Crankshaft is what helps in conversion of reciprocating motion to rotational motion and vice versa. The flat plane crankshaft is the simplest design for the crankshaft, which was very popular in earlier designs of V8 engines. The straightforward design was easy to build and apply for the powerful engines despite its various limitations.
A crankshaft has journals arranged at varying degrees against each other, to which connecting rods are attached. The journals move along an axis in rotational motion and the connecting rods move in the reciprocating motion. That is how inter-conversion between the two motions occurs.
In a flat plane crankshaft, the journals are arranged against each other at an angle of 180 degrees. That means that the journals are so arranged that they are on a single plane. Contrastingly, journals in a cross plane crankshaft are arranged at ninety degrees against each other, thus forming the shape of a cross when viewed head-on.
A flat plane crankshaft is great in power and design. It takes less place and does not require any counterweights, which increases the speed of its spins and hardens its rev. But due to a number of issues with its rotational balance, the birth of the cross plane crankshaft became pertinent.
When one says rotational balance, it refers to the secondary vibrations from the engine – an issue not known to plague the new arrangement. It also came with an additional advantage. The four journals balance each other well in a cross plane crankshaft, but the firing pulse they create on each bank is uneven. This unevenness gives the cross plane crankshaft a fine, rhythmic sound that everyone has come to enjoy in modern day V8 engines. However, despite overcoming a major issue faced by flat plane crankshaft engines, the cross plane crankshaft could not compare to its predecessor in terms of weight and size. It was considerably more balanced and smooth, but required heavy counterweights and a heavy crankcase, which messed with the weight and centre of gravity of the car.
So, flat planes are generally used and seen in small, light race cars where pure power is given more importance than the experience of driving the car. The vibration problem of the flat planes becomes more pronounced with higher capacities. So, it can be easily used effectively in smaller engines without causing much trouble. So, it is also seen in four cylinder donks and boxer four/ six engines such as in Porsche 911s.
Talking about power, some people have estimated that the flat plane engine can go as high as 8000 rpm. That V8 on a race track would be insane, never mind all the noise and vibration. Leading car makers like Ford and Ferrari agree. Ferrari makes all its V8s with flat planes. And Ford has demonstrated how the vibrations can be controlled by intelligent use of light weight materials and creative designs.
Another simple reason to prefer flat planes is its efficiency in the usage of cylinder exhausts. Its performance and efficiency in this regard is much greater than other variants.