So, the real question is: if efficiency were doubled in the ignition spark, would that lead to doubling of MPG? Doubling of power?
Use two plugs?
Some manufacturers have used two plugs (My 1984 Nissan 200sx had two plugs). I think Ford did this on a few engines too. I can see how this would be very useful with lean mixes since they burn slower than mixes that are stoich. correct.
Maybe one stronger spark could better ignite a slightly lean mix- but IMHO, unless you are having misfires, changing plug wires to get a stronger spark wouldn't change fuel economy because all that matters is that the mixture gets ignited.
My grandmother had a old Datsun pickup with dual plugs, I assumed it was for reliability. And, that truck was REALLY reliable. Ran great till the chassis was worn completely out, and after sitting for over a year it still started, without even jumping the battery.
If I remember correctly the dual plugs in the Nissan trucks were for a power boost when needed. When you needed the extra power all 8 plugs fired and when on the open road cruising only 4 of them fired.
I'm pretty sure I remember a Ford Ranger 4 cylinder engine, maybe from the late 1990s, that had DOHC and dual spark plugs per cylinder. IIRC the spark plugs were configured differently to be used along with the different cam configurations.
The Nissan engine was called a NAP-Z and the dual plugs were more for emissions than power, although they ran good and got excellent mileage. Plugs were on opposite sides with single intake and exhaust valves, in a Hemi head configuration.
Something very similar is the honda idsi engine which uses two valves and dual plugs. Seems to be similar.
The NAP-Z Nissan engines first came out with carburetors, which also had heated screens in the base plates to help with fuel atomization. Used a suction operated reed valve arrangement to eliminate the air pump.
This was the last stages of development before fuel injection. FI started out in the 1986.5 Hardbody pickups with throttle body 3 valve engines then went to port injection and DOHC in 1991 I think.
There are many aftermarket ignition systems on the market that produce multiple sparks across a longer degree of crank rotation. In fact, I have one on my car (necessary for converting to distributor-less ignition). It doesn't give you any measurable increase in FE.