[DougD]

The material used (aluminum or steel) is less important than the actual weight of the flywheel. There are lightweight steel flywheels out there that approach the weight of aluminum.

For a street car, the rule of thumb I've been told is that the flywheel weight should be approximately 1% of the car's weight. A 4000lb car should have a 40-pound flywheel. A 2500lb car needs a 25 lb flywheel, etc.