What gains do you get by doing this?
Responsiveness?
Marginal power increase?

Interested to know

The are no negative effects to lightening your stock flywheel unlike replacing it with an ultra-lightweight Aluminum flywheel. Most manufacturers make the stock flywheel very heavy. This makes the engine very smooth and enables it to retain energy at part throttle cruise and up long grades for better fuel economy.

An Aluminum flywheel goes completely the other direction with almost no weight. This lets the engine rev up very quickly and allows the engine to work easier due to the reduced weight spinning around on the back of the crank. This is fine as long as you are at full throttle and wide open throttle all the time like in a Road Race or Drag Race only car. As soon as you let off the gas the engine RPM drops instantly and the car slows down. In a road car this causes surging and bucking at cruise speeds and poor driveability and clutch engagement.

A lightened stock unit on the other hand gives you the best of both worlds. It is a compromise between the overly heavy OEM unit and a Too light Racing unit. You get the benefits of both with none of the bad qualities.

Turbo cars are exceptionally critical to flywheel weight. Too light a flywheel will make the car rev faster and possibly come on boost faster but that is only half the issue. When you let off the gas to shift the car will drop off of boost just as quickly causing a poor transition when you come back on the throttle.

We have found that a lightened stock unit will generally be 8-12 lbs. lighter than the original yet still give the driveability needed for street use. The Mitsubishi Starion is an extreme example of an overweight stock flywheel. They are 33-35 lbs stock. We lighten them down to 20-21 lbs with nothing but positive effects and no loss of strength or reliability.

All the weight is taken off the back side of the flywheel so the surface is still the same height and your clutch still fits and works the same. We try to remove the weight as much from the center of mass as possible and leave a ring of weight around the outside for better inertia. We keep the center hub thick for extra strength.

How does a lightweight 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 occur. 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 happens 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 pounds from the flywheel a noticeable difference to a 3000lb+ car’s acceleration will be made. In fact the difference is quite noticeable and the secret behind this is hidden within the gearbox. Everyone knows that cars accelerate at a greater rate in low gears, this is because a car’s gear box basically a mechanical lever 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 car's mass as only around say 250lbs but the engine internal mass would still remain around 45lbs.
http://www.uucmotorwerks.com/flywheel/h ... _works.htm