I believe it was gasoline issue and a reduction of cubic inches. I believe they are all 355 cu in now. Where as before there were 427, 426, 428, 429, 430. IN 1966 ford withdrew its support from nascar teams due to nascar made the Fords add 1 pound of weight per cubic inches when they introduced the 427. Therefore the fords were 427 # heavier than all other makes, but that is nascar. This was all documented in either a car & driver or road & track magazine back in the day.