this might sound stupid but oh well.... when poeple compare a 13b to a piston engine they usally say it like a 2.6 liter...so should a 20b be compared with a 4.0 liter?? i have a street port if that makes any difference in the sizing. I'm trying to figure this out because im trying to find out what turbo will fulfill my needs best. thanks.

 

Forget the whole comparing a rotary to a piston engine. You can't compare apples and oranges. It's better to ask people who have used different turbo's on a 20B and look at their results or take the time to try different ones out yourself. Maybe this thread will help you some.
https://www.rx7club.com/20b-forum-95/quest-choosing-good-turbo-713423/

 

First of all, if you are trying to choose a turbo for an engine, then you need to contact the turbo manufacturer and/or your local rotary engine speed shop if you want a truly accurate assessment. Believe it or not, this is the method used by the pros.

OK, with that out of the way, you can compare engine airflow requirements to get a general idea of a comparison. See this link for the math. Use 80 cid for 13B displacement, or 120 cid for 20B displacement. As you suspected, porting increases the volumetric efficiency, which you can read about in the link.
http://www.gnttype.org/techarea/turbo/turboflow.html

 

Porting only allows more air to enter the engine. It doesn't change the size of the combustion chamber. So no, it doesn't increase displacement.

Without confusing yourself with formulas and such, this is the easiest way to figure out rotary displacement.

Your average piston engine needs 2 rotations of the crankshaft to move enough air to fill it's full displacment for each spark/power event.

Mazda has always measured it's rotary displacement off 1 rotation for each chamber per spark/power event.

Soooooooo this means:

13b 2 rotor moves 2.6L of air during 2 rotations of the e-shaft and will have had 4 power events.
20b 3 rotor moves 4.0L of air during 2 rotations of the e-shaft and will have had 6 power events.
13b based 4 rotor (26b) will move 5.4L of air during 2 rotations of the e-shaft and will have had 8 power events.

 

Its actually-
654cc per rotor x2 = 1308cc then x2 = 2.6
654cc per rotor x 3 = 1962cc then x2 =3.9
654cc per rotor x 4 = 2616cc then x2 = 5.2

To get overly technical