Recently engine stopped and when investigated found that...
Original (M20B25 MY 88 with distributor) BMW/Bosch coil discharge (burn) time is quite long (few ms) -> coil has no enough time to discharge. Note that saturation time is also quite long -> well over 3 ms. (coil (+) terminal 13.9-14.0 V)
Power (current) gets high (mean power around 10A and peak around 15 A) this fries ignition driver (yes, this happened) very soon at sustained faster revs (fex. rally cars and other competition vehicles).
- adjust charge 'coil on' time shorter*. This is a quick one but not recommend (too weak spark).
- Use faster coil (burn/discharge time faster).
- drive coil by switching two fets in parallel (fex. IGN00 and IGN01).
- adapt wastespark (3x2) coil -> helps also in situations where intended maximum revolutions are high.
fex. At 6400 1/min revolutions takes 9.375 ms per rotation.
Four stroke 6 cyl has 3 events per rotation hence available time 3.125 ms per event (when using distributor in spark delivering) to charge and discharge coil.
- One can adjust coil charge time smaller, but then available spark current (power) drops, causing misfire at higher revs. Specially on turbos and other engines where pressure is high and mixture rich, also lean air/fuel mixtures need higher spark power.
Problem could be worked around little bit by using faster coil or wastespark coil, but then still 'you hit a wall' with high revving engines.
High revving engines need better dwell control (min discharge time, dwell table and/or overlapping dwell) strategy.
1.1.x Firmware does have overlapping dwell, used daily with missing tooth N-2 and N-1, with and without camsync.
Note that same counts fuel injection also, available time to inject gets shorter and when high amounts of fuel is needed you need larger injectors even if the injector 'flows enough at 80 %/6400 1/min'.
Minimum coil charge time is adjustable with ign_dwell14. Min discharge time is constant 160 usec (IIRC) when the same ignition output follows next (that is, when distributor is used). Is this not enough for some coil and setup ?
Yes - 160 us is not enough. More preferable minimum discharge time should be at least around 5-600 us. Standard type (Bosch) TZI coils (in BMW, Peugeot, Mercedes et al.) are rather 'slow'.
Conclusion... off we vent to wire and configure 3x2 double fire coil (Bosch 0 221 503 002 item), but this does not completely cure the 'available time problem**'.
'Bosch 0 221 503 002'
Dwell time (ms)
Ubatt 5A 6A 8A
8V 6,0 8,5 12,0
10V 3,8 4,9 7,0
12V 2,8 3,5 5,0
14V 2,3 2,8 3,9
16V 2,0 2,4 3,0