1 7/8 headers on 69 dart?

on our 340 stocker, changed from 1 3/4 to 1 7/8 dropped 2 tenths and gained 2 mph, average to average , best to best, etc. friend of mine runs a 340 6 pac stocker, changed from 1 5/8 to 1 3/4 step headers to 1 7/8 with 3 1/2 collector, gained 35 hp. just food for thought. my son changed from 1 7/8 to 2 ' headers on his 440, gained over 3 tenths lower.