Stock piston weight: 720 grams
Stock pin weight: 160? grams
Stock connecting rods: 758 grams

Typical sftermarket HP stuff:
Piston weight: 500 grams
Pin weight: 120 grams
Connecting rods: 650 grams

As the car accelerates, the engine also accelerates its rotation. Accelerating the heavier rods, pins and rods eats some of the power that could be used to accelerate the car.

R.