Would this be doable using just the rotors (including bearings) from an '89 engine in a 84 or 85 13b block? Or would there be mismatches between e.g. the rotor bearings and the eccentric shaft, or the rotors and the counterweights, etc.? Just curious if you can take advantage of the higher compression 89-91 rotors in a pre-86 13B without getting too complicated or expensive? Thanks.

 

I don't think that you can use JUST the rotors from the 89 in the gslse engine without balance problems.
In the last engine that I built for my friends 84 gslse I used the complete rotating assembly from a 86 rx-7(rotors,counterweight,eccentric shaft&the flywheel) the statinary gears were changed also but its not needed.

 

You have to keep rotors, front counterweight, and flywheel/rear auto counterweight together as a balanced set. Change all 4 parts and it will be just fine.

I ran GSLSE rotors in my 89 turbo setup, and I have put other rotors in GSLSE engines before. Just keep the balanced parts together and it will be fine. You can run any 13B rotor in any 13B engine with it's matched balances.