Re: MSD Ignition - Question for Techs
This may not answer your question but I will try to give you some insight into what I see happening on your waveforms. <br />First It is clear that you have an adaptive spark type msd unit, each sucessive spark shown is different based on what is happening in the cylinder. <br />Your first spark did not produce ignition (thats OK)you can see the difference in the negative part of the cycle,<br />there is no "ringing or oscillation" in the negative pulses, the ignition is re balancing all the energy that it used in the spark itself and re plating or transferring any electrode material that the spark plated over (most of it anyway).<br /><br />In the second spark the ignition knew that the first one did not produce ignition,<br />it adjusted the energy a bit higher and you can see the change in the waveform. The voltage is higher and the negative portion is more of a sine wave and actually develops a slight positive voltage as the impedence of the burning mixture changes.
<br />Right after the positive "peak" on the negative portion the ignition knows the mixture has ignited
<br />and it quickly hammers a bunch of negative spikes to rebalance the ignition ready for the next pulse.
<br />On the third pulse the ignition sends the same energy to the plug (remember it worked last time so why not try again) the voltage is a little lower because the fuel mixture has ignited,<br />flame spread has occured
<br />and the impedence of the mixture has gone down.<br /><br /> On the negative pulse of this wave form you can see that the slope of the curve as it goes positive is just slightly longer,<br />the mixture has changed impedence slightly and the ignition knows that the mixture is fully ignited.
<br />You can see slight variations in this waveform as each cylinder fires, there is always tiny variations in each cyl, each stroke, with variations in temp, fuel mix, valve overlap etc etc. You can see them here, they are normal.
<br />The fourth pulse is redundant, the ignition knows it has achieved combustion, it fires anyway and effectively captures a "profile" of the results of ignition ready to adjust for the next suck squeeze bang and blow.<br />The ignition rebalances itself again and you can see the large number of negative pulses to balance all the positive energy that has been expelled. The waveform after the last pulse is "ringing" and is just the residual energy dispersing itself in the highvoltage circuit (coil secondary).<br />With regard to plug gap I would follow the builders reccomendation (.028) The builder obviously knew this was a marine application and that the engine would be run at high rpm under high loads. Since it takes more time to make a spark jump a larger gap, and you have less time to do it at high rpm the smaller gap is called for.<br />Larger gap is useful for automotive applications for fuel ecomomy at cruise.<br />As far as timing goes, talk to the builder, he will know how much total advance he wants.
