Plug gaps go with combustion chamber design and air fuel mixtures.
The wider gaps were part of a emission package and very lean air fuel mixtures.
The high energy ignitions were developed due to the "no tune up required" regulations with certified emission for 50,000 miles . ( today its 100,000 miles)
A old engine ( lets say flathead ) doesnt show any need for wider gaps as most ( especially modified cams and dual carbs), all you gain is higher coil output and overheated parts . Short life etc on ignition components......
There may be some changes in performance from time to time with plug gap changes , however if so its because you fixed a problem in another system etc...
Heres a scenario i like to use .
Scope hooked to stock good running flathead running at idle shows a actual need of 5,000 volts per plug at idle. Proper air fuel mixture of 13-1 per the day of manufacture. .030 plug gap .Plugs are taking approx 2,000 to jump the gap and ignite the hydrocarbons of fuel. The other 3,000 is used up with rotor gap and resistance in the sytem etc.
Now lets add two carbs making the air fuel mixture richer (lets say 12-1) Scope now shows 4,000 volts as richer air fuel mixtures take less . More fuel hydrocarbons are easier to ignite requiring less output from the ignition.
Now lets widen the gap to the plugs to .045. The engine will now use 6,000 volts to fire the longer duration spark based on the wider gap and the jump off the plug point on the scope will show it now requires 3,000 volts to jump the plug etc.
And the story goes on and on. A compression increase ( milled heads) will cause a increase in spark out put.
So lets say you mill the heads (increased output needed) and added twin carbs ( lower output needed) then we would be back at the orginal specifications..
Makes it all clearer right ??????
