Exedy Lightweight Flywheel Subaru STI 2004+

Lightweight flywheels reduce rotational mass in the engine which
means more horsepower, and faster revvs. The Exedy lightweight flywheel
combines low weight, low inertia, and high thermal capacity. Can be used
in all different types of motorsports whether it be drag racing, rally,
road racing or autocross. These could also be suitable for regular
street use. 
These are constructed from a solid-one piece billet chromoly steel
forgings, and improves the engine response. Other special design
features include airflow design enhancements to improve the cooling of
the clutch. The ring gear teeth are integrated onto the flywheel unlike
an aluminum flywheel where the ring gear is pressed onto the flywheel
which has the possibility of separating from the flywheel due to the
different expansion coefficients of aluminum versus steel.