So the following graphs show two options of ignition curves for the same engine. Based on my, apparently, very limited understanding of two stroke ignition timing and advancing ignition, besides the fact that the first map offers advance above 15000rpm, I fail to see how the first ignition map would be more performant overall (as is the case in the real world).
Please enlighten if you have enough knowledge to explain where these two curves will most likely make their power (low, med, high on the curve), as well as where both will most likely be the weakest (again, low, med high on the curve).
Map 1
Map 2
Amount of and difference in advance between each map
What makes me extra confused is that if someone runs the coil with map 1 then they also run 3mm more advance on their stator. I don’t know if that additional three degrees is represented in the map, or if that’s a totally separate thing.
If it wasn’t already obvious, I’m trying to swim way above my pay grade here, lol.
Map 1 generally seems to have less timing than map 2 over their active range. So an additional 3mm of advance at the stator for map 1 relative to map 2 seems reasonable. It would tend to push the entire curve of map 1 closer to the reference curve. I recall running the rok gp at about 3.5mm stator timing. A quick trig calculation shows that going from 3.5mm to 6.5mm at the piston is about an additional 6 degrees at the crank. Which seems about right based on the maps shown
Hi @Ed_H and thanks for your reply! I had to look up the “ref curve” and now maybe I have a better idea of what I’m seeing. My interpretation is that the refer curve is sort of the “safe theoretical ideal” amount of advance for any given engine speed, so theoretically, the closer to it you can stay throughout the curve the better - yet choices/sacrifices must be made to tune for desired powerband characteristics, as well as detonation and heat?
One thing seems clear is that both curves will likely drive much differently and I’m guessing I might need a 2-4 tooth increase on the rear sprocket if I want to maintain some of the mid acceleration that map 1 seems like it might give up to map 2. Hm.
This is what that ends up looking like then in terms of final curves (taking into account the amount set at the rotor for each map as well).
EDIT: DISREGARD the chart below - the difference numbers are all wrong as pointed out in comments farther below. Leaving for historical context.
Happy to help. Agree they will behave differently. The most obvious difference is the big timing dip centered at about 12k rpm for map 1. This lines up fairly closely with where the power value opens. I suspect the dip is to help soften the punch when the valve opens.
With respect to comparing maps with the stator timing advance/offset included, it is important to make the distinction between the units of measurement. The maps and reference curve appear to be in units of degrees before Top Dead Center. Whereas, the additional stator advance is measured in mm of piston travel prior to TDC. To make a proper comparison the piston travel to crank angle relationship needs to be included. The sketch below shows the geometry and math to make the comparison along with a range of results below that.
@Ed_H - Doh! I actually knew that mm do not equal degrees in regard to advance, but somehow entirely forget this fact along the way - thanks for pointing out this (enormous) error.
@KartingIsLife - completely agree at some point the thing just needs to be driven to really understand the changes, but at the same time I think/hope an overall/general expectation of how the engine might behave can be found by comparing the ignition curves (holding all other things equal - which in this case they are). Attempting to arrive at the track with rough ideas that I can compare against to know if things are, more or less, behaving as expected as well as for hints about where to look first for tuning (particularly gearing). Plus all this activity keeps me from going nuts until I can actually make it out to the track again!
