I want to check if I remember something correctly,
Obviously 10a, 12a, and 13b have 3 combustion chambers per rotor for 6 total.
The way CC is measured, doesn't add the volume of all 6 chambers together. Only one from each rotor.
So the 13b is 654cc x 2 = 1308cc right?
20b and 26b use the same size housings as the 13b.

Why didn't Mazda use the volume of all combustion chambers (6 in a 13b), the same way piston engines are measured?

No mathy formulas please (eg r=pi*2 squared*mc9.2^) it goes over my head

 

Lower taxes for smaller displacements is the actual reason.

a 13B can only take 654cc per rotor at a time. A theoretical piston equivalent would be an engine that has 3 pistons that take turns in 1 cylinder.

 

Because the rotors spin at 1/3 of the RPMs of the E-shaft/crank. So for every single 360* rotation of the E-shaft, only one of the 3 rotor faces has a combustion event/power stroke. So the 13b displacement is 654cc x 2 = 1308cc