Expansion Chamber tuning

Dogtown Burner · Oct 6, 2011

Has anyone came up with a good "low end torque" header length for the SBC Xchamber? VMB said shorter the better, but I have a heavy micargiMustang and Im 200lbs of fun. I live in a hilly region, and prefer the low end torque (when I raced 2 stroke 250s back in the day, I preferred the lower end of the power curve hondas and Kawis, and Hated the revvy suzukis.

sooo....Id like to have cake and eat it to. loads (haha) of torque and great top speed. im guessing 36t sprocket and a header pipe on the "closest to stock " length. ???

F_Rod81 · Oct 7, 2011

9 to 11 inches is a good balance between good low-end torque and top-end HP. Don't include the header in your measurement. You should measure from the end of the header and 1/4'' before the chamber starts.

culvercityclassic · Oct 7, 2011

This is the chamber that I made and has great low end torque. Against everyone’s theory about header length (18 to 20in) it works great. The motor has other mods to help. The bottom line is I don't think you are going to achieve what you want with your combo. The 36 sprocket is not going to cut it. I would say a 44 with that bike...this way you will have some low end for the hills and the bike should Cruz around 30mph.

http://motorbicycling.com/f13/expansion-chamber-rat-rod-18511.html

Dogtown Burner · Oct 9, 2011

Thanks for the replies fellas. Interesting about the varying lengths of header for torque. I will just do what fits the bikes lines, which will most likely ne around 1o inches. So, are
The 36 tooth guys non stretch bikes? 44t better for the heavier, long bikes? Being an old mx'er, I'd rather make it like a CR or KX pipe, that's why all the questions. Plus, I don't wanna lose top speed with a 44t sprockett

Joseburgos · Oct 10, 2011

culvercityclassic said:
This is the chamber that I made and has great low end torque. Against everyone’s theory about header length (18 to 20in) it works great. The motor has other mods to help. The bottom line is I don't think you are going to achieve what you want with your combo. The 36 sprocket is not going to cut it. I would say a 44 with that bike...this way you will have some low end for the hills and the bike should Cruz around 30mph.

http://motorbicycling.com/f13/expansion-chamber-rat-rod-18511.html

How is the price of the chambers????

Venice Motor Bikes · Oct 11, 2011

Actually, I said a shorter the header pipe will give more high RPM power & a pipe of around 12" will give better low RPM power.

If you don't care about keeping up on the LA rides?, use a 44t rear sprocket & you'll have plenty of hill pulling power... But if you wanna keep up with us?, use a 40T.

Dogtown Burner · Oct 11, 2011

Venice Motor Bikes said:
Actually, I said a shorter the header pipe will give more high RPM power & a pipe of around 12" will give better low RPM power.

If you don't care about keeping up on the LA rides?, use a 44t rear sprocket & you'll have plenty of hill pulling power... But if you wanna keep up with us?, use a 40T.

right on, thanks for the clarification. still waiting on the custom tank, and then I'll drop by. save those wheels and the sprocket for me!

his196 · Oct 12, 2011

F_Rod81 said:
9 to 11 inches is a good balance between good low-end torque and top-end HP. Don't include the header in your measurement. You should measure from the end of the header and 1/4'' before the chamber starts.

Hey there FRod,

I'm running a 48cc Skyhawk, with stock SBP Ex. Chamber and Boost on the RT Carb. (Mikuni Knock-Off).

It's idling ok, and from start to about 20mph it just sputters trying to get up to speed, with low torque, and then POW, at about 20mph, (once warmed up too), the XChamber and 44T Sprocket kick in, and I can go up to about 28-30 with NICE torque and sweet sounding POP POP WAAAHHH! Hardly any vibration in this range.

But man, from 0 to 20mph, sputter, tug, tug, sputter sputter, pop, pop, jerk.

Sooo, that said, should I increase the length of the header to gain low-end torque?

You mentioned measuring from where to where?

Do NOT include the short, exhaust manifold attachment (header) mounted on the Jug, but measure from THAT point forward to about 1/4 inch before Xp starts right?

And if I'm currently at about 12-14 inches, should I go longer?

See pics. Thanks in advance!

https://picasaweb.google.com/lh/photo/-aGNseWmBYnN6SnjGeEpyWn4lcev8lfwo7dkIouNagc?feat=directlink

https://picasaweb.google.com/lh/photo/tLzWl_DA4z-zhJK0EuEUWGn4lcev8lfwo7dkIouNagc?feat=directlink

brnot

Dogtown Burner · Oct 12, 2011

his196 said:
Hey there FRod,

I'm running a 48cc Skyhawk, with stock SBP Ex. Chamber and Boost on the RT Carb. (Mikuni Knock-Off).

It's idling ok, and from start to about 20mph it just sputters trying to get up to speed, with low torque, and then POW, at about 20mph, (once warmed up too), the XChamber and 44T Sprocket kick in, and I can go up to about 28-30 with NICE torque and sweet sounding POP POP WAAAHHH! Hardly any vibration in this range.

But man, from 0 to 20mph, sputter, tug, tug, sputter sputter, pop, pop, jerk.

Sooo, that said, should I increase the length of the header to gain low-end torque?

You mentioned measuring from where to where?

Do NOT include the short, exhaust manifold attachment (header) mounted on the Jug, but measure from THAT point forward to about 1/4 inch before Xp starts right?

And if I'm currently at about 12-14 inches, should I go longer?

See pics. Thanks in advance!

https://picasaweb.google.com/lh/photo/-aGNseWmBYnN6SnjGeEpyWn4lcev8lfwo7dkIouNagc?feat=directlink

https://picasaweb.google.com/lh/photo/tLzWl_DA4z-zhJK0EuEUWGn4lcev8lfwo7dkIouNagc?feat=directlink

wow that chain tensioner is one heck of a lesson in physics. took a minute to visualize the action of it. ingenious!

his196 · Oct 12, 2011

Hey Dogtown...

Yeah, the chain tensioner is spring loaded on the bottom end, for torque, and simultaneously loaded for 'counter-action' on the vertical side for 'slap', and tension, to not 'override' the drive sprocket. Works pretty well.

ZERO lateral 'slap' on the topside, and near-zero 'over-ride' on the entry.

Now that the chain has stretched, I MAY get rid of the spring-loaded top side, and just go with a solid-state, anti-lateral-slap roller. We'll see.

~Chris B.

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Expansion Chamber tuning

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