Building an Expansion Chamber

[LRSimons]

Dang, that's a nice looking pipe! Must have been fun doing all the welds. Unfortunately, I found out the software existed just about as I was wrapping up. The project started out as a experiment (throwing crap together whenever I had a few free minutes), but after the time it took to stitch it all up and run nicely I could have made a nicer pipe.

I'm thinking it could be fun to throw the cone templates in CAD and cut out a pipe in 20 minutes on the plasmacam . Will have to try for the next one. Heck, one 4X4 sheet of steel could probably make ten pipes.

I notice for the header section, the diameter increases slowly all the way from the start. Does this have any benefit as opposed to a constant diameter from the exhaust port to the first cone?

Also, did you roll the header out of sheet from lack of access to tube bending equipment, or the reason above?

Thanks.


Logan

 

[LRSimons]

I just walked out to the mailbox and found my fresh magneto there. The bike will be back online and I'll have a chance to do some more testing tonight.

Logan

 

[Deleted member 12676]

a tapering header may have some benefit for high reving motocross bikes but not for these engines unless they are modified for high revs

 

[HeadSmess]

it DOES have a straight header! its just that its only about 3omm long!

the tapered header is a function of the particular software i use... find me one motocross 2stroke with a straight header pipe. or any decently tuned twostroke, for that matter.

i wont try to explain the pros or cons, but shall simply do some cutting and pasting...

Diffusers should have an 8-
degree included taper for maximum energy recovery, and an outlet area 6.25-times that of
their inlet. Thus, a diffuser to be attached to an inlet having a 1.5-inch diameter should
have an outlet of 3.75-inch diameter.

Although these diffuser diameters, tied to the 6.25 constant, remain the same,
diffuser length may be varied, as there are reasons for using diffuser tapers other than 8-
degrees. That taper does the best overall job of energy recovery, but it is possible to get a
stronger inverted wave with diffuser tapers greater than 8-degrees, at the expense of wave
duration. Conversely, one also may extend the wave duration by accepting some
diminishing of its amplitude with shallower tapers. A long wave duration spreads an
engine's power band; a short-duration wave with high amplitude is best for maximum
power at peak revs. Diffusers having tapers of more than 10-degrees return a wave of
such brief duration as to be almost useless even for a road racing engine coupled to the
rear wheel via a multi-speed, ultra-close ratio transmission, and also are rather inefficient
in terms of energy recovery. For that reason, I do not recommend that you use a diffuser
taper greater than 9-degrees even when planning an expansion chamber for a road racing
machine, as you may otherwise find it impossible to keep the engine operating within its
power band. At the opposite extreme, do not try to use anything below a 5-degree taper
diffuser in an expansion chamber for an off-road motorcycle. You will find that even a
5-degree taper results in a diffuser that is almost impossible to accommodate within the
system's tuned length, and that it returns an inverted wave to the exhaust port too feeble
in amplitude to be very effective in scavenging a two-stroke engine.


there you are... from the horses mouth.

ok... if you have a constant diameter header, you cannot retain a desirable taper to achieve required diameters, whilst keeping the length within the restraints imposed by the desired RPM of operation. the tapered header allows for a wider powerband as the angle of taper is kept within limits...

it also allows for several increases in taper, that adds to the kick of the band. its hard to tell, but that particular pipe is made up of 3 different tapers.


wide, abrupt angles...hell of a kick, but restricted rpm range. very restricted.

straight, almost nonexistent angles... barely any power gain, spread over a wide range.

use of several tapers... hell of a kick over a wide range

thinking about it, due to the "standard" single speed on the average MB... we want tapered headers! make the taper less than 8 degrees...less overall gain, but over a far more useable RPM range.



the ideal header is actually a ...ummmm...logarithmic? gradually increasing taper, like a trumpet, but owww... too hard to fabricate.

(P.S. i did eventually have a go at trying a hydroformed pipe using the pressure washer... initial results were promising, but the welding of the two profiles together poses the hurdle! the edges need to be pre-rolled before welding so the seam isnt being forced open. once the seam splits, its a prick to re-weld the tiny little pin holes. plus i no longer have an operating TIG...)

 

[HeadSmess]

my christmas wish list.... a new plasma cutter, and a new tig welder... yep, you could cut out the profiles in a matter of minutes, make ten pipes a day!

 

[LRSimons]

That's fantastic! It makes sense that the powerband is adjustable in location and duration.

The overall diffuser angle for my pipe is probably leaning towards the useless range. The powerband may be too wide to note a definite point of the pipe kicking in. Wish I had a tach to collect some concrete data. I'll update when I have it running nicely. I enlarged the jet a tad too much and have to dig around for some solder to resize.

Hydroforming is awesome! Seems like it has the possibility of being faster than rolling cones. Either way, it's a better option than bending 1" tube!


I wonder if you could turn a couple of old trombones into an expansion chamber...
It seems like most open too aggressively for our purposes. That would be hilarious though.


Logan

 

[zippinaround]

I built one of jaguars pipes a while back , just recently added a belly stinger and capped it off , but it seems to be even louder now then without not sure maybe the stinger isnt long enough . I tried upload a pic but it came out sideways haha

 

[Deleted member 12676]

something is wrong because the wave pressure at the belly is next to nothing whereas at the end of the baffle cone it is strong. that pressure equalizing into the atmosphere is what creates the sound and so the higher the pressure the more sound (noise) there will be. A belly stinger is always quieter. Maybe there is a leak. show me a picture or make a drawing using Paint.

 

[zippinaround]

Before

• zippinaround
• Aug 1, 2016
• 1

this is before i was using a wd40 can muffler didnt last long tho

Bike

• zippinaround
• Aug 1, 2016

torque pipe

not the best pic but yeah i dont have any fancy equipment i made with a hammer and a few buts of round bar its not exactly to your spec but does significantly increase power compared to the stock garbarge , that was my main concern noise i dont mind once there is power hahah , but my avatar has the exhaust fitted its just sideways haha will try upload something better, this might be better http://prntscr.com/c0uzqe my stinger is only a few inches long but i didnt insert in the belly just welded to the outside flush

 

[butre]

that pipe is looking a little skinny. I couldn't tell you jag's measurements off the top of my head but I'd wager those measurements have nothing to do with his design.