17-18 is good/OK
I think I get at least 20 on a my 1960 261.
I see Internet posts of up the 24 inches on late-model good-condition 235s.
You are talking about at idle, right?
It would be tough to get much over 10 if you have your foot on the gas at all. That is why the vacuum wipers are such a pain, especially when under a moderate load like when accelerating or going up a hill.
Carl
