most dyno headers are 2"- 2 1/4" tube diameter. big cu. means bigger tubes, most marine headers are 2 1/4 on the inner tube. The jetting and tuning items done on the dyno will be slightly affected by header size. If you are doing a race application, use your headers for max performance tunning. If you build a poker run, pleasure motor, tune it to max with good dyno headers, then back up to the rich side tune up. Your motors will live a lot longer.