As well as accelerating the car, the engine has to also accelerate its own mass, the lower the inertia of the engine the faster it will accelerate itself, this improves the acceleration of the vehicle as a whole, especially in the lower gears where the mass of the engine is a greater proportion of the overall mass. The rotating moment of the flywheel is quite high so removing mass, especially at the outer reaches of the flywheel has a profound affect. oily