I just realized I should explain the last comment as there may be some who don't understand why that is so. Sparks in an internal combustion engine have to jump an air gap. Air is an excellent electrical insulator, so electricity doesn't want to travel through it unless it has no other choice (or just can't at that voltage level). So we build a low resistance well insulated path that leaves the spark plugs air gap as the ignition voltage's only choice, at least that's the plan. When the planned path or insulation fail the voltage may find a easier way to get to ground or just never get there at all. That being said when a spark ignition engine is running at idle or light load its intake is throttled and therefore at a lower air pressure than ambient air pressure. So even with compression the amount of air in the actual plug gap may be less resistant to a spark than say a leaky boot, insulator, or cable path in the outside air, or a just starting carbon track from a broken insulator that's still a longer distance from the shell than the gap. But as soon as the throttle is opened and charge pressure rises, much less the charge is boosted above atmospheric, with the rise in charge pressure more air molecules are crammed in the spark plug gap. At this point the spark can't get through there as easy as before and as the voltage rises suddenly the failed insulation point with what ever amount of standard pressure air in it's "gap" to ground is the easier path. This is what happens when an engine runs smooth at lower loads but starts to miss as the charge pressure is increased.
In Justin's theory where with multi-spark may misfire on the first strike but ignite the charge on one of the subsequent sparks he's saying that as the piston starts retracting from top dead center (when the charge pressure is it's highest) and the spark went another direction at some point the charge pressure drops to the point where now the actual plug gap is the easiest path to ground again the gets the spark. Therefore igniting the mixture before it's too late. A single spark system would only fire at the first desired crankshaft degree point for maximum efficiency when the charge pressure was too high to over come in the weak system.
By the way Dieletric grease is a better insulator than air and air is one of the things I use the grease to seal the system from.
Justin, I have a question for you, do you know how many crank degrees the BMW DME multi-fires across? I've seen MSD list this in their literature before but I haven't read much on the DME.
Vernon
