Not Ranked
One of the most enlightening talks I ever heard was when Smokey Yunick discussed headers. He said he had over 100 pairs hanging on his walls. He would design them then test them on different engines.
He said it all boils down to this: make the turns as soft as possible and try to flow everything smoothly. If you do that, you will get 90% of the benefits of a header.
I wouldn't worry about the headers. There have been many times an unequal length header has produced more HP than an equal length.
Some basic suggestions:
Make sure the headers are appropriate for the ports in your heads. If you have a choice between long and short tubes, long tubes will give you better performace in the RPM range you are probably looking for. Tube diameter will be more important to keep up velocity. A good ceramic coating will also maintain velocity better.
If you need that last 5 to 10 HP to be competetive in a racing class, long tubes designed for your RPM range might be appropriate.
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