I bought my current rx7 a short while back, 85 GSL with a carbureted 13b in it.
As far as I can tell the motor is a very early (late 70s) version out of a cosmo or rx4.
Are there many differences between these and the more common later EFI motors?
Fairly certain the were 4port not 6port?

 

There are some differences. Your engine is a 4 port. Some would say it's a superior to the later 6 port designs, and I'd agree with them.

 

mine from a 74 rx4

 

Mine with 74 ported nitrided R5 irons and J-spec rotor housings with a GSL-SE exhaust port timing.


Right after stacking

 

its basically a 12A with wider rotors, the later EFI engines have a bunch of differences that make them, well different.

 

so clean