my main question is i have no clue the breathing needs of my motor and the last thing i need is to make a header that is to big, small etc...

now i'm not aking for build plans so to speak ( not that i would expect that or want that). but i'm more looking for a good start to work off of.

specs:
81mm b18a1 w/ pr3 pistons, (11.4:1)< zeal auto comp. calculator, stock head/cams, modded gsr intake manifold/t body, stock gsr intake, and a 2.5" header back exhaust.

only thing i will be getting in the future will be a set of crower 404's and valvetrain.

ok guys lmk what you think

well i know im not going to master making n/a headers over night here ppl.
but i'm more then capable of making a header as i make turbo manifolds and related peices daily, it is my job. with that said it is pretty hard for me to spend $600-1000 on a custom header from one of the known companies, when i know i can do similer work for cost of matarial and my time.

so with that said i'm not looking for a step by step diy. i just want to know where to start, like tube size and legnth for primary and secondary. i can figure out the rest from that.

also what would be a better layout 4-1 or 4-2-1?

In theory, the 4-2-1 headers should make more torque, at the expense of some top end, but in real life, you are not going to see the advantage of that design on a motor like that. They work great on V8s, but small motors make power by reving and 4-1 headers make more power at higher rpms. Yes, I know its an LS, not a B16, but it is still a small motor in the overall scheme of things. Go with a 4-1.

As for the tube diameter - you must know someone with a decent header; throw a caliper on there and see what the pipe diameter is.

My .02.

Best of luck with your project.

EDIT --> if you do go with a 4-2-1, copy the HyTech layout which makes the 'Y' really looooong. Again, small motors rev to make power and they like the long tube exhausts.

Modified by Top Ramen at 10:09 PM 5/30/2008

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yeah i thought about measureing tube diameter. but primary and secondary length is what i'm stuck on.