http://www-staff.lboro.ac.uk/~elvpc/progs/expch.html
I suggest the following workplan
- use the sums to get you in the ballpark
- make a set, see how it perfoms on the dyno, measure the gas temperature in the three sections
- make another set using the revised temperatures and knowledge of power delivery to lengthen/shorten teh sections
- go back to 2 and repeat adinfinitum, or at least until you are satisfied with the performance
There are also software available which does the calcuations if you look hard enough on the internet - try typing expansion chamber design software into google, like this.
A better decription of expansion chamber design, tied to port time area, is available in the book by John Wood at http://www.lortim.demon.co.uk - if he can get a 200cc villiers engine from 8bhp to 40+bhp, he must know his onions.
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NSRless again...