Title says it all. Mostly in regard to electronic ignition but it would be good for the general knowledge of whoever winds up here from a google search.

Stock gap is .045 but would one expect a rough idle or other bad things on .035 or .055?

 

One school of thought is the correct plug gap is dependent on combustion chamber design and doesn't change due to an ignition upgrade. One thing to keep in mind, it won't necessarily run rougher at idle with a wider plug gap, what you're concerned with is misfire at high speed or under heavy load, climbing hills, or towing that's where the potential for misfire comes into play. Wider plug gaps increase the voltage requirements to fire the plug, and so does heavy load, causing voltage rise inside the coil, and this is where arcing to ground, or crossfire, or burning up rotors etc., comes into play.

It gets complicated because everybody is using different coils and voltages and different distributors, different rotors, wires, etc. An older small diameter points style distributor definitely won't allow for too wide a plug gap without breaking down.

 

Ditto - the reason high load causes misfires in edge-cases is because the compressed air/gas mix makes it that much harder for the plug to fire, and the high voltage might bleed off somewhere else, as Tedster9 stated. In turbo/super-charged applications, this becomes important as the boost goes up - for example, in the Ecoboost tuning world, smaller gaps are used to make sure the arc is not "blown out" by high boost.

But in normal daily-driven, mild performance FE's, I'd use what the manufacturer says to use for that year's ignition. Stock small-distributor w/points, small-distributor w/Duraspark, or large distributor with Duraspark, I'd follow the stock gap spec.

If you're messing around with higher voltage non-stock coils, and aftermarket distributors, well, all bets are off.

 

I've mentioned this before, but the spark plug gap isn't the only air gap that affects coil voltage. The widest air gap in the secondary circuit is what determines the coil firing voltage, so it's either the spark plug gap, or the rotor gap, whichever is greater.

The rotor gap has exactly the same effect as plug gap as far as that goes. '57-73 distributors (at least) use the same part # rotor, but they are supplied with markedly different lengths depending on when they were made. I think smog era motors and everything made since then use wider gaps, or maybe that started when points were no longer in use. I had problems with Pertronics Flamethrower 40k coil and full battery voltage, bypassing the ballast. Crossfire and arcing. This was mainly evident in extreme humidity.

After studying the intertubes a while and buying NOS rotors I finally found a couple that have a lot closer rotor tip and terminal clearance. The earliest black rotors. This calmed everything down, along with a fresh set of plug wires. It seems to me the rotor tip and terminal clearance should be at least roughly equal or maybe even less, than the plug gap. At least when using a points style distributor. The takeaway is that ignition systems are more complicated than they seem, it all works together as a system, can't change one part without affecting other parts of the system. Beware replacing earlier pre-smog components with smog era components.

The way to see these kinds of changes is with an ignition oscilloscope, the coil firing voltage is displayed on the firing line, a properly tuned ignition system only provides what's needed to fire the plug, and the remainder is held as reserve voltage for high load or high demand conditions. Unnecessarily high voltage spikes just burn up parts, cause carbon tracking, or cause misfires, crossfire, or arcing to ground. The spark belongs at the plugs, and nowhere else.