Quote:
Originally Posted by Carl Froehlich
I’ll have to disagree. The Lightspeed or the pMag timing will be the dominant condition. The mag is just along for the ride...
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Not the complete answer. PCP timing is a
result of a composite of existing spark timing. In simple terms, if two flame fronts are started at opposite sides of the piston at the same time (as with a matched dual system), then the flame front should meet in the middle and PCP is achieved in X time. Dump one of those plugs entirely and the flame front will travel twice the distance to get all the way across the piston. PCP will still be achieved, but the time required to reach peak will be increased. This resulting effective PCP timing will be retarded from the above example, and explains changes in EGT and engine performance when doing the "mag check". A similar condition exists when you have two plugs firing but at different timing. The advanced plug will start a front that starts making its way across the piston before the lagging plug, but there is a good chance that the two fronts will still meet somewhere on the piston. For the sake of simple argument, the lead flame front may meet the lagging front 75% across the piston (75/25), which will result in a PCP timing somewhat retarded from that seen with a matched dual, but still advanced from a single spark event. In other words, the magneto is not generally "along for the ride" - it's contributing something to the party.
That said, there are certain high and lean conditions where it's likely the electronic ignition is
so advanced that it outpaces the contribution of the fixed magneto, but anything short of that, the mag is being seen in the composite. And on the flip side, the magneto timing at 100% power is most likely the leading actor, since (based upon my testing) its data plate setting is more advanced than the engine needs (for 100% power). I would expect a properly tuned EI/Mag combo to have the EI retarded several degrees compared to the mag. In this case, the EI would be retarded even more than in a dual system as an attempt to drag the composite timing lower to the optimum for PCP.
In summary, a completely adjustable EI is a simple thing when coupled with a fixed magneto. The test is straightforward, and the results predictable and repeatable. Mixing two "sort of" adjustable EI's gets much more complicated - particularly when neither of the subject EI's have the required range to begin with.