Basic Differences

• In its most basic sense, a V-6 engine is just a V-8 with two cylinders hacked off. While most V-6 engines don't directly share components with V-8s from the same manufacturer, they tend toward the same architecture. However, some V-6s are indeed just modified V-8s; evidence the GM 4.3-liter, a V-6 derivation of the 350 V-8, and the Chrysler 3.9-liter Magnum derived from the 5.2-liter Magnum V-8.
  
  

Smoothness

• A V-8 engine is inherently smoother than a V-6 engine of the same displacement. Part of this has to do with the size of the individual cylinders, since a V-6 engine will require much larger cylinders -- and thus a larger and more powerful combustion event -- to match the V-8's displacement and power. The other major factor has to do with the cylinder count on one bank, as the V-6 is essentially a pair of in-line-three-cylinder engines. This odd cylinder count creates an inherent imbalance not present in a V-8 or in-line-four engine.