* Manufacturer: Different manufacturers have different designs and materials, leading to varying weights. A 3.9L V6 from General Motors will likely weigh differently than one from Ford or Chrysler (assuming such an engine even exists from all three – a 3.9L V6 is not a common displacement).
* Materials: The use of aluminum vs. cast iron in the block and heads significantly impacts weight. Aluminum is lighter.
* Accessories: The weight will change depending on whether the engine includes accessories like the alternator, power steering pump, intake manifold, etc. A "bare" engine will weigh less.
To find the weight, you would need to specify the exact make, model, and year of the vehicle the engine came from. Then, you could potentially find a service manual or parts catalog for that specific engine. Even then, finding the exact weight might be difficult.
