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My engine is a mild 2.0L-125hp or so. I built a 1.5" header with 42" primary length and 2.25" collector to focus the power band from 3500-5000pm and still pull to the 6200 rpm redline.
Primary tube size is all about keeping the velocity up without causing restriction of flow....use the smallest tubes necessary to get the job done or you sacrafice performance.
I've been sucking up header design knowlege from every source I can find for the past 2 years and have managed to learn a lot but still thirst for more. I don't profess to be a header whiz but I have learned some amazing things from several people with a lot of design/fabrication/dyno time. I realized I made several mistakes in my youth when it comes to header choice and left signifiacant amounts of HP on the table because of those choices. Ed Henneman and Jerr Stahl have both been in business over 40 years designing/building/testing custom competition headers and are suprisingly free and open with information on what really works.
For comparison, with the 1.75" primary tubes Eurolook71 mentioned, a v8 engine would need to make about 450hp to take advantage of primarys that big. Whak that engine in half and you've got a 225hp 4 cyl.... not a bunch of those cruising the streets.
I have been compiling all the information from books, magazine articles, phone conversations (Ed loves to talk!) and websites into one place. I would like to write an article for a magazine to publish on header design and a second on header construction with some dyno testing thrown in.
Sorry for the long response but I truely love the science/art of header design,
ken
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