i was thinking....
 

does anyone kno why mazda didnt use the carburated 13b from the REPU in the 1st gen RX-7s instead of the 12A? i was thinking about this today, because the 13b from the REPU had more power right? and they eventually switched to the FI 13b, so why didnt mazda just use the REPU 13b in the RX-7 to begin with?
i donno....just something i was thinking about today....

The 13B in the REPU stopped being produced in the seventies. To answer your question, there was a nitrided 4 port carburated 13B which also had a high volume oil pump and electronic ignition. Mazda had some funny ideas and unfortunately this nice engine didn't find its way into the 1st gen.

^That's exactly what I'm putting in my RX4. :)

To address the original question: I think it had more to do with fuel consumption than anything else, but that's just my opinion.

Does it say R5 in the side plates? Does it only have 17 tension bolts? Do the spark plug holes have semi-circle shrouds to try to prevent the use of short spark plugs? Did it come with a counterweight instead of a flywheel? Does it lack an oil pressure sending hole? These a just some of the unique features of these engines.

I have one in my REPU with mild street ports and another with monster street ports waiting for a 1st gen. I also have the rotating assembly from one that will get some RX-4 rotor housings and 1st gen Y side plates and will find its way into my rotarded MG Midget project. :)