Don't want to hijack Pash's very interesting engine model thread, as he's on intakes at the moment.
But we briefly touched on exhausts.

Although it's been pointed out to me that cylinder volume has no correlation to exhuast header volume I found it quite interesting today to find the volume of the headers on my X1 are 2400 cc ! Coincidence?

Are there any rule of thumb calcs for header sizing? Discuss?

http://www.exhaustvideos.com/faq/how-to ... -diameter/

and http://www.eng-tips.com/viewthread.cfm?qid=104735

Should be enough info there

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2017 KTM 690 Duke
2001 M2
2019 Street Triple RS
2004 Monster S4R
Wilier La Triestina

The eng tips website is good isn't it. I'd already read through that thread.
I must admit I got lost!