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June 15th, 2016, 06:19 AM   #31
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It's a good question matey, but I'd say a difficult one to answer. There are too many variables now, electronic advancement as you say, tyre advancement too. Fuelling would certainly be electronically injected, with sensors for everything just about killing off the black art of setting up carburettors on strokers in different climates.
There are also the cases of riders adapting although most of the top guys cut their teeth on strokers.
Cue the "Vale would win easily, blah blah blah" LOL
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June 15th, 2016, 08:06 AM   #32
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It's a good question matey, but I'd say a difficult one to answer. There are too many variables now, electronic advancement as you say, tyre advancement too. Fuelling would certainly be electronically injected, with sensors for everything just about killing off the black art of setting up carburettors on strokers in different climates.
There are also the cases of riders adapting although most of the top guys cut their teeth on strokers.
Cue the "Vale would win easily, blah blah blah" LOL
Good points.

I really don't know much about the 2-stroke GP bikes at large. Was wondering if you could answer this, why is/was the exhaust piping design the way it was on the 2-strokes rather than the way it is on a 4-stroke?
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June 15th, 2016, 10:20 AM   #33
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Complicated but basically back pressure.
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June 15th, 2016, 12:09 PM   #34
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Good points.

I really don't know much about the 2-stroke GP bikes at large. Was wondering if you could answer this, why is/was the exhaust piping design the way it was on the 2-strokes rather than the way it is on a 4-stroke?
Good question.... I love two strokes!! Personally I really enjoyed watching the mixture when two strokes were racing with the four strokes for a short period when MotoGP switched in 2002. And if they had continued developing the 500 it would be as good as a modern four stroke MotoGP I reckon

I could talk two stroke all day cos I'm sad, but two strokes expansion chamber design is an art. From what I know It's to do with back pressure from the sound waves produced from the engine. The length of the chamber is critical to the power band width and the varying diameter and where the diameter varies alters the pressure. at the exhaust port from the engine the diameter is smaller so spent gases come out of the engine quicker, when the diameter increases, the pressure drops and exhaust gases slow up. when the engine sucks to draw in fuel/oil and air it also pulls some of the exhaust gases back into the cylinder, boosting pressure and power.

MZ discovered the benefits of expansion chamber design I think.
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June 15th, 2016, 02:25 PM   #35
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I made an expansion chamber once for my mate's 100cc Kawasaki Aggy Bike. We got the formulae out of a dirt bike magazine. The idea is to accelerate the exhaust gasses during the time that the fuel air mixture is drawn into the engine - actually drawing it into the header of the exhaust pipe as well as the combustion chamber - and then to push it all back into the combustion chamber, just before the piston blocks the exhaust port, to maximise the mass of mixture induced.
This is achieved by the geometry of the expansion chamber, the basic configuration is: header pipe, diffuser cone, baffle cone, tail pipe.
The diffuser cone creates a negative pressure wave to draw the mixture in and the baffle cone produces a positive pressure wave to reverse the flow of the gasses to push mixture back into the combustion chamber.
The timing of all this is a function of the gas velocity, which I guess is more or less constant and the stroke timing, which is of course determined by the RPM. The timing of the negative and positive pressure waves, as determined by the geometry of the exhaust system, is therefore only optimal for a narrow band of RPM and this is the reason two strokes have a so called power band.
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June 15th, 2016, 03:22 PM   #36
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Quote:
Originally Posted by JPSLotus View Post
Good points.

I really don't know much about the 2-stroke GP bikes at large. Was wondering if you could answer this, why is/was the exhaust piping design the way it was on the 2-strokes rather than the way it is on a 4-stroke?
https://en.m.wikipedia.org/wiki/Expansion_chamber
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June 16th, 2016, 02:55 AM   #37
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Not a bad wiki. The animation is good.

Kaaden rightly gets a lot of credit for the spanny, but DKW were running them on their V3 in 1951. Amazing bit of kit
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June 16th, 2016, 03:03 AM   #38
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Quote:
Originally Posted by Richy56 View Post
Good question.... I love two strokes!! Personally I really enjoyed watching the mixture when two strokes were racing with the four strokes for a short period when MotoGP switched in 2002. And if they had continued developing the 500 it would be as good as a modern four stroke MotoGP I reckon

I could talk two stroke all day cos I'm sad, but two strokes expansion chamber design is an art. From what I know It's to do with back pressure from the sound waves produced from the engine. The length of the chamber is critical to the power band width and the varying diameter and where the diameter varies alters the pressure. at the exhaust port from the engine the diameter is smaller so spent gases come out of the engine quicker, when the diameter increases, the pressure drops and exhaust gases slow up. when the engine sucks to draw in fuel/oil and air it also pulls some of the exhaust gases back into the cylinder, boosting pressure and power.

MZ discovered the benefits of expansion chamber design I think.
As another 2 stroke nerd, I have to clarify some of this (sorry). It's not just sound waves. It's better thought of as pressure waves carrying a mass flow. Which makes things interesting. Much easier to bounce air back and forth, but the mass of fuel also needs consideration. Which is where is gets difficult.
Not to mention that the speed of the gas depends on the temp in the pipe....so a hot pipe is "shorter"...
Such a "simple" engine...yet so much to consider.
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June 16th, 2016, 04:29 AM   #39
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Thanks guys...that is so fascinating the way acoustic waves are used for benefit in a 2-stroke bike engine. Thanks for linking to the diagram povol.
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June 16th, 2016, 05:58 AM   #40
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Thanks guys...that is so fascinating the way acoustic waves are used for benefit in a 2-stroke bike engine. Thanks for linking to the diagram povol.
Simple engine that took a lot of knowledge and practice to learn how to tune. Like someone said earlier, it really is/was a black art. the slightest change in humidity, elevation, temp, etc would have big affects on their performance. Very finicky. I watched a tuner at VIR dynoing tuning 125's for race teams and it was amazing how much more power he could extract from those engines that were already race tuned by another. He was finding 2-3 hp on just about every bike that i watched him tune.
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