B1 modified exhaust chamber homebuild

[CookMySock]

Can anyone provide a starting point for building my own exhaust pipe from scratch? We can cut, bend, roll and weld, but don't really know where to start otherwise.

edit: for two-stroke pocket bike engines

Steve

[Kwaka14]

This has some information on it http://www.burnsstainless.com/TechArticles/Theory/theory.html

[CookMySock]

This has some information on it http://www.burnsstainless.com/TechArticles/Theory/theory.htmlThanks. Yes I had seen that. It's quite interesting, and I think I'll use their idea of varying collector length for tuning, though I wonder how practical that will be on a two stroke.

cheers,
Steve

[SS90]

Can anyone provide a starting point for building my own exhaust pipe from scratch? We can cut, bend, roll and weld, but don't really know where to start otherwise.

Steve

It's a really cool idea to make your own expansion chamber there Dangerous bastard......

In the "old days" (as peoples true under standing of expansion chambers was quite limited).... there was not really much variation in pipe design.

Nowadays, our understanding has improved somewhat, and what design you use for your expansion chamber is dependent on quite a few factors.

I'll try to make this quite simple (not intended as an insult, but bullshit baffles brains, and I don't want to confuse the situation!):laugh:

Now, the two stroke engine is really a simple piece of kit (mechanically), but it is the tuner who understands the intricate details the best who achieves the best results!

When it comes to the pipe design, you want to take the following things into account.

What sort of transmission does it have (3 speed 4 speed 5 speed 6 speed... or CVT (like your pit bike)

This is THE FIRST thing I consider when building a two stroke engine.

An engine that has 3, 4 or 5 available "gear ratios" (gears, or "speeds") has a distinct disadvantage (over a 6 "speed"), or CVT.

The problem is, such gear boxes have (generally) the same final drive ratio in top gear (that is to say, 5th gear (in a 5 speed engine) will be roughly the same "ratio" (amount of teeth) as 6th gear (on a 6 speed engine)

The reason such transmission ratios exist, is (generally) for "road" or "touring" bikes (be they 125cc commuters, or even bigger (Harley's, moto guzzi etc..... it doesn't matter if it is 2 stroke or 4 stroke.....the type of purpose the engine is designed for dictates how many ratios is has.)

The reason for this is, (generally speaking) a commuter bike (or "tourer") is (generally) designed to have a "broad spread of power", that is to say (if it is a 2 stroke commuter, as an example), it will have no expansion chamber, just an exhaust and muffler, as well as low cylinder compression low exhaust port height (time area) (and high primary compression, which, by the way is a VERY important thing to understand)

The result of such a set up, is an engine that, due to those factors (Low cylinder compression,low exhaust port height, high primary compression, no expansion chamber) will not produce startling amounts of power, but it will produce a VERY long power curve, and make good power from just above idle)

This is in stark contrast to your pocket bike (for example), which (has an expansion chamber)

Now, back to expansion chamber design!

When you have a two stroke engine (like your pocket bike) the sort of "characteristic" your require is quite different to that of, say a 125cc road going commuter two stroke.

That is to say, it must produce alot of power, and all of it at the top end.

There is essentially no "mid range" and certainly no bottom end to worry about.............you open the gas (full), the centrafugal clutch takes care of the rest....it pretty much stays at the high RPM.

So, with such an engine, peak power (in a very narrow range) is all that is required!

This, (in contrast to other engines) is a little unique!

Because, the type of "set up" that best suits this sort of power characteristic is actually the same sort of set up that was used in the late 70's!

I'm serious!

Such an engine wants high exhaust port timing,high cylinder compression,high primary compression, and all these are in complete contrast to what is used today!

Due to the above criteria, you would end up with an exhaust that is really "old school" (to say the least!)

If you look at a pocket bike expansion chamber (in particular the length of the last diffuser....the tapered bit before the "stinger" (the bit that connects the muffler), and then looked at a well set up 125cc motocross bike (or even a chamber from an Rg150), you will see that even though they have a massive size difference, the geometry is totally different!

To be 100% honest, you could piss around for YEARS trying to develop an expansion chamber for your pocket bike that would work better than the one it came with, and maybe actually go backwards in the process, but by all means, give it a shot!

The best reading I can recommend is called "The two stroke tuners handbook" by Gordon Jennings....(you can download it off the net, but it would be way cooler to buy one at a technical book shop...... stealing sucks).in particular read the chapter on "Crankcase pumping".... this can be the most important part of developing a two stroke (other than port times), as such things (the crankcases ability to act as an efficient pump) effects your pipe design.

Don't be baffled by other peoples nonsense.....read this book (a few times...even though it was printed in the late seventies if you read it carefully, it gives you the basics to design a complete two stroke engine (up to todays standards).

Good luck!

[geoffm]

AG Bell's 2 stroke book is online as a scanned PDF if you look around, as is Gordon Jenning's book. Search and the truth shall be found :-)

[CookMySock]

To be 100% honest, you could piss around for YEARS trying to develop an expansion chamber for your pocket bike that would work better than the one it came withCough, I doubt it. It's a little diamond-shaped piece of shit, LOL. I expect you could put together a much better shaped chamber in about 20 mins from a dog food tin, a vice, and hammer and pop rivet gun.

Anyway, I found some windows software to design exhaust chambers. It seems to run quite well on Ubuntu+wine. See Pipe_dsn.zip on this page. Theres a bunch of other stuff there too, but I haven't waded through that yet.
http://tzr250.com/Downloads/Pipe_dsn.zip

Steve

[SS90]

Cough, I doubt it. It's a little diamond-shaped piece of shit, LOL. I expect you could put together a much better shaped chamber in about 20 mins from a dog food tin, a vice, and hammer and pop rivet gun.



Steve

Ah, you see, there in lies the peculiar thing about pocketbikes!

The B1 is the water cooled one I think Is that correct?

I rode one a few years back I think it was called a B1.

Went pretty hard!

Anyway, here's how I would do it.

Get a electronic rev counter from a Kart shop. There are lot's available, and ones for Kartsport generally have a data logging function....that is they can record things like Maximum RPM, minimum RPM, and AVERAGE RPM (for each session on the bike)

The following numbers are just an example (I don't know what they would really be)

let's say, the average range in RPM you use in a race is from 10,000 to 11,500 (just an example, but I think it would be someting like that.....I suggest that maybe they could rev more, but the range would be about 1,500 RPM, no matter what the number are)

So, for this example, this two stroke engine has an operating rev range of 1,500RPM...... and operates between 10,000 RPM and 11,500 RPM (these are important factors in expansion chamber design, and with such an engine, high primary compression is required (small crankcase volume)....the complete OPPOSITE of what is needed for a "modern" racing two stroke)

So, if you where to design an expansion chamber to have an operatiing rev range of 1,500RPM.....(and between 10,000 and 11,500RPM).....you would end up with just a header pipe, connected to a small tapered volume, then on to a small stinger, then the muffler (which is actually just part of the stinger)

Possibly just like you have now.

If you where to make a miniature "expansion chamber" (like something from a motocross bike....which has an operating rev range of about 5,000 RPM) you would end up with something with the power delivery similar to a Yamaha Jog!:laugh:

Don't forget, expansion chamber design is dependent on quite alot of factors....... one of which is operating RPM

Just compare a 125cc motocross bike expansion chamber to a 125 cc GP125
bike chamber.......

There are VERY important reasons they look totally different.

As a point, due to the fact that there is such a narrow RPM range, AND such Operating rev range of a two stroke expansion chamber are almost entirely a function of it's LENGTH.......a piece of pipe (the correct length) with a bake bean tin in the middle (providing a resonance frequency that works at that RPM/rev range) would probably work ok!:laugh:

Serrious!

It is my experience (and opinion) that you would more that likely end up with an expansion chamber of the almost the same volume (and design) as you started with!

[Gadgit]

DB
I havent seen you bikes but they are not that old and at a guess have the later model expansion chamber on them. If your going to build a new one model it off an older chamber from say Loosies B1. The new style has a weak power band and only seams to let the engine rev to 12500rpm, where as the old one has a much larger headder into the chamber and the whole pipe is larger this pipe will open the engine up to 14000rpm.. with some other engine mods you maybe able to get it to rev to 17900rmp like my little 50 dose lol.

[CookMySock]

@Gadgit, ok we will look at Loosies bike and note the differences. We'll try and model something in software too and see if it comes out similar.

@SS90, hrm all is not what it seems.. or is it..

We'll strip the engine (have to anyway - waterpump) and take some measurements and have a play with this software and see what pretty pictures it draws for us.


Cheers all
Steve

[geoffm]

@Gadgit, ok we will look at Loosies bike and note the differences. We'll try and model something in software too and see if it comes out similar.

@SS90, hrm all is not what it seems.. or is it..

We'll strip the engine (have to anyway - waterpump) and take some measurements and have a play with this software and see what pretty pictures it draws for us.


Cheers all
Steve

While you have the barrel off it, measure the ports very carefully. Ypu need to calculate the port time-area, duration and opening point. Have a look in AG Bells book for what to do with it.
All my 2 stroke books are out on loan at present (must get them back), so can't give you the target numbers.
I used to have IMT MOTA, for modelling 2 strokes, but some cnut stole the disk and dong;e as part of a breakin so never ahd a chance to use it. It works well if you measure everything very carefully.
Geoff

[mohawk]

Are the kids gunna have a bike for the 5th Steve? we should be there next time.

[CookMySock]

Are the kids gunna have a bike for the 5th Steve? we should be there next time.Probably not the open bike.. Just the usual standard daytona and the standard B1. Slightly broke at the moment.. :oi-grr:

Always good to see you fellas there. Quite a good turnout at the moment.

Steve

[mohawk]

cool az. Got another 2 riders talked into coming down aswell (grc rep, d2 ).
Should be a bit of a laugh.