Actually. a flat crank V-8 fires every 90 degrees, it just has a different firing order than a crossplane crank V-8. In order to fire two cylinders together every 180 degrees, four cylinders would have to come to top dead center at the same time, two of them firing and two of them reloading. With a 90 degree V angle of the cylinders, when either the two middle pistons or the two outer pistons of the left bank are at TDC, all the pistons on the right bank are at mid stroke.
The actual source of the infamous “4 cylinder buzz” is the fact that piston motion is not sinusoidal. The piston acceleration at top dead center is much higher than the piston acceleration at bottom dead center. The motion from mid stroke to tdc is not a mirror image of the piston motion from mid stroke to bdc.
The asymmetrical motion of the pistons prevents the inner cylinders from perfectly balancing the inertial forces of the outer cylinders. Boxer fours are a different story, they lack the “four cylinder buzz” even though they also have a 180 degree firing order just like inline fours because the inertia of a piston at bottom dead center is balanced by another piston at bdc an the inertia of a piston at tdc is balanced by another piston at tdc.
This picture I drew illustrates what connecting rod length does to piston motion.
