BillO
Husqvarna
AA Class
Okay, I’m starting to put it together.
The WR graph is relative to a fixed starting point and the CR graph is absolute.
5 degrees to ease starting. The CR definitely starts easier than the WR, which demands a more aggressive kick.
The 28 degrees makes sense with the PV, I should have known that one. I see it tapers off right away though. It may work better if it hung in there, or even went to 24 degrees at 2K then increased linearly to 28 or 30 at the point where the PV opens, then let off quickly to about 23 until 9K
The reduction beginning at 9K makes sense in light of this:
It’s a trade-off. The higher temperature allows the expansion to tune at higher frequencies due to lower density and heat of the exhaust gases. It does have less time to burn in the cylinder, but that is more than offset by the better exhaust scavenging. To bad someone could not design a pipe that had variable tuning. That might help get the best of both worlds. In any case, some gains might be made in this region to suit riding style or type of event.
This:
Only comes into effect because the exhaust is not working well any more. It’s a symptom rather than the real problem. If the exhaust tuned better at high RPM, there would be little in the way of pumping loss. Retarding the ignition allows the pipe to tune better, as above.
Now it all makes more sense, and I realize that I should have been able to figure this out. Just need to get my head out of my ass and think.
Thanks for your input everyone!
The WR graph is relative to a fixed starting point and the CR graph is absolute.
5 degrees to ease starting. The CR definitely starts easier than the WR, which demands a more aggressive kick.
The 28 degrees makes sense with the PV, I should have known that one. I see it tapers off right away though. It may work better if it hung in there, or even went to 24 degrees at 2K then increased linearly to 28 or 30 at the point where the PV opens, then let off quickly to about 23 until 9K
The reduction beginning at 9K makes sense in light of this:
The scientific reason for the shift of the powerband to extremely high rpm, is because the temperature in the pipe increases with the retarded timing, and that enables the pipe’s tuned length to be more synchronous with the piston speed and port timing of the cylinder.
It’s a trade-off. The higher temperature allows the expansion to tune at higher frequencies due to lower density and heat of the exhaust gases. It does have less time to burn in the cylinder, but that is more than offset by the better exhaust scavenging. To bad someone could not design a pipe that had variable tuning. That might help get the best of both worlds. In any case, some gains might be made in this region to suit riding style or type of event.
This:
Advancing the timing has the affect of increasing the midrange hit of the powerband, but makes the power flatten out at high rpm. The reason is that the relatively long spark lead time enables for a greater pressure rise in the cylinder before the piston reaches TDC. This produces more torque in the midrange but the high pressure contributes to pumping losses at extremely high rpm.
Only comes into effect because the exhaust is not working well any more. It’s a symptom rather than the real problem. If the exhaust tuned better at high RPM, there would be little in the way of pumping loss. Retarding the ignition allows the pipe to tune better, as above.
Now it all makes more sense, and I realize that I should have been able to figure this out. Just need to get my head out of my ass and think.

Thanks for your input everyone!