<P> In a typical V - twin with a shared crank pin, (e.g. Ducati), the strong vibration of the 360 ° - crank parallel twin is divided into two different directions and phase separated by the same amount of degrees as in the V angle, with 13 . unevenly spaced firing as well as the imbalances 4 . Plane imbalance on reciprocating mass, 6 . Plane imbalance on rotating mass, 14 . Plane imbalance on torque generation and 16 . Plane imbalance on compression . These four kinds of imbalance are also known as "rocking couple". </P> <P> A boxer engine is a type of flat engine in which each of a pair of opposing cylinders is on separate crank throws, offset at 180 ° to its partner, with 13 . an evenly spaced firing . If the pistons could lie on the same crank rotational plane, then the design is inherently balanced for the momentum of the pistons . But since they cannot, the design, despite having a perfect 3 . phase balance largely cancelling the non-sinusoidal imbalance, inherently has 4., 6., 14 . and 16 . imbalances due to the crank pin rotating planes being offset . </P> <P> This offset, the length of which partly determines the strength of the rocking vibration, is the largest on the parallel twin with a 180 ° crankshaft, and does not exist on a V or a flat engine that has a shared crank pin with "fork and blade" conrods (e.g. Harley - Davidson V - twin engine . See illustration on right). Other configurations fall in between, depending on the bigend thickness, crank web thickness, and the main bearing width (if they exist in between the throws). </P> <P> Inline 3 with 120 ° crankshaft is the most common three cylinder engine . They have 13 . evenly spaced firing and perfect 3 . phase balance on reciprocating mass, with 4., 6., 14 . and 16 . imbalances . Just like in a crossplane V8, these first order rocking couples can be countered with heavy counterweights, and the secondary balance is comparable to, or better than an ordinary inline 4 because there are no piston pairs that move together . </P>

What kind of engine noise is most noticeable at idle when oil pressure is at its lowest