I will be having the rotating assembly for my 85 12A clearanced and balanced and am thinking of having them lightened as well. The car will be NA for a while but is likely to be boosted in the future. I'm concerned with taking away the rotor's ability to absorb heat and also compromising its structure. I don't think it is absolutely necessary to lighten the rotors because I will already be using a 9 lb Turbo flywheel and aluminum main pulley, plus the most that the engine will see will be 8500-9000, less when boosted. It just may be worthwhile since I will be sending the rotors out anyway.

Opinions of people who have EXPERIENCE with boosting on lightened rotors is very appreciated.

 

Why not just remove a little from the tub,that way youll lighten them and lower the compression a bit for boosting?
For the record,I run a S5 TII motor and I can really tell the difference between them and the heavier S4 TII rotors I used to run.A few lbs does make a difference,especially with a light flywheel.

 

I will at least be smoothing the tubs out and putting a slight radius on the edges of the tubs. I'll cc them to make sure they are identical. I also will be putting a decent sized bevel on the opening edges of the rotors to get a little earlier opening and better airflow than I could get with a normal streetport (I don't want a bridge). This will all be done before having everything balanced. It should add up to about 9.0:1 which should be safe enough to run 12-15 psi on with a standalone. I will probably go for a T04B setup with a nice, divided, equal length manifold. My biggest concern now is whether the housings will be strong enough to support that much torque. I'm going to make another thread to see if anyone is having issues with 12A rotor housings under boost.

 

Sounds good.Thats basically what Im running,except with a 13B.
The rotors have been known to collapse at the tub,but I highly doubt it would happen in the 12-15psi range,especially if you drop your static compression ratio down a peg.The rotor housings should be OK,Id worry about the irons.They have a tendency to crack on the early TII engines,until Mazda reinforced the rear plate.But thats only an issue above 400HP.But it also depends on your tuning.If you go heavier on the streetporting,you can get more HP with less boost.The tradeoff will be less lowend,which can be a good thing since lowend grunt tends to be harder on driveline components.
With a standalone and a TO4,you shoud be able to squeeze 300+ crank HP out of a 12A with minimal internal mods.Just get your tuning right and dont beat it.Over that,Id start to worry about structural integrity with a mostly "stock" engine.A race engine and a daily driven engine are two different beasts.

 

300 RWHP would be SWEET, even if it only happens on rare occasions like when I take it to a dyno or track and boost a bit extra.

For the street I would probably limit boost to 6-8 psi.