It's a myth. The problem with poor combustion was limited to a few extremely lean-burning engines in the late 1970's/early 1980's that needed a gap of .060" or .080" to assure there might be some combustible gases between the electrodes when the plug fired. Cadillac was one of the worst about developing a miss with .045" gaps, and a .080" gap cured the problem.
I managed shops and taught hundreds of trade school and high school students the intricacies of the HEI system for over 30 years. Once catalytic converters got efficient enough to clean up slightly richer mixtures without violating EPA rules, the manufacturers went back to a .035-.045 gap without a problem. The lie about HEI needing a wide gap dies hard, like the old idea that a hot engine needs to be running when pouring water into it. That advice went out of date when they put a water pump on the 1928 Ford Model A.
Jerry
