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I think thats the basics, I could be wrong about a few points. I'm not 100% sure if the volume of the collector has to be included too. It might just be the cross sectional area of the runners needing to be equal.
TK
The collector volume does matter to some extent. In some cars (a good example is an E46 M3/M3 CSL) the volume of air moved is huge, and the engine benefits in higher RPM's from having a larger volume of air available (the CSL has this), but in some cases you can treat the volume as "atmosphere" because the air movement is minimal.
