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theflanman

Two Stroke Exhaust

16 posts in this topic

Has anyone put a header from a 250F on a two stroke 250? I'm getting a bike for my 16 year old to ride in the woods and I'd like the torque of the 250, but he doesn't need all that top end. I was looking at a YZ 250 with the header off a YZ 250F in the hopes of around 30 HP and no expansion chamber to dent. Any ideas?

Thanks in advance.

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Modern MX two-strokes are almost completely dependent on the wave tuning of the expansion chamber to even run. If you stuck a straight pipe on an otherwise stock MX two-stroke, I'd be surprised if it made enough power to get out of it's own way.

You would be much better off if you sent the head and cylinder to Eric Gorr for his low-to-mid porting.

Edited by Chokey

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Has anyone put a header from a 250F on a two stroke 250? I'm getting a bike for my 16 year old to ride in the woods and I'd like the torque of the 250, but he doesn't need all that top end. I was looking at a YZ 250 with the header off a YZ 250F in the hopes of around 30 HP and no expansion chamber to dent. Any ideas?

Thanks in advance.

Kinda like Chockey said...

But if he is a real novice simply close down or shut off the power valve so it dosn't open,it will still make the same power down low but not get the "hit".

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They do depend heavily on wave tuning, but you might be surprised to discover that an open megaphone exhaust dimensioned a little differently than the typical head pipe/divergent cone portion of an expansion chamber is also an effective way to accomplish that. Gordon Blair's writings describe this, and anyone who remembers the Greeves 250 Challenger with the "blooey pipe" will have seen (and heard) one in action. They aren't as effective as a full EC exhaust, but they can accomplish the same thing in principal. Noise, of course, is another matter. :smirk:

As far as the idea of using the 250F pipe, I think you'd actually be better off with the pipe from a WR250F. The straight through 4-stroke racing exhaust would be so completely incorrect for the two-stroke that using a baffled muffler would probably work better.

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They do depend heavily on wave tuning, but you might be surprised to discover that an open megaphone exhaust dimensioned a little differently than the typical head pipe/divergent cone portion of an expansion chamber is also an effective way to accomplish that. Gordon Blair's writings describe this, and anyone who remembers the Greeves 250 Challenger with the "blooey pipe" will have seen (and heard) one in action. They aren't as effective as a full EC exhaust, but they can accomplish the same thing in principal. Noise, of course, is another matter. :smirk:

That type of pipe will scavenge the combustion chamber, but it won't produce the return wave that helps modern two-strokes make so much power by essentially supercharging them.

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look at a trials bike pipe, or a chainsaw. it can be done, but not with yz250 cyl and yzf pipe. had an old twostroke water sump pump that used basically a straight pipe, you could start it up reverse direction too, by wrapping the starter cord around the opposite way,

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look at a trials bike pipe, or a chainsaw. it can be done, but not with yz250 cyl and yzf pipe. had an old twostroke water sump pump that used basically a straight pipe, you could start it up reverse direction too, by wrapping the starter cord around the opposite way,

Of course it can be done. On an engine that was designed from the first sketch to run with that type of exhaust. On a modern MX two-stroke? Good luck ever getting it to run well enough to actually be able to ride it off road.

I remember reading an article in a magazine (can't remember which one) probably ten or fifteen years ago where they where doing dyno tests (I believe it wan on an RM250), and just for grins they did a test run without the pipe installed, The engine wouldn't even rev over about 3500 rpm or so.

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141_0509_250shoot_18_z.gif

See the little dip in the power curves of all the bikes at just over 6000 rpm, and then the sudden rise in power immediately after? That's where the power valves are opening. So yes, you would get your thirty hp that way, but you would also lose about 3000 rpm on top.

Edited by Chokey

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141_0509_250shoot_18_z.gif

See the little dip in the power curves of all the bikes at just over 6000 rpm, and then the sudden rise in power immediately after? That's where the power valves are opening. So yes, you would get your thirty hp that way, but you would also lose about 3000 rpm on top.

Perfect! Thanks a lot.

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That type of pipe will scavenge the combustion chamber, but it won't produce the return wave that helps modern two-strokes make so much power by essentially supercharging them.

Yes, it actually does, as a matter of fact. Read Blair on the subject.

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Yes, it actually does, as a matter of fact. Read Blair on the subject.

I think the only book I still have is Jennings. But it's my understanding that the return wave is generated by the convergent cone. How is a straight megaphone going to create a reflection?

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I believe Jennings explains this also, but possibly not. I can't recall sometimes which one of the two said which particular thing. As the exhaust port opens, a shock wave precedes a positive pressure wave down the length of the pipe. As the pressure wave enters the divergent cone, it expands, aided by the heat of the exhaust, just as in an expansion chamber. And, just as in an EC, there is a trailing negative pressure zone behind that. But the difference is that with an open megaphone, the pressure wave, on reaching the end of the pipe, bursts out into the open air, exaggerating the negative pressure behind it. That leaves the interior of the pipe all the way back to the port a low pressure area that causes air to rush into the pipe in response, creating a positive pressure wave that is accelerated by the convergence (going backward) of the megaphone.

The same sort of thing occurs on a four-stroke exhaust, but the lengths are considerably different, since the goals are reversed. In the 4T, you want the low pressure wave to arrive at the exhaust port as the valve closes. There's a "reflected" positive wave that follows, but it's not useful, and arrives after everything's closed down. With the 2T using the open megaphone, the exhaust typically ran straight under the bike and terminated under the engine, so it was short enough that the reflected pressure wave arrives in time to stuff the exhaust port as it closes.

As I said, this is nowhere near as effective as a modern expansion chamber is, which is why megaphones were phased out long before every 2T had a silencer on the stinger. It wasn't about noise, although God knows the megaphones were incredibly loud, EC's just work better. But megaphones do work.

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another pic of the power curve, this one shows the valve rigged closed, then another with the valve rigged wide open. where they cross is where you want the valve to open if you have the ability to test and control it. Sometimes guys go the wrong way thinking rigging it closed will make the power softer, especially in slick conditions. When actually they would be better off taming the low end with the valve wide open, reducing the quick response on the low end., Same with pipe, see guys run gnarlys where they'd been better off with a rev pipe, just for the more controllable low speed traction.

rave.jpg

Edited by highmarker

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One thing most people don't realize is that the exit point of any piping system also creates backpressure. A pipe welded into the side of a tank wall and emptying into open space generates more backpressure than even a 90 degree short radius bend. But this is all too basic to explain what goes on in a 2-stroke exhaust pipe. You need to look into standing waves to even start to touch on the science behind exhaust systems. 4-strokes rely heavily on the high speed low pressure description of gas flows but two strokes rely heavily on standing waves. As frequency changes within a fixed length exhaust system changs the location of nodes and anti-nodes change wih the frequency. A 4-stroke likes a node or low pressure in the exhaust port to scavenge the cylinder while a 2-stroke likes an anti-node or high pressure to hold the supercharged air charge in the cylinder.

So a convergent outlet may work well to bounce a wave back at the cylinder. A divergent outlet can also generate a pressure wave. The convergent outlet nozzle may work better for bouncing back a pressure wave at lower flow rates such as those seen a more conservatively tuned off-road engine that requires a wide and early pwoerband while a wildly tuned roadrace engine which requires peak power at high rpms over anything will benefit from the high flow rates available through a divergant exit nozzles.

Just a random bit of info you may or may not care about... in steam talk, convergant-divergent nozzles are used for supersonic flows and convergent nozzles are used for subsonic flows. This probably has something to do with gas engines but I haven't figured it out yet.

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