18a would be custom excentric. Or heavily modified 20b...
the Difference in cappacity between a 13b (1300cc) and a 12a (1200cc) is gained but the rotor housings being different widths.. So a 13b is longer than a 12a..
meaning a normal 20b crank would be too long if you built a three rotor engine using 12s housings..
And where the turbo .???
Friends 20b is now making 1300rwhp.... imagine your 86 with that...