from Jaguar via a friend:
Exo, your understanding of a typical 2 stroke timing curve is not quite right. You should visit my site to read up on the subject and look at the timing curve graphs. I think you are referring to an older type of CDI of a different design.
I have read everything on your site, multiple times.
I have also read most of what Gordan Jennings has written, as well as books by Graham Bell.
I have researched SAE papers written by the engineers at Honda, Suzuki, Yamaha, and Kawasaki.
I have also found a forum where the Aprilia factory engine builder for their MotoGP team in the late 80's and 90's when they were heavily developing the 2 stroke engine was answering questions.
What exactly is not quite right about this 'understanding' of 'typical' two stroke timing curve:
1. A two stroke engine favors high initial advance.
(Even the image you attached shows the RM250 CDI is already at 15 degrees advance @1800 RPM's)
2. A two stroke will generally favor a slight increase in advance as the RPM's rise towards the Torque Peak RPM.
(Again, the image you attached shows the RM250 CDI continues to add advance as RPM's rise, in fact hitting about 22-23 degrees @3500 RPM's)
3. A two stroke will favor a CDI Curve that RETARDS timing after the Torque Peak RPM.
(Again, the image you attached shows the RM250 CDI does in fact RETARD timing, starting @3500 RPM's -witch I can reasonably assume IS the Torque Peak RPM of an RM250- It continues this retarded curve until it is only adding 3 degrees of advance @10,000 RPM's -witch I can reasonably assume is the Red Line RPM of an RM250-)
This is because of two dynamic effects.
1. The position (in degrees, or in mm's before or after TDC) the piston is in when the spark fires determines where the 'heat of combustion' is going to go.
-A. If the timing is sufficiently advanced, the entire combustion event will happen INSIDE the cylinder, and the CYLINDER will absorb all of this heat. This will create a scenario where the CYLINDER receives more of the 'heat of combustion' than the expansion chamber.
Because the expansion chamber is 'relatively' cooler in this scenario, and gas travels SLOWER when temperatures are LOWER, an ADVANCED timing curve is preferable at lower RPM's.
At lower RPM's, the gas travelling SLOWER through the expansion chamber, LOWERS the tuning frequency of the expansion chamber, witch means you get 'on the pipe' earlier. I.E. Low-End Torque.
-B If the timing is sufficiently retarded, the entire combustion even will NOT be completed INSIDE the CYLINDER. SOME PORTION of the combustion process will continue burning INTO the expansion chamber. Thus, a scenario is created where the EXPANSION CHAMBER receives more the 'heat of combustion' than the cylinder.
Because the expansion chamber is 'relatively' hotter in this scenario, and gas travels FASTER when temperatures are HIGHER, a RETARDED timing curve is preferable at higher RPM's.
At higher RPM's, the gas travelling FASTER through the expansion chamber, RAISES the tuning frequency of the expansion chamber, witch means you 'stay on the pipe' longer. I.E. Top-End Run Out.
I am completely open to a better understanding of the dynamics going on here, you just have not provided one.
If you see any flaws in this understanding of the 'typical' two stroke timing curve, please let me know.
from Jaguar via a friend:
1. RM250 has peak torque at 7,000 rpm so your assumption of timing needing to be most advanced to match the peak torque is wrong.
2. all of the combustion happens inside the cylinder but the remaining gases retain a lot of the heat. How else can the pipe get hot enough to burn your exposed skin?
3. the speed of the pressure wave from the exhaust pulse varies very little with different temperatures. I’ve measured around a 33 degree increase in exhaust temps for every 1000 rpm increase.
4. timing is retarded at high rpm because the rate of combustion is dependent on the turbulence the intake charge experiences which rises with higher rpm. Replace the expansion chamber with a straight pipe with muffler and you’ll see that the engine still does prefer the same spark retard at high rpm.
5. the RM250 graph I showed has a 7.5 degree BTDC as its most retarded timing, not 3 degrees.
6. the timing graph you showed was not from a modern analog CDI. They always demonstrate a very gradual change in timing. The Grubee engine depends on an analog CDI because there is no trigger coil to tell a digital CDI what the timing reference is.
Your presentation so far demonstrates the modern axiom that “a little knowledge is a dangerous thing”.
1. Correct, I made an assumption, and I was off as to the number, but you will notice the the curve is indeed RETARDING after the 7k Torque peak as well. Suffice to say, we can agree, a 2 stroke engine would not prefer MORE advance, beyond the torque peak (like the grubee CDI). Obviously, one curve (the RM250 as our current example) does not represent the only 'right' way. The specific RPM that the CDI begins Retarding will be determined by the manufacturer based on MANY variable. But that does not change the dynamic I described; optimal results will be achieved with a timing curve that begins to 'retard' at some point in the mid-range, and will continue to retard all the way to Red Line.
- I also did not say it MUST be most advanced at the Torque Peak, I don't know where you got that from.
2. Did you notice the term 'relatively' ?? OBVIOUSLY the expansion chamber is hot while the engine is running (REGARDLESS of ignition advance) it is about relative heat.
3. "The speed of the pressure wave from the exhaust pulse varies very little with different temperatures." Absolutely wrong. This represents a complete misunderstanding of the laws of thermodynamics. The gas speed absolutely changes to a significant enough degree to effect the exhaust system tuning frequency. A straight pipe still experiences resonance effects. And obviously a straight pipe will still be effected by the laws of thermodynamics. So OBVIOUSLY you will still experience the effect with a straight pipe.
4. Yes, gas turbulence during the intake charge effects burn characteristics. But you have not explained HOW retarding the timing effects intake charge turbulence. You have also NOT explained how the changes in intake charge turbulence effect power output. Or why the turbulence change that results from 'retarded' timing is beneficial as RPM's rise.
-Are you talking about the turbulence of the 'intake charge' or the turbulence of 'combustion'? Your statement makes perfect sense if you are referring to the turbulence of 'combustion'. But the 'intake charge' event has already happened by the time the spark is ignited. And further retarding the timing means the spark event happens even LATER; WELL after the 'intake charge' event has taken place.
5. Who cares if it was 7 or 3, the point is, it continued to take timing out all the way to Red Line.
6. No, it was not from an analog CDI, why does that matter? It was a common 4 stroke curve that showed the curve advancing all the way to Red Line, witch demonstrated my point sufficiently.
AND as a matter of fact, the PW80 CDI I gave you the part number for in the forum thread IS an ANALOG CDI, and as you can see it has multiple steps that are anything but 'gradual'. Yamaha apparently figured out how to create such a curve with an analog CDI. They have since gone to digital (Post-2003), with a Hall Sensor added to the magneto in order to send the trigger signal.
Our China Girl magneto (obviously) does not have a Hall Sensor, so you must use the Pre-2003 ANALOG Yamaha CDI, if you want it to work with the china girl stock magneto.
“a little knowledge is a dangerous thing”
Indeed it is.
And your 'presentation so far' demonstrates your misunderstanding of thermodynamics, witch thus leads to a misunderstanding of the dynamics involved.
You have provided no evidence to support your belief that: "The speed of the pressure wave from the exhaust pulse varies very little with different temperatures."
"I’ve measured around a 33 degree increase in exhaust temps for every 1000 rpm increase."
This statement is a complete non-sequitur. It does not disprove my assertion (that of thermodynamics) nor does it support your assertion.
You have also provided no description of the dynamics that demonstrate: "Timing is retarded at high rpm because the rate of combustion is dependent on the turbulence the intake charge experiences which rises with higher rpm."
You have provided me with NO REASON to change my understanding of the dynamics involved.
But please provide an explanation of the dynamics involved in this statement: "Timing is retarded at high rpm because the rate of combustion is dependent on the turbulence the intake charge experiences which rises with higher rpm."
You have not explained HOW the turbulence changes as a result of ignition timing (I.E. Advanced VS. Retarded)
You have not explained HOW these changes in turbulence effect power output at high and low RPM's.
If you do not understand these effects yourself, how do you expect me to just 'take your word for it'???
and good vibes. 
