When should you move from 1 5/8 header tube size to 1 3/4? I know if you go to big with a mild motor you'll actually kill some power so if there is any rule of thumb for choosing between the two I'd greatly appreciate it. Thanks.
the 1 5/8's will probably be best for a mild engine. the smaller primary's will allow your motor to make more torque then the 1 and 3/4's. that being said if you plan on upgrading your setup in the future maybe the 1 3/4 would be better. after all the power/torque you lose you will never know about because you haven't swapped between the 2 parts. I have 1 5/8's on my 351w and i am told that my application would prefer 1 3/4's.
A different intake, or intake work would make more of a difference. If you've already got 1-5/8s I would stick with those until you get stroked or blown.