S4 (1986-88) n/a 24lbs
S5 (1989-91) n/a 22lbs
S4 TII (1986-88) n/a 28lbs
S5 TII (1989-91) n/a 22lbs
S6/S7/S8 (1993-2002) 19lbs
Source: Racing Beat, www.racingbeat.com
Flywheel Explaination:
In order to perfectly balance the rotating assembly of a rotary engine there is a front and rear counterweight. On manual transmission RX-7s the OEM flywheels are "internally balanced." What this means is that in order to balance the rotating mass of the engine, the flywheel does not have have the same weight symetrically distributed around its circular surface. Automatic transmission RX-7s use a flexplate with symetrical weight distribution around the circular surface. In other words on their own, without the rest of the engine, flexplates are balanced and flywheels are unbalanaced. In order to properly balance the rotating assembly of engines with automatic transmissions, an external rear counterweight is employed. If you look at the "engine side" of a stock flywheel you will notice that there is a partial ring of metal portruding out towards the engine. This portrusion is the internal balancing.
All aftermarket flywheels (with the exception of one from HKS) have no internal balancing. They are simply symetrically balanced by the manufacturer and require an external counter weight like a flexplate. The counterweight used with an aftermarket flywheel is the same as used on automatic transmissions. In other words, if you want a counterweight for your flywheel, you can probably find one from someone who is parting out a car with an automatic transmission or is upgrading to a manual transmission.
All S4s use the same counterweight while all S5s and FDs use the same counterweight. This might be contrary to some logic, but it's true. For instance S4 n/a rotors are different from S4 TII rotors, but they weight the same amount. N/A and TII flywheels are NOT the same. TII flywheels are larger in diameter. The S5 and FD rotating assemblies have the same weight and counterweight. Interestingly even though the S5 TII clutch is different from the FD clutch, the lighter stock FD flywheel can be used on a S5 TII. In other words you can pay $200-400 for a ~17lbs steel flywheel for your S5 TII or you can buy a used 19lbs FD flywheel for ~$30. In terms of aftermarket flywheels (externally balanced), you can use any TII flywheel on any TII or FD model, as long as you select the correct counterweight for your engine. I'm not 100% certain if the same is true for aftermarket flywheels for n/a models.
The benifit of a flywheel is that it reduces the drivetrain's rotating mass and thus allows for less of the engine's power to be wasted on it's way to the wheels. Unfortunately this benifit is substantial during the lower gears and rather small in 5th gear. Reducing the flywheel's weight too much can result in reduced drivability for street driving.