I was a designer /fabricator for hooker headers, and did many ,many hrs of dyno time on my own chassis dyno,
I'm just providing the truth, not trying to convince anyone, you can spend your $$ as foolishly as you choose.
on a under 400 cu in motor for STREET USE, 1 5/8 IS OPTIMUM.
1 3/4 you loose some low end torque. and 1 7/8 a noticeable loss in low end torque
my cars seldom are driven "like I stole them"
I value the cars, and the cost involved. so I don't abuse them .
yes, they will rev well past 5000 rpm, but why?
I'm not in a contest.