is beneficial to the end product. and MORE fun!

Best
rc


...and I'll see if I can locate the link I've got for a 'different' method of building expansion chambers...

.

....and GOSH do u NEED brakes!
.

RustyCase ... Don't worry about offending me with suggestions for improving the bike!! It seems motorized bicycle are like houses and cars, there is always something else to do to them. The things you mentioned are on my list of improvements, but the exp. chamber is on the top of the list. I have a Huffy Cranbrook motorized bicycle that is my regular ride, and the Hiawatha is an on going project for looks more than day to day driving. The exp. chamber on my Huffy works much better than the Hiawatha one as far as power, but the smaller one on the Hiawatha is so quiet that you can hardly hear it when its right next to you. Niether one is the best they can be. I'll be making changes to both of them, and any suggestions on what to change will be helpful. For starters, the quietness of the smaller chamber could be from its size, or because of the black cat muffler. The larger chamber gives more speed and torque, but makes the bike 4 stroke moderatly, and the Briggs muffler is louder than I want. I think the lead-in pipe on both of them are too long. Now that I have the two bikes to ride, I'm going to be able to make changes on one, while I ride the other one. It takes me a couple days to cut, weld and install a new exhaust system, and because of all the changes that can be made, I've now got something to compare it to. Thanks for checking out the Hiawatha so completely! I appreciate that you took the time to look it over so well, and what you mentioned about it is absolutely right! I do want to mention that my Huffy has the CNS carb (red filter) and with lots of patience, I had it working great until I put my homemade exp. chamber on and it now 4 strokes slightly. The Hiawatha has the newer CNS carb (performance filter), and so far has been great without any changes to it.

I'll be back when exp. chamber changes are made..... Porch