difference between 12A and 13B
 

Just looking for an explanation of the difference (if there is one) between a 13B and 12A

welcome to the board. :)

they are indeed different and the difference is size.

12A = 1146 cc, 70 mm rotors/housings
13B = 1308 cc, 80 mm rotors/housings

the 13B is physically about an inch longer than a 12A. generally, more torque.

Thanks for the info!

13B came in 3rd gen models as we'll only

Old School's were both 12's and 13's. It can get confusing.

not sure what you were trying to say here, but 13Bs have been available for US consumption since 1974. they disappeared with the introduction of the Rx-7 and came back in 1984 and hung around in one form or another until production of the Rx-8 ended.