I'm currently in the process of ordering a 509ci big block soon (read my other posts for info). I'm wondering if I should go with the heavy steel, or light aluminum. I put a hays heavy steel version behind my high revving 357. It caused the RPM's to drop less between shifts, hit the tires harder, and slightly affect the torque output.
This big block is going in the same car, a fairly light 68 Nova, 4-speed car. I deffinetly won't need help in the torque department, but it wont rev as quick as the small block. Will a 15lb. aluminum flywheel be better than a 30lb. steel version for this engine?

 

I built 2 cars in the last few years, one for me and one for my son. They used almost identical drivetrains with about the same weight bodies. Both engines were Pontiac 455s with equivalent power (380 rwhp/ 470 rwtq) with Tremec TKO 5 spd trans, Moser 9" 3.70 rears, Denny's driveshafts, Lakewood bellhousings, CF dual Friction clutches, etc. Only real difference was my car had a 15 lb aluminum flywheel and his had a 30 lb steel flywheel. His car would pull the front wheels about 4" every launch and run better 60 foots. However, mine would pull slightly better ETs and more mph. From a 50-70 mph roll, I would always walk away from him. The lighter flywheel hurts you a bit at launch but helps you every where else. We both prefer driving the car with the lighter flywheel. Hope that helps.

 

I've got a 35lb billet steel flywheel behind my 350, and I'm thinking of trying a lightweight Fidanza next time. I think I'd prefer the snappier revving of the lighter setup.

 

IMO the more weight you take off the rotating assembly/drivetrain the better!