[HarryD]

The flywheel helps the engine keep it's momentum when you engage the clutch. For a race car, the ligher fly wheel means you need to accelerate less mass, and gives you a slighly better rate of acceleration as your power goes to the wheels instead of overcoming the inertia of the mass in the flywheel. Also, the lighter flywheel, since it has less inertia, means it is easier to stall when you first start moving the car from a dead stop.