Expansion pipe header length

[mew905]

So I been thinking... every single expansion pipe I ever see for any 2 stroke bike has a relatively short length. Even though I "calculated" mine out to be 36 (ish) inches from piston to baffle cone, I gave it extra length to set the power band lower, but it seems that my "power band" never hits. I run an SBP pipe, and maybe its the flex pipe (more than likely, leaky, rough internals...) but I never feel the boost until I hit around 7000 RPM. Pipe is tuned for 8000 IIRC. But every single expansion pipe I've ever seen, for these bikes or even GP/street performance, isn't nearly this long. are my calculations way off?

 

[bairdco]

yes. according to graham bell's two stroke handbook, the header pipe length should be 6-8 times it's width. that's about 5"-7".

the outlet pipe (stinger) should be .6 the diameter of the header pipe.

the sbp pipe is good for a stock motor, but it's not the ideal shape and size. and neither is any of the other aftermarket pipes although the arrow pipes are pretty close.

optimal pipe configurations will vary depending on engines and modifications.

i've got a basic formula somewheres. hang on...

 

[bairdco]

i can't find my notes, but my end result is a 6" long, 3/4" header pipe, the baffle cone has an 8° taper to a 4.5" diameter cone, then a 16° taper to a 8"long, 1/2" diameter outlet pipe.

the KTM pipe is easily modified for those dimensions. "easily modified" means you need a bandsaw, welder, extra pipes, a drill, grinder, metal plate to make an exhaust flange, and the eye to cut the pipe in the right spots and twist it around to fit on your bike.

my race bike with the pipe is the only expansion chamber i've had that actually works.

 

[mew905]

Okay, yeah then mine is waayyyy too long. Mine uses a flex pipe, wraps around the downtube and the expansion chamber is hanging on my seat clamp pointing down. I think I measured it to be 33" or something like that, I have my calculations here somewhere, it has to do with sound resonance at ~520 m/s, at 8000 RPM or something it comes out to around 36" but thats way too long from the looks of it. IF you can find those notes, I'd be very interested.

 

[Davezilla]

Same here, my modified KTM pipe is the best running pipe I've used so far... My first pipe was made by modifying a header from a stock pipe and slip joining it to an old pocketbike pipe, this one gave me good low end but fell flat on top because the stinger was too small, but the KTM pipe, even as short as it is gives me decent bottom end and strong top end...

Before... (modified pocketbike pipe and 15" header)

After... (modified KTM50, no extra header length)

 

[Theon]

I think a longer pipe designed right is getting me some really good Mid range grunt.
On the Goat I made my header about 12" x 1", my diffuser also 12" with a 3" x 1 1/2" belly, main belly is 3" x 3" ,baffle cone is 4" and stinger is 6" x 5/8" id.
The motor is falling flat on the top end but has awesome wheelie pulling mid range grunt, making for a very 'ridable' power delivery with powerband kicking in from 4000 RPM and pulling strong to 7500. I realize there is quite a bit more power to be had by raising the jug and shortening the pipe, but that's for my other bike.