Oh I think what you need down in Chch is another fast 2 stroke to give you all the learn. I'll put a man on it.Originally Posted by Buddha#81
No8 Wire Extraordinaire
Oh I think what you need down in Chch is another fast 2 stroke to give you all the learn. I'll put a man on it.
Don't you look at my accountant.
He's the only one I've got.
Forum whore
at least this bike runs and dosent stop unlike the trusty FXR which brake conrods
"Instructions are just the manufacturers opinion on how to install it" Tim Taylor of "Tool Time"
“Saying what we think gives us a wider conversational range than saying what we know.” - Cullen Hightower
Forum whore
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Crankcase Compression Ratio
Let V1 be the swept volume of the cylinder.
V2 be the crankcase clearance volume, that is, the volume below the piston crown at BDC excluding the volume of the transfer duct(s).
Crankcase Compression Ratio = (V1 + V2)/V2
The Data:-
.................................................. . = 250cc CR = 1.50
Std GP125 crankcase clearance volume = 275cc CR = 1.46
Chamfered Crank Inlet Side ............... = 280cc CR = 1.45
Long Rod 14mm Spacer under the Barrel= 375cc CR = 1.33
I understand from Graham Bells book (1) to increase torque and reduce engine peekeness, I need increased crankcase volume to lower the velocity of the transfer streams and increased transfer period. And from John Dixons book (2) that a ratio of 1.5 is good which is what SS90 also said.
These results look wrong to meI used Bells formula (1) and got similar results. It looks like I need to reduce the crankcase volume to get a CR of 1.5. I thought I would be close or have to increase the volume on the GP. Even the STD GP125 engine is a bit low.
Totaly opposite to what I expected.
(1) Two-Stroke Performance Tuning. A. Graham Bell. First Edition. Pages 41-45
(2) The High-Performance Two-Stroke Engine Dr John C. Dixon First Edition page 67
Graham Bell http://www.kreidler.nl/artikelen/per...raham-bell.pdf
Gordon Jennings http://www.edj.net/2stroke/jennings/
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Bucket Racer/Crasher
The crankcase volume has very little to do with getting the mixture from under the piston to on top of the piston. The pressure differential across the transfer ports at the moment they open at operating speeds is mostly due to tuning specs - pipe/ex port. Gas velocity, and therefore inertia, in the ports also contributes and what is trapped/returned to the cylinder after the transfers close is due to the pipe design again and the port. The crankcases are more or less just somewhere to store air/fuel mix until it's sucked up the transfers.
I'm sure studs are waisted for a different reason than what is being suggested here.
Forum whore
I thought it was to stop the barrels seizing on to the studs, I had a H1 triple that I nearly wrecked because the buildup of corrosion had mede the fit close to interference
Forum whore
Heres the Gordon Jennings article , its from the 70's when TZs were new and just the shit
Forum whore
I tried reading one of the pages but it seems to have lost some readibility, I can email them to some brainiac who knows how to fix such things, if needed
Forum whore
Its readable, Great stuff Yow Ling. Page 3 center column midway down talks about piston seizure.
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Forum whore
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Great info on crankcase delivery ratios and inlet timing for rotary disk motors. The end of page one talks about a wide low inlet port being better than a tall narrow one of the same area.
"Case of Rotary Disc-Valve Inlet Port Engine"
http://www.edj.net/2stroke/jennings/delivery_ratio.pdf
By Kazunari Komotori and Eiichin Watanabe. Keio University.
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Forum whore
Speedpro, I think you are mistaken here.
The crankcase is more than "just a place to store the air/fuel mixture till it's sucked up into the transfers."
CRANKCASE PRESSURE IS RESPONSIBLE FOT THE FACT THAT THE AIR FUEL MIXTURE FINDS IT'S WAY THROUGH THE TRANSFERS INTO THE CYLINDER IN THE FIRST PLACE..............
I'll elaborate.....
The very fact that the Air/fuel mixture even finds it's way into the transfers (actually, it's not "sucked", but more on that later....)
is because of the fact that there is a pressure differential between the cylinder (which is somewhere near atmospheric), and the crankcase is approx. 1.5 atmospheres (22p.s.i)
Now, this principal is not a secret, it's how any carbed internal combustion engine gets it's air fuel mixture into the combustion chamber.....
Think of it like this.....
When you watch the weather report on telly, if there is a big LOW pressure sitting around NZ, that means there will be what????
Wind (and rain, booo!)
The reason that there is wind, is because physics dictates that a high pressure will "rush" to a low pressure to equalise it.....
The same is true in 4 strokes, or 2 strokes.....
(this scenario is not including the fact that you also have the benifit of the extractor like effect from an expansion chamber, so for this purpose, we will ignore that, just for BASIC information on how an engine works, ignoring all the facts and figures that confuse everyone.....)
If there was 3 p.s.i in the cylinder, and 3 p.s.i in the crankcase...... nothing would happen, simply, the engine would never run, it never could......
Try this....
"THE AIR/FUEL MIXTURE GETS INTO THE CYLINDER/COMBUSTION AREA AS A RESULT OF A PRESSURE DIFFERENTIAL BETWEEN THE CRANKCASE (IN THE CASE OF A TWO STROKE, OR INLET MANIFOLD IN THE CASE OF A 4 STROKE)
The air/fuel mixture sitting in the crankcase is in a higher pressure state, it travels through the transfers, to the combustion area, because the combustion area is at a LOWER pressure than the crankcase
There is a DEPRESSION created in the combustion chamber, (caused by the fact that the piston is going down, creating that depression)
Underneath the piston, a positive pressure is being created, because the area under the piston (the crankcase) is being compressed by the piston.
Herein lies our Primary compression ratio.
Primary compression ratio= case volume at TDC
case volume at BDC
THE CRANKCASE BEING A HIGHER PRESSURE (APPROX. 1.5 ATMOSPHERE) THAN THE CYLINDER/COMBUSTION CHAMBER (APPROX 1 ATMOSPHERE)
Or, if we now take into account the extractor effect of the expansion chamber, more like NEGATIVE .5 atmosphere....
As such, I argue that cranckase pressure is VITAL in regards to how a 2 stroke runs, infact, that it runs at all..... and as such, careful attention to its ratio is paramount to it's overall efficiency!
No mate, the crankcase is more than "Just a place to store the air/fuel mixture"
Remember, that the important thing about this process is that we are trying to make this engine more efficient, and this is an important part.
If you have too low primary compression, then the pressure differential (the pressure difference between the crankcase and the cylinder/combustion chamber) is not high enough to efficiently "extract" the air/fuel mixture into the cylinder/combustion chamber (and keep it sufficiently atomised, which is critical for efficient combustion)
If you have too much primary compression, then the gains you make by having "more" are used up in the "pumping losses" (the energy (horse power" it takes to compress the air/fuel at that higher rate)
AS such, I will clarify my reasons for suggesting that this may be a way to improve power, AND run cooler....
Once again....
Fujio Nagao (one of the men responsible for the early Yamaha 125cc rockets back in the day researched this very subject extensively), and I quote..
"That any deficiency in air delivery due to a crankcase volume too great for an engine speed id fairly well compensated by properly tuned intake and exhaust pipes"
This engine does not have "properly tunes intake and exhaust pipes", (particularly because 1 the carb is too small, and, because they are constantly changing the port timing for an individual track, and not the pipe)
But, mainly the carb (due to the rules) is too small.
I believe that as such, determinating the current ratio correctly is prudent, (primary tests indicate that it's much below the 1.5 that is the "standard", as it is indicated that it may be more like 1.33 or so.......
So, I say, if you cannot "compensate for the advantages of a high primary compression ratio" by tuning the intake (bigger carb), then consider raising the primary compression........
I know it sounds old school, but in this case, I believe it has merit.
At least determine what it is to start with!
Remember, it is considerably LOWER than an original GP125, due to the fact that it has a spacer under the cylinder, as well as the fact that the intake side web has been chamfered, all increasing the crankcase volume, and lowering the ratio.......
Forum whore
Some times pictures help.....
http://science.howstuffworks.com/two-stroke2.htm
Forum whore
Speedpro once explained the benifits of a larger crankcase volume to me.
He told me that it was easier to quickly scoop up a full cup of water from a larger bucket than a smaller one. Or fill a cylinder from a larger crankcase than a smaller one.
It made sense. especialy when you relate it to the gas law P1V1= P2V2
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No8 Wire Extraordinaire
Mike has now answered my question from the page before. That is why I don't think of 2 stroke as just a pump. Pipe extraction & gas inertia.
As far as primary compression is concerned Honda & Yamaha have often gone in different directions on this. It will be a compromise, but the more modern approach is that as pipes have gotten more effective, lower primary ratios have been beneficial. I don't shoot for high primary ratios.
However that isn't to say that it is possible to go this way all the time. Running super effective pipes on engines with transfers that are not curved enough produces bad results over scavenging.
In this caseman may have a point. Within class rules a good compromise leaves no one happy.
Don't you look at my accountant.
He's the only one I've got.
Forum whore
Yea, I have to admit I felt silly even suggesting "raise the primary compression", as it is the battle cry of the "I can tune a two stroke, but don't know jack squat" guys,
How many times has some guy asked you...
"That's fast mate....did you polish the ports and raise the primary compression?"
"ermmmm..... no"
"oh, you should do that, it would really go then!"
But by what I see from your results so far, there needs to be an alternative solution to this problem, and possibly, in this case it would help.
I was thinking as well, whenever someone has tuned these sorts of engines normally any mod is open bar fuel type....but in this case, your two restricting factors are air cooled and carb size....... I suggest that VERY little research and testing,has been done.
Because in reality, you would have STARTED with a bigger carb, day one!
I also believe that when you get it sorted, you will perhaps be in a field of your own, (as far as undercarbed 125cc aircoold disc valve two strokes go that is!)
And yes, F5 is quite right, current trends are towards lower Primary compression......and to some VERY big carb sizes as well....... I wonder if there is a pattern there....... Maybe not, but... HE CAN'T CHANGE THE CARB SIZE (dammmmmmmm Ittttttt!)
Forum whore
Sorry SS90, there has been so much to look at that I have not been able to keep up.
I am pleased that the primary compression is so low on my long rod GP as now I can fill in the area above the crank wheels and improve the flow at the entry to the transfer ports next time I have the motor apart.
Why did I use a long rod. Becouse it was all the go with TZ's back in the day and I had a box of RD400 rod kits, same with the pistons, had a box of them at the back of the shed. This is buckets, do the work your self make use of what you can and learn as you go.
What have you done with your SS90?
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