Ahh, racing. Where horsepower is good and weight is evil. Or is it? When you think about it, it's the weight on the tires that gives us traction. In the simple analysis, the friction force (F) between the tires and track is proportional to the weight on the tires (W) and friction coefficient (u) from F= Wu. So weight is good.
However, when you try to accelerate your car, weight becomes a problem. According to Sir Newton, force equals mass x acceleration, F= ma. Written another way, acceleration is proportional to the force divided by the mass, a= F/m. Notice we don't show "weight" here, only "mass," so what's the difference?
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Picture a flywheel, 10 inches in diameter and 1/2 inch thick. Also picture a shaft, 1 inch in diameter, and 50 inches long. Both will have the same mass, but the flywheel will have 100 times more rotational inertia than the shaft, purely due to the location of the mass, farther away from the axis of rotation. | 
Here are the aluminum drums next to the stock iron drums. The aluminum is used on the heat sink (fins) around the outside of the drum and the mounting face. Since the heat sink weight savings is at the farthest point from the rotational axis (axle centerline), the improvement in rotational inertia will give a greater performance effect than just the pure weight savings alone. |
Mass is, well, mass. It's a measure of the amount of "matter" you have (count up all the protons, neutrons, and electrons in your car next time you're bored). Weight (W) is a function of mass and the acceleration of gravity (g), so W= mg. Now as long as we race only on Earth, the gravity is fairly constant, so weight and mass are routinely used to describe the same thing. However, they're not.
If we decided to hold the "Solar System Nationals" on, say, Jupiter (ignore for the moment that Jupiter doesn't really have a "surface" per se), the gravity would be much greater. If you hadn't changed the car since the last time you ran it on Earth, the mass you have to accelerate on Jupiter would be the same, but due to the increased gravity on Jupiter, the car's weight would now be much greater on Jupiter. Therefore, traction would be greatly improved, compared to Earth. Assuming the same power levels on Jupiter (yeah, I know...maybe if you brought your own air)--since you still have the same mass to accelerate, but now with more traction from the increased weight--your car could run much quicker on Jupiter than on Earth. However, don't expect any actual drag testing on Jupiter as part of this article, since we don't have that kind of travel budget.
It's difficult to see now with the drums sandblasted (and painted), but checking out the inside of the aluminum drum shows a cast-in iron liner used for the friction surface. In other words, the aluminum drums will not wear out any quicker than the original cast-iron drums. | 
Using our old fishing scale (yes, we had a life before GTO restorations consumed every minute/dollar), the factory cast-iron drums weighed in over 13 pounds each. | 
...and in this corner, weighing in at a featherweight 8 pounds, is the Aluminum Contender... |
There's more: It's not really mass that's the problem--it's "inertia," which is a function of mass. Inertia can be described as an object's resistance to acceleration, so something with inertia (i.e. anything with mass) will resist being accelerated (or decelerated for that matter). More mass equals more inertia equals more resistance to movement (acceleration). You don't see fat guys winning the 100-meter Olympic sprints, and similarly, you don't see too many successful skinny Sumo wrestlers, for whom more inertia is a distinct advantage.
So far, we've really just been talking about stuff moving in a given direction. When it comes to rotating objects, it gets a bit more complicated. For a rotating object, its "rotational inertia" is determined by not only the mass, but also where the mass is in relation to the axis of rotation. If the bulk of the mass is located far from the axis of rotation, the object will have much greater rotational inertia.