The 2nd order harmonic vibration of the flat plane V-8 and 4 cylinder engines.
At any given instant, the velocities of the pistons traveling upward does not equal the velocities of the pistons traveling downward.
The net velocity of all eight cylinders is therefore not equal to zero, which causes vibrations, felt as buzziness to the occupants of the vehicle.
Contributing factors that make up this 2nd order harmonic are
1) Stroke,
2) Reciprocating mass, and
3) Rod to Stroke ratio
Shorter strokes, less reciprocating mass and longer rod to stroke ratios all help to reduce that 2nd order harmonic
Most 4 cylinders are small enough and have a large enough rod to stroke ratio that the 2nd order buzz is generally small enough it hides under the firing pulse buzz, but is still there
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