I was pulling some 305s apart and found that the 86 from an LG4, T-5 was waaayyy heavier than the one I pulled from an 87 LB9 T-5. The 86 is solid, where the 87 is hollowed out on engine side. Seems to be almost half the weight. since 86 was first year of the 1 piece RMS, does this mean the 86 flywheel will only work for an 86 crank? Or can you put the lighter 87 one on the 86 crank without screwing up the balance? Or was the lighter flywheel a performance option?

 

The later flywheels should be fine on the '86, but the LG4 probably has a 3.08:1 axle, while the LB9 probably has a 3.45:1 axle. The smaller number the rear gear ratio, the more a heavier flywheel will help driveability in town. The reverse is also true. If you're going to a bigger-number axle ratio, the easier it is to like a lighter flywheel.

 

The run-of-the-mill cars (LG4 and TBI) got the heavy flywheel, to help the motor be able to hulk the poorly-geared car out into traffic without killing the motor. The performance cars (L69 and TPI) got the lighter flywheel, since with more power and better gears, they didn't need to store up as much energy to launch the car.

The difference is about 16 lbs vs about 22 lbs.

Yes they are completely interchangeable. Get rid of that half-ton thing and use the light one.

 

At this level, power doesn't enter into it.
Even the final LB9s, at 230 HP, still had good off-idle torque, compared to the torque-cammed LO3 and LG4 versions.
Now, if you were gonna run a 292-duration (or similar) cam, in traffic, you might do better with the heavier flywheel, even with 4.10:1 axle gearing. That 292 cam will be dramatically down on torque off-idle.
Light flywheels were invented for road-course-style racing.

 

Great, thanks guys. I was planning on using the lighter one with the 87 engine in the project anyway. The 86 is for sale, dont have the engine anymore.