Ok, the purpose of this post was partially to get all this written down in hopes that it helps someone, and to generate some discussion about whether I'm thinking about all these effects correctly, and to see whether the take-aways match peoples' experience when tuning.

First, some background things to keep in mind:
- The faster the mixture burns (as long as its after TDC), the more efficient the engine will be at that point (everything else held constant)
- As the chamber pressure and temperature decrease (as load goes down & intake is more throttled), the time it takes for a flame to spread throughout the combustion chamber increases. So, at low load, you need more spark advance because the flame spreads from the spark plug more slowly. At high load, you don't need very much advance, since it spreads quickly.
- The richer the mixture (up to 13:1 AFR or so), the faster the flame speed.
- At high loads (in boost) with elevated peak cylinder pressures and temperatures after compression, you're more likely to auto-ignite some of the mixture (knocking)
- At all but the lowest speeds, turbulence in the engine increases proportionally to the engine speed, meaning that most of combustion takes approximately the same number of crank-angle degrees as speed increases, not the same amount of time
- At the lowest speeds (<2000 RPM), turbulence doesn't increase as quickly with engine speed, so spark timings should not be as advanced as at higher speeds. In addition, lower speeds leave more time for the fuel/air mixture to "cook" at high loads, increasing the likelihood of knocking.

These pieces of background information shape the "ideal" spark timing map across the speeds & loads we're interested in. + is more advanced, - is more retarded than mid-speed, mid-load (~20 deg bTDC timing at 3000 rpm, 100 kPa, stoich AFR)

Low Engine Speed, Low Load:
++ low-load flame speed
- low-speed const. turbulence
- Rich AFR needed for idle stability, higher flame speed

Low Engine Speed, High Load (in this case, not much over 100 kPa with a turbo):
- high-load flame speed
- low-speed const turbulence
- knock prevention

Mid - High Engine Speed, Low Load:
++ low-load flame speed
+ leaner operation at low loads (if you run leaner than stoich below atmospheric)

Mid - High Engine Speed, High Load:
- high-load flame speed
- knock prevention

Now that we've got this "ideal" timing roughed out, lets think about the best way to fire the two plugs in two locations that we have to work with. Just for clarity's sake, I'm defining the spark "split" the same way as MegaSquirt and all other references to it I've seen... positive split is the number of degrees the trailing (higher on the housing, exposed to fresh charge as rotor spins first) plug fires AFTER the leading plug (lower on the housing, exposed to fresh charge as the rotor spins last).

I've found that a good way to think about the split is in terms of what your ideal spark timing is, and which spark plug is closest to the center of the rotor's combustion chamber cut-out as it spins. At TDC and slightly before it, the leading plug is closest to the center. About 10 deg before TDC, the leading & trailing are equidistant from the center, and earlier than 10 deg before TDC the trailing is closest to the center. Thus, all things being equal, you'd want to fire the trailing plug first if your ideal spark timing was 15 deg bTDC or earlier (so fire the trailing plug at 15 deg, then fire the leading after that as it gets closer to the center of the combustion cut-out), fire them at the same time around 10 degrees, and fire the leading plug first after 10 deg bTDC and later.

However, the trailing plug hole is MUCH smaller than the leading plug hole, meaning that its a harder and it takes longer for any ignited mixture to escape the trailing hole than it would to escape the leading hole. This biases you towards using the leading hole first more often to ensure a stronger early flame growth unless the spark timing needs to be very advanced and the leading plug is on the edge of the combustion chamber "dish" carved in the rotor, where it will have a hard time spreading to the rest of the chamber & may be quenched by the rotor. The practical take-away for this effect is that you'd want to fire the leading & trailing at the same time if your "ideal" timing was 20-25 deg bTDC. In addition, if your ideal timing is earlier than that (say 30 deg bTDC), you should fire the trailing plug even earlier (so 35 deg bTDC) to account for the extra time the burning mixture will need to escape the tiny leading hole and begin to spread.

In addition, at the highest loads, you don't want negative or zero split because not only is the leading plug closest to the center, but you don't want two competing flame fronts heating up the un-burned gas/air mixture between them, risking some severe knock.

What this ends up looking like then is:

"Ideal" timing (in degrees bTDC)

200 kPa:...5.......10......15......15
100 kPa:..15......20......25......25
35 kPa:....20......30......35......35
...RPM....1000..2000..3000..4000+

"Leading" timing (in degrees bTDC)

200 kPa:...5.......10......15......15
100 kPa:..15......20......25......25
35 kPa:....20......25......30......30
...RPM....1000..2000..3000..4000+

"Split" timing (in degrees after Leading)

200 kPa:..15......15......15.....15
100 kPa:..10.......5........0.......0
35 kPa:....-5......-10.....-15....-15
...RPM....1000..2000..3000..4000+

So? Thoughts?

 

what happened to this? this was a good thread!