Aluminium Flywheels

This may help,it is from another site but I think it applies,generally light is for road racing as the engine needs to come to a lower speed for corners and the heavy is for drag racing as the engine needs to stay in it's power band. At one time I raced a normally aspirated RX7 with 350 HP that ran low 10's in the 1/4 and went to a heavier than stock flywheel to help keep the engine at its 9200 rpm band.

"FLYWHEELS: The actual horsepower of the engine does not change related to the weight of the flywheel. However, to increase the engine RPM, and accelerate the car, all rotating weight (as well as the entire car) must be speeded up. This requires power, and the heavier the part, the more power needed to speed it up. The less power needed to speed up the flywheel (and rotors, clutch, driveshaft, wheels, tires, car, etc.) the more power available to speed up everything else. The question we are asked frequently - "How much more horsepower will my engine have with a light flywheel" - is not the right question, because the answer is NONE. What you will have is more available horsepower to accelerate the car and yourself down the road.

The trade-offs are as follows: the heavier the flywheel, the smoother the idle, the easier the starting is from a stop, and the smoother the cruising/slight throttle changes are. BUT - the slower the acceleration is. The lighter the flywheel is, the harder it is to get the car moving from a stop (just requires a somewhat higher RPM before the clutch is let out - which wears the clutch faster), depending on porting, intake, and exhaust, the idle will be somewhat rougher, and some "jerkiness" may be noticed while cruising. BUT - you will be able to accelerate faster. This is true to a much lesser degree on fuel injected engines than on carbureted engines."