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movable expasion chamber

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I thought that this topic would have already been covered but has anybody thought about putting a movable expansion chamber on a mx bike. Ive seen it on a couple things like race boats but not on a bike. what do you guys think

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As to "moveable" do you mean that lengths, etc. can be adjusted easily for testing and adjustment?

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i mean like the length and width can be adjusted on the fly to suite the particular rpm. like it would shorten up at high rpm to make more power. kind of like a power valve opens at high rpm

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I've seen RC 2 stroke pipes that do it, using a spring loaded internal baffle

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I have also thought about this before, its packaging that would make it difficult, its hard to explain what i thought you could do.

Basically have a normal size pipe, or one more suitable for bottom end power, ie, longer, then have a 'funnel' piece that can slide forward and back from the rear cone, internally.

You would need a 'straight' section to be able to do it, and that would be hard to do on a mx bike.

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i don't think you'd necessarily need a straight section just a carefully crafted sliding mechanism. i really think the biggest disadvantage would be the weight of the whole thing. If you could make it really light it'd be nice. maybe just make the entire expansion chamber out of aluminum with a carbon fiber guard in the front for rocks and stuff.

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If you are really going to do it, Titanium would be the way, then Ceramic coated (if you can do that on Ti?) for heat retention.

I wish i was clever enough, and had the time/skills/money and equipment to make something like this.

The other thing i thought, which might be totallllllly out there, would be a single cylinder outlet, that splits in to 2 smaller diameter pipes with the correct combined volume, but differing lengths/ cone angles.

I think maybe peak power would be down, but power spread could be better?

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The dimensions of the pipe determine the resonance frequencies. When the resonant frequencies of the pipe and frequency of the engine (RPM) jive, the bike is "on the pipe". Most of the manufacturers have found it simpler to use the powervalve to alter the resonant frequencies of the pipe. This is the purpose of the sub-ports on 3 exhaust port powervalve 2-stroke engines. This was also the concept behind Honda's ATAC power valve on the 84-85 CR250. It is also the reason Suzuki has a solenoid valve to introduce cool air into the exhaust stream of their 250 2-strokes. Changing the length of the pipe is only one way to change the resonance of the pipe to give a broader power band but there are many, (while not simpler) easier to implement ways to do it.

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Hi there,

just got a 250 sx for play´n around. Would like to do some endurocross with it and some 3 or 4 hour hard-enduro. We altered the bike a little. Put a rekluse in ( like in my regular enduro CRF X ), let the cylinder down by allmost a mill and drilled out the head for the same amount. Then we put a havier flywheel and the bike feels good. But still i´d verry much like the ajustable exhaust.... Could it be done by a sort of membrane or diafragma on the exhaustchamber like on a GasGas?

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