v6's tend to run more smoothly because you have more of a curve when you look at the combustion impulses. If you were to think of a sin wave you can cut it up into six sections to represent each combustion. With a 4 banger you can only cut that up four times. You get a better curve fit with 6 combustions than you do with 4.

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Nissan Versa HB 6MT 1.8L I4 Magnetic Grey