Attached to the engine is a huge heavy metal disk, the flywheel. Its presence actually helps to keep the engine running smoothly. We know it takes effort to accelerate or decelerate an object and this law applies to flywheels. Due to its weight it resists changes in engine speed which can be a good thing and also a bad thing.
The advantage is that on long journeys at cruising speed the car will maintain it speed more easily and requires very little in the way of throttle inputs and adjustments. The undulations of the road, hills, bumps and camber have little effect on a cars velocity primarily due to the flywheel.
On the downside it takes great effort to accelerate the flywheel and when you lift off the throttle it takes a while for the revs to drop. In a race situation you need much faster responses to request from a driver for a change in engine speed. Power is also sapped to keep the flywheel spinning.
Fitting a lightened flywheel addresses these two issues. A car will feel more powerful the moment a lighter flywheel is fitted. It will also start to get harder to maintain a constant speed on a road, especially on a hill and you will feel as though you are losing power. In reality you just need to respond more quickly with the throttle if you want the car to continue at its present speed. When you rev the engine it will pick up speed much more quickly and it will also lose its speed much sooner after you release the throttle.
How light should you go? The choice is up to your needs. You have to balance daily drivability with performance. The balance of the flywheel is essential otherwise serious fault will become apparent elsewhere in the engine. This requires professional attention, just drilling holes in your flywheel will almost certainly affect it’s balance. Off the shelf lightened flywheels are avaiblable and some specialists offer a custom lightening service.
Just remember that what is right for the track is often totally wrong for the road. To get the best of both worlds only get a flywheel which is a few kg’s lighter.
