Use of a lightened flywheel frees up power by reducing rotational mass and inertia.  In other words, the motor revs faster!  The stock flywheel weighs in at around 30 lbs. - lightened flywheels are significantly lighter versus the stock flywheel.

There are two kinds of lightened flywheels...

The lightened steel flywheel weighs in just under 20 lbs. - that's at least a 1/3rd weight difference versus the stock flywheel.  The lightened steel flywheel is a good upgrade for street and drag racing applications.  The (slightly) lighter weight of the lightened steel flywheel does not compromise starting from a stop, but it does let the engine rev significantly quicker to redline.

The lightened aluminum flywheel weighs around 10 lbs - that's a 2/3rds drop in weight!  Because of it's low weight, the engine revs a lot quicker to redline.  Contrary to popular belief, the lightened aluminum flywheel is not entirely made out of aluminum - in addition to the steel, outer ring gear, a steel insert is secured with rivets as the contact area for the clutch disc.  This steel insert can be replaced when worn past it's service limits, thus the unit is rebuildable.  These aluminum units are mainly used by road racers; unlike drag racers, road racers do not require a priority on good, standing starts.   Lightened aluminum flywheels are not recommended for drag racing (or for street) due to it's inability to launch from a stop easily.  There are people who do street lightened aluminum flywheels, but most shops do not recommend this.

Use of any lightened flywheel requires the use of a proper automatic transmission counterweight.  The lightened flywheels do not have the appropriate counterweights attached, so be sure to track down the appropriate rear counterweight when trying to replace your stock flywheel.  Also, six bolts are used to secure the rear counterweight to the lightened flywheel - be sure to have these on hand when doing the install.

(insert pics of lightened steel flywheel)
(insert pics of lightened aluminum flywheel)
(insert pics of various counterweights)

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