Calculating optimum CFM for rotaries

[scarsofcarma]

Originally Posted by dr.occa

I've been going over some math in determining various carb options for the rotary motors. I'm very familiar with the tried and true IDAs (running one on my 12A), DCOEs & 44PHH (running a pair on my 4AG) and the 4 barrels: what I'm scratching my head about is do I use the cc displacement at the taxed 720° rotation or the actual full 1080° rotation in calculating optimum CFM?

Here are the answers for my 12A using both displacement numbers (in cubic inches):

((8000[rpm] x 140cc[rounded up from 139.866421]) / 3456) X .85 = 275.46CFM


((8000 x 210cc[rounded up from 209.7996322158]) / 3456) X .85 = 413.2CFM

Which should it be?

Your latter number sounds better, but I think it should still be a bit higher... You were right to question the effective degrees of rotation because the rotary motor breathes more like a 2-stroke than a 4-stroke. I did some rough calculations before but I don't have the numbers in front of me as I write this. I remember getting something between 600 and 700 maximum CFM's for a 13B, but that number will change depending on the redline value you plug into your calculation, and wether or not it is ported.