I was just wondering, how great of a difference, or how noticable of a power increase would I see if i replaced my flywheel with a lightened flywheel. I just want to make sure that it is a " worth it" mod before I spend the money to change it out. I plan on getting a lightened flywheel, and new clutch next summer, but only if I will definatly notice a performance increase.

 

It can be a 'worth it' mod, but not because of any power gain. The engine will still make the same amount of power, and the power delivered to the wheels at a constant rpm will be the same. The difference a lighter flywheel makes will only be noticed when the engine rpms are accelerating, i.e. spinning faster and faster. A flywheel stores energy in the form of rotational inertia, which makes it possible to start from a stop, something you really need in normal driving and drag racing. (It can also smooth out engine speeds, which can be good for various reasons) The benefit of a lighter flywheel is that the engine rpms can change faster, thus getting the engine into the power band faster. Thus the engine will be more responsive to throttle changes, which is certainly a plus for a car like the RX-7 with its power band at really high rpms.

Note that too light of a flywheel can be bad if your car is used on the street. Also note that most of the aftermarket flywheels for RX-7s require the use of a counterweight from an automatic.

Remember that it is not just how much lighter the flywheel is, but also how far the weight is from the center of rotation that determines how much rotational inertia it posseses. Thus it is possible to have two flywheels of equal weight that can have entirely different effects.