Help Sizing Exhaust Headers

Hi all,

So I have chosen to have a full custom exhaust for my 2litre built in the aim of getting as much low end torque as possible. I chosen to go with 2.25" for the cat-back with an under axle design splitting into a muffler either size with both mufflers having two tips on each. (Before the catback will be a sports cat).

The main issue I'm having now is choosing how to size the headers, I was set on having either 1 1/2" to 1 5/8" as the primary header size but after coming across this website which says that my primary pipe length should be, there or there abouts, 1" I am now un-sure (that is for peak torque at 4500 rpm). Can anyone shed any light on primary pipe diameter please, I know thinner and longer is good for torque but how thin is too thin?

Other questions:

I know my collector will be 2.25" but what about the secondary pipes where the four merge into two, what size should those be?

Any thoughts on the primary pipe length? I know longer is better for torque but what is too long?

Any help would be greatly appreciated as this is the last thing I need to figure out for my Tib and then I can focus on getting things done!!