Why do two strokes have pea shooter exhausts?
Discussion
I've been wondering this for a while, and I suppose it has to do with back pressure in the exhaust?
The subject comes to mind lusting over RS250's, RGV Gammas, NSR125's and RS125's etc etc. They're often equipped with two small end cans with tiny exits. Why not just one big can, with a big exit, as per four stroke sports bikes? I know two stroke exhausts are a different kettle of fish with their massive expansion chambers, but do they use a small exit to keep internal pressure high to prevent blowback into the cylinder when the port is open?
On the two stroke throttle I've tried (I've not yet ridden one, sadly), a slight twist of the throttle grip produced an instantaneous response. It revved round so freely, you'd have never guessed it was a single cylinder. Compared to the KTM Duke 390 with an aftermarket exhaust I was looking at the other day. That sounded amazing, but each twist of the throttle, in neutral and from a closed throttle, felt like there was an pressure to overcome. From an open throttle, it'd rev freely but from a closed throttle it lagged and the change in gas pressure from the wide exhaust exit was really obvious, almost like closing the throttle created a vacuum of atmosphere into the end of the can. I know the two aren't exactly comparable because the four stroke Duke has fuel injection and the two stroke was on carbs so would rev more freely, but I hope I have conveyed what I mean about the 'inertia' which must be overcome on the Duke.
And I'm guessing because I've not seen a 2T with a 4T style exhaust that there is a reason they're designed the way they are. I know many 4T exhausts have a wide exit with a sleeved down pipe inside, but some look like you could roll a pool ball down them!
Pea shooters:
The subject comes to mind lusting over RS250's, RGV Gammas, NSR125's and RS125's etc etc. They're often equipped with two small end cans with tiny exits. Why not just one big can, with a big exit, as per four stroke sports bikes? I know two stroke exhausts are a different kettle of fish with their massive expansion chambers, but do they use a small exit to keep internal pressure high to prevent blowback into the cylinder when the port is open?
On the two stroke throttle I've tried (I've not yet ridden one, sadly), a slight twist of the throttle grip produced an instantaneous response. It revved round so freely, you'd have never guessed it was a single cylinder. Compared to the KTM Duke 390 with an aftermarket exhaust I was looking at the other day. That sounded amazing, but each twist of the throttle, in neutral and from a closed throttle, felt like there was an pressure to overcome. From an open throttle, it'd rev freely but from a closed throttle it lagged and the change in gas pressure from the wide exhaust exit was really obvious, almost like closing the throttle created a vacuum of atmosphere into the end of the can. I know the two aren't exactly comparable because the four stroke Duke has fuel injection and the two stroke was on carbs so would rev more freely, but I hope I have conveyed what I mean about the 'inertia' which must be overcome on the Duke.
And I'm guessing because I've not seen a 2T with a 4T style exhaust that there is a reason they're designed the way they are. I know many 4T exhausts have a wide exit with a sleeved down pipe inside, but some look like you could roll a pool ball down them!
Pea shooters:
Baryonyx said:
And I'm guessing because I've not seen a 2T with a 4T style exhaust that there is a reason they're designed the way they are.
Yep. Because of the way two-smokes work, the combustion chamber needs very, very good evacuation to produce decent power - and that means the exhaust needs to be bob-on. Four-strokes have various back-pressure requirements, but they're much more forgiving.Discounting the fact that a 2 stroke has a power stroke every 2 strokes rather than 4 alsp.making it rev up.more quickly and depending on the size of the pistons etc you'll have different reciprocating masses changing the speed it'll rev up also.
As far as exhaust design goes I know it's a dark art in itself which is why if you're good at it, you're minted.
As far as exhaust design goes I know it's a dark art in itself which is why if you're good at it, you're minted.
Gentlemen , there is a book called "STEALING SPEED: The biggest spy scandal in motorsport history "by
Mat Oxley . I urge you all to read it . It's a mixture of a spy novel and a race documentary and the most amazing thing is it's all completely true ( I'm being deliberately vague as to not give away the story ).
It's around bike racing , two strokes and the iron curtain . It focuses on an engineer that tries to work out how best to make 2 strokes run , the man was a genius and the book is so well written . Anyone with even a passing interest in the subject of motor cycle racing will love it , friends I have recommended it to that don't like motorcycle racing have loved it too .
A bonus of reading this book is that you will have a great understanding of two stroke motorcycle exhausts and performance by the end of it .
Mat Oxley . I urge you all to read it . It's a mixture of a spy novel and a race documentary and the most amazing thing is it's all completely true ( I'm being deliberately vague as to not give away the story ).
It's around bike racing , two strokes and the iron curtain . It focuses on an engineer that tries to work out how best to make 2 strokes run , the man was a genius and the book is so well written . Anyone with even a passing interest in the subject of motor cycle racing will love it , friends I have recommended it to that don't like motorcycle racing have loved it too .
A bonus of reading this book is that you will have a great understanding of two stroke motorcycle exhausts and performance by the end of it .
Edited by wtdoom on Tuesday 28th April 11:05
The shape of a two stroke exhaust is so important for it's performance.
There's formulas to determine the taper size of the down pipe, first part of the expansion chamber, the volume of the exhaust and the final taper shape all relating to the bore size, port heights/timing & at what rev's you want the power.
The can at the end is simply to control the noise volume - look at some early expansion chamber designs and there's just little pipe sticking out the end of the chamber - and that little pipe size and length has an effect on performance!
There's formulas to determine the taper size of the down pipe, first part of the expansion chamber, the volume of the exhaust and the final taper shape all relating to the bore size, port heights/timing & at what rev's you want the power.
The can at the end is simply to control the noise volume - look at some early expansion chamber designs and there's just little pipe sticking out the end of the chamber - and that little pipe size and length has an effect on performance!
wtdoom said:
Gentlemen , there is a book called "STEALING SPEED: The biggest spy scandal in motorsport history "by
Mat Oxley . I urge you all to read it . It's a mixture of a spy novel and a race documentary and the most amazing thing is it's all completely true ( I'm being deliberately vague as to not give away the story ).
It's around bike racing , two strokes and the iron curtain . It focuses on an engineer that tries to work out how best to make 2 strokes run , the man was a genius and the book is so well written . Anyone with even a passing interest in the subject of motor cycle racing will love it , friends I have recommended it to that don't like motorcycle racing have loved it too .
A bonus of reading this book is that you will have a great understanding of two stroke motorcycle exhausts and performance by the end of it .
A very good book!Mat Oxley . I urge you all to read it . It's a mixture of a spy novel and a race documentary and the most amazing thing is it's all completely true ( I'm being deliberately vague as to not give away the story ).
It's around bike racing , two strokes and the iron curtain . It focuses on an engineer that tries to work out how best to make 2 strokes run , the man was a genius and the book is so well written . Anyone with even a passing interest in the subject of motor cycle racing will love it , friends I have recommended it to that don't like motorcycle racing have loved it too .
A bonus of reading this book is that you will have a great understanding of two stroke motorcycle exhausts and performance by the end of it .
Edited by wtdoom on Tuesday 28th April 11:05
I've read it...I can't remember any of the technical details...
Two strokes are very different animals to four strokes - they don't even need to have valves (although many do), in the most simple form relying instead on ports in the cylinder that are opened/closed by the cylinder moving past. Obviously they also get through a whole combustion cycle 'suck-squeeze-bang-blow' in one revolution rather than two. Thus they struggle rather more than a 4 stroke to both fill the combustion chamber and scavenge the exhaust gas - largely because you're trying to move the mixture in a much shorter time.
The exhaust design has a pretty major effect on efficiency - tuned pipes are quite common; decent article here explains the concept better than I can:
http://en.wikipedia.org/wiki/Expansion_chamber
Note particularly that you're 'tuning' the pressure wave to arrive back at the exhaust port in time with when you want to stop the outflow. Obviously these are effective over a narrow range of RPM.. two strokes tend to be peaky anyway - largely because there is much less flexibility available to control gas flow through the engine than there is with a 4 stroke, so the design will be 'optimised' around a particular powerband, outside of which things aren't really working very efficiently.
Edit - found the expansion chamber.. it's hiding in the belly pan - they are what I'd call a tuned pipe..
As for having separate pipes, pulse tuning is a lot harder (and probably less effective) when you start with N->1 manifolds etc. Probably far more effective to each cylinder as a separate unit.
[edit to add link, and correct my lack of observation]
The exhaust design has a pretty major effect on efficiency - tuned pipes are quite common; decent article here explains the concept better than I can:
http://en.wikipedia.org/wiki/Expansion_chamber
Note particularly that you're 'tuning' the pressure wave to arrive back at the exhaust port in time with when you want to stop the outflow. Obviously these are effective over a narrow range of RPM.. two strokes tend to be peaky anyway - largely because there is much less flexibility available to control gas flow through the engine than there is with a 4 stroke, so the design will be 'optimised' around a particular powerband, outside of which things aren't really working very efficiently.
Edit - found the expansion chamber.. it's hiding in the belly pan - they are what I'd call a tuned pipe..
As for having separate pipes, pulse tuning is a lot harder (and probably less effective) when you start with N->1 manifolds etc. Probably far more effective to each cylinder as a separate unit.
[edit to add link, and correct my lack of observation]
Edited by upsidedownmark on Tuesday 28th April 11:29
Yeah, really great book "stealing speed" the story of the MZ race team behind the iron curtain...and the contribution to the success of the Japanese manufacturers in 2 stroke racing of the 70's onwards....
also the Stan Stephens book is good for 2 stroke tuning history
http://www.amazon.co.uk/Mechanic-Lucky-Stan-Stephe...
also the Stan Stephens book is good for 2 stroke tuning history
http://www.amazon.co.uk/Mechanic-Lucky-Stan-Stephe...
There was a time when 2-strokes had (deafeningly loud) 4-stroke type exhausts.
DKW split-single
https://www.youtube.com/watch?v=uRAp8PUoS1Y
DKW also came up with the first expansion chambers.
1953 DKW 350-3
Then Walter Kaaden & MZ, and everything up to the present day
DKW split-single
https://www.youtube.com/watch?v=uRAp8PUoS1Y
DKW also came up with the first expansion chambers.
1953 DKW 350-3
Then Walter Kaaden & MZ, and everything up to the present day
You can run a two stroke on large diameter pipes, it would just give you different performance characteristics.
The tuning of the backpressure in a high power output two-stroke exhaust system is as key as the tuning of the fuelling on a four stroke.
The expansion chamber method of tuning didn't arrive until the late fifties, early sixties. Until then, two strokes had normal diameter pipes and if anything were noted for their torque; hence Scott (two stroke pioneers in the early 20thC) only ran two gears. That's all you needed with a 500cc two stroke
The tuning of the backpressure in a high power output two-stroke exhaust system is as key as the tuning of the fuelling on a four stroke.
The expansion chamber method of tuning didn't arrive until the late fifties, early sixties. Until then, two strokes had normal diameter pipes and if anything were noted for their torque; hence Scott (two stroke pioneers in the early 20thC) only ran two gears. That's all you needed with a 500cc two stroke
bass gt3 said:
y2blade said:
The fact that this question is being asked makes me sad.
^^^^^ This....Alex@POD said:
bass gt3 said:
y2blade said:
The fact that this question is being asked makes me sad.
^^^^^ This....
And to be blunt, I could sit anyone from this site with a chap I know who used to design 500GP exhaust systems and they'd be baffled ('scuse the pun). It's one of the most complicated things I've ever had anyone try and explain to me! I get the principals, but actually applying them is another world of complicated so I don't think people can be blamed for not understanding it!
Even worse than not knowing about the exhaust is that I literally have zero knowledge about anything 2stroke. I've never ridden one, never really seen one. I've seen the odd one on the road.
I've only been riding bikes for 4 years and no one in my family is into bikes so I've not known any older history really.
If it was made before 2000 I'm completely lost which seems like a shame.
I've only been riding bikes for 4 years and no one in my family is into bikes so I've not known any older history really.
If it was made before 2000 I'm completely lost which seems like a shame.
Tim85 said:
Even worse than not knowing about the exhaust is that I literally have zero knowledge about anything 2stroke. I've never ridden one, never really seen one. I've seen the odd one on the road.
I've only been riding bikes for 4 years and no one in my family is into bikes so I've not known any older history really.
If it was made before 2000 I'm completely lost which seems like a shame.
It is a shame, but understandable.I've only been riding bikes for 4 years and no one in my family is into bikes so I've not known any older history really.
If it was made before 2000 I'm completely lost which seems like a shame.
I caught the tail end of the two-strokes by buying an RS250, and I'd recommend you try and have a go on one, they're immense!
srob said:
Alex@POD said:
bass gt3 said:
y2blade said:
The fact that this question is being asked makes me sad.
^^^^^ This....And to be blunt, I could sit anyone from this site with a chap I know who used to design 500GP exhaust systems and they'd be baffled ('scuse the pun). It's one of the most complicated things I've ever had anyone try and explain to me! I get the principals, but actually applying them is another world of complicated so I don't think people can be blamed for not understanding it!
It's sad because 2 strokes are dead and as a result there are people that don't even know about them or how they work anymore
This question would never have been asked or needed to be asked a few years ago.
Of course I wasn't referring to someone asking for an answer to a question because they don't know.
Alex@POD said:
bass gt3 said:
y2blade said:
The fact that this question is being asked makes me sad.
^^^^^ This....Sadly 4T for new bikers is pretty much the only option and I think thats sad. Little (and big
) 2T's have their place...As for why, here's a brief explanation...
Whilst a 2T has a power stroke every revolution, they are horribly inefficient because for a large period of the stroke, the inlet AND exhaust ports are open at the same time. This causes fuel to pass right through the cylinder and into the exhaust, unburnt and having given no work.
The expansion chamber is actually a resonance chamber in that it creates a resonance wave that pushes the unburnt mixture back into the cylinder, thus aiding the cylinder charge. So as the exhaust gases leave the cylinder, they expand. As they expand they slow down and this expansion and speed is controlled by the dimensions of the pipe. Finally, the gases hit the rear most cone and are reflected back. When this reflection is times to coincide with the exhaust port opening, the pressure wave stuffs the unburnt mixture back into the cylinder, hence making more power.
Whilst the dimensions of the expansion chamber can vary, there's several factors to the sizing of each part of the pipes, from port window to belly diameter and length and end cone length and angle, each of these will affect the 'power band' accordingly. As such, they're always a compromise between screaming top end with a very narrow power band to more torquey designs that sacrifice absolute top end for a better midrange and spread of power. These are all dictated by other factors like engine bore, stroke, exhaut port window dimensions, power valves... the list is long!
As for the stinger, the tiny diameter pipe out the back, it doesn't actually have to be connected at the very rear, it can betaken from almost any part of the pipe. The rear cone is the reflective cone, it's job is to bounce the pressure waves back to the cylinder. So the stinger provides an exit route for the gases, however, it too has key dimensions. To narrow and the cylinder will overeat, too fat and you lose power. Its size is again a function of the overall port window and pipe requirement. At the end is the can. Not totally necessary except for noise considerations.
As said, the design of a good 2T pipe is a real science, although there's some excellent books on the subject
Everyone should have a little 2T in the garage...
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