There has been a lot of talk lately regarding engine mods and bang for your buck. One of the modifications discussed was the old lightened flywheel. The mod has been used by motoring enthusiasts almost since the car was invented and even with today's modern cars, is still in use today. Now, lightening a flywheel is all well and good but, how many people really know what are the benefits, how are they produced and what must be sacrificed when bolting on a lightened flywheel.

Firstly, lightening a factory cast iron flywheel can have its dangers and is not recommended. The process of casting iron produces a metal component with a hard, fatigue resistant surface and a particularly soft core. If this hard surface is removed during the lightening process ( which it often is ) then this will seriously weaken the flywheel and could result in the wheel coming apart at high rpms, possibly taking the gearbox and your legs with it.

So how does a lighten flywheel work? Amongst the majority, there are two schools of thought concerning light flywheels. The first is that they do not contribute to power output. The second is that they do. Which thought is correct? In fact both, in a way, are correct.

If we measured the power output of an engine first with light flywheel and then again with the standard part on an engine dyno, no change in power will be seen to occure. At first it appears that the light flywheel has done nothing and was a total waste of cash. This is not the case. A dyno that shows max power at constant revs does not demonstrate what occurs to an engine's power output in real life situations - like acceleration. If an engine is accelerated on a dyno ( we are talking about a rate of around 2000rpm a second ) it would show a power output of around 20%-25% less than at the constant rev state.

The reason for this is that when accelerating a vehicle the engine not only has to push the total mass of the car but the internal components of the engine need to be accelerated also. This tends to absorb more power as the extra power is used accelerating the internal mass of the engine components and is why a motor accelerating on a dyno will produce less power than at constant revs. Also it must be remembered that the rate of acceleration on the engine internals is much greater that the rest of the car. This would then suggest that by lightening the flywheel, less power would be required to accelerate it and therefore more power would be available to push the car along.

Now, it may seen unbelievable that by removing a few kilos from the flywheel a noticeable difference to a 1000+ kg cars acceleration will be made. In fact the difference is quite noticeable and the secrete behind this is hidden within the gearbox. Everyone knows that cars accelerate at a greater rate in low gears, this is because a cars gear box basically a mechanical leaver and just like when using a leaver to lift a heavy object, the gearbox reduces the mass of the car that the engine sees. For example, in first gear an engine will see the cars mass as only around say 100kg but the engine internal mass would still remain around 20+kg .

It is now easier to see were the extra performance comes from when you lighten a flywheel. In some cases, and not necessarily extreme ones, you could effectively " lighten " a car by 20% in first gear just by removing mass from the flywheel. As the gear used increases this "lightening" effect is reduced. This is why carÉs acceleration reduces in higher gears, to almost no effect in top gear. Great for drags and tight race tracks but will not increase a cars top speed one bit.

Now if having Lightweight flywheels work so well, why are they so heavy from the factory? The answer to this is quite simple. When a lightweight flywheel is installed, smooth idling almost always suffers - more so if standard cam timing is used. If a longer period cam is used, the situation is not quite as bad. With such a cam the valves close much later in the cycle so it doesnÉt take as much flywheel energy to run the piston up on the compression stroke and the idle is by nature smoother.