MegaSquirt-II has a 'base dwell' (aka. maximum dwell duration) and a voltage correction table (the time to charge depends on the applied voltage, and this table lets you correct for that).
You can try values below 2.0 for the dwell, but the engine might not start. If you try the higher value (3.0) at idle, there's little risk of burning out the coil, because the duty cycle - the time the current is applied versus the time it is shut off - is so low.
Two parameters affect the running dwell (in additon to the voltage and base setting):
- the rpm, which sets the time available between ignition events, and
- the maximum spark duration, which is the amount of time MS-II tries to wait after a spark before starting to charge the coil again.
MS-II tries to fit the dwell time + max. spark duration into the time between sparks. If it can fit them, then the full dwell time is used. If the time is too short (because the rpm is high) to fit both the dwell and the max.spark duration, then both are shortened proportionally.
If the charging starts too early, no energy is put into the coil, it simply dissapears into the ionized current at the plug. So you want the max spark duration to reflect the actual spark duration, not shorter.
If the max spark dration is too long, however, then the actual dwell value gets shortened at a lower rpm, limiting the spark energy at higher rpms.
A value of 2.0 for max spark duration is typical, but you can try as low as 1.5 if you have high rpm misses.
I don't know how your motronic ECU handles the dwell, perhaps the voltage/rpm dwell values are hard-coded into a table rather than being calculated on the fly?
Thanks Lance. Yeah because I am running an MS-1 V3 MSnS-E I prob don't have that choice of spark dwell adaptation. The stock motronic uses a hardcoded map, I managed to extract it:
I am going to start with a 2.0ms time I supposed and go from there. I am finishing up my MS-1 V3 with 6 COP in wasted spark, but I'd like someone to look at my board to let me know if I did the wiring correctly. That's for another thread. Thanks for answering my (and future) question!