dual spark heads

mcnoople said
The average carb street engine there isn't really any advantage to dual plugs. Ford used it for years on the 2.3L ranger motor and chrysler is using it on 3rd gen hemis and later 4.7's, but for all normal purposes 1 plug per cylinder will get the job done just fine.

Any hemispherical head will run better with dual plugs and this was indeed a good idea that because of cost and bulk wasn't implemented on the Street Hemi. Chrysler engineers designed those castings from the start with this capability, although I think some were not cast with the needed boss in place. I am not an expert on nor a fan of the 426 Street Hemi for several reasons, but I can attest to the benefits of dual plugs on hemi combustion chambers.

You can prove this easily if you have a way to constant load the engine and disable one of the ignition systems. Power will drop. On aircraft engines they do a "Mag Check" before takeoff and it is required to have a drop in a certain range of rpm and for the drops to be even. If one mag is dead the engine quits. The fact that there is always a drop means that the engine runs less efficiently on one than on both.

I had BMW motorcycles for years and when gas got crappy in the eighties we had detonation issues. Some guys added avgas or ethanol. Some people put in spacers at the cylinder base to lower compression. But what worked best was having the heads machined for twin plugs and a second coil and wire set added. The engine then made more power and detonation ceased.

The Harley guys did this for awhile too and then the Evo came out with a different combustion chamber design.

Is the hemi combustion chamber particularly good for power and efficiency? Not so much which is why it has gone out of favor for pentroof and high squish designs. The Modern Hemi has a shape that is not hemisperical but hourglass shaped.