My dad bought a new 4dr 122 in 1964, in Delaware.

My Grandfather liked my dad's car so much he bought a new 2dr for himself in 1966 (the car that I currently own).

My dad insists to this day that his 64' would run rings around my grandfather's 66', and he could never figure out why, back in the day.

My dad tuned the carbs on both cars (he's really good at it too!)

I've often wondered if my dad somehow wound up with a P1800 motor in his car; he said it came with the phenolic spacers between the carbs and the intake manifold. I had associated these items with the higher compression ratio motors, but perhaps I am wrong.
The only other things I could think of would have been (possibly) a vacuum advance distributor or a different rear axle ratio.

Any ideas?

Bill