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Rotating mass is everything that rotates due to the engine turning. Engine internals, pulleys, the flywheel, transmission internals, driveshaft, diff internals, axles, brake discs, wheels, and tires. These all represent a significant amount of weight, and power spent turning all these things around is power that could be going to the ground to move you forwards. Obviously, some things you're not going to be able to change. However, you can lighten the pulleys, flywheel, driveshaft, brake discs, and wheels pretty easily. Less weight requires less power to move the weight and allows more power to be put to the ground.
But there are tradeoffs. A heavy (OEM) flywheel has a lot of inertia, so when you push the clutch in to shift, the RPMs drop slowly instead of plummeting. This makes for smoother shifts with less fucking up. A lighter flywheel has less mass, so less inertia. Now when you push the clutch in the RPMs drop like a rock. Your shifts need to be on point, every time, or they're going to suck.
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