Cylinder head discussion 340 with stock above deck pistons

toolmanmike said
I tried that back in the day. I put headers on my 340 Challenger, opened them up at the track and went slower. Without rejetting of course.

Generally most folk after header install, open, reported 2 to 4 tenths better, with tune, not always, but generally amongst breeds, street machines.
Extension had different effects on diff size engines .