V8 - Crankshaft Design

Crankshaft Design

There are two classic types of V8s which differ by crankshaft:

• The cross-plane or two-plane crankshaft is the configuration used in most V8 road cars. The first and last of the four crank pins are at 180° with respect to each other as are the second and third, with each pair at 90° to the other, so that viewed from the end the crankshaft forms a cross. The cross-plane can achieve very good balance but requires heavy counterweights on the crankshaft. With counterweights, the cross-plane V8 is a fully balanced configuration, so it can be scaled up to large displacements without causing vibration. However, the use of heavy counterweights makes the cross-plane V8 a slow-revving engine that cannot speed up or slow down very quickly compared to other designs because of the greater rotating mass. While the firing of the cross-plane V8 is regular overall, the firing of each bank is LRLLRLRR. In stock cars with dual exhausts, this results in the typical V8 burble sound that many people have come to associate with American V8s. In all-out racing cars it leads to the need to connect exhaust pipes between the two banks to design an optimal exhaust system, resulting in an exhaust system that resembles a bundle of snakes as in the Ford GT40. This complex and encumbering exhaust system has been a major problem for single-seater racing car designers, so they tend to use flat-plane crankshafts instead.
• The flat-plane or single-plane crankshaft has crank pins at 180°. They are imperfectly balanced and thus produce vibrations unless balance shafts are used, with a counter rotating pair flanking the crankshaft to counter second order vibration transverse to the crankshaft centerline. As it does not require counterweights, the crankshaft has less mass and thus inertia, allowing higher rpm and quicker acceleration. The design was popularized in modern racing with the Coventry Climax 1.5 L (92 cu in) V8 that evolved from a cross-plane to a flat-plane configuration. Flat-plane V8s on road cars come from Ferrari (every V8 model they have ever made, from the 1973 308 GT4, to the new 458), Lotus (the Esprit V8), TVR (the Speed Eight), Porsche 918 Spyder and McLaren (the MP4-12C). This design is popular in racing engines, the most famous example being the Cosworth DFV.

In 1992, Audi left the German DTM racing series after a controversy around the crankshaft design of their Audi V8 DTM. After using the road car's cross-plane 90° crankshaft for several years, they switched to a flat-plane 180° version which they claimed was made by "twisting" a stock part. The scrutineers decided that this would stretch the rules too far.

The cross-plane design was neither obvious nor simple to design. For this reason, most early V8 engines, including those from De Dion-Bouton, Peerless, and Cadillac, were flat-plane designs. In 1915, the cross-plane design was proposed at an automotive engineering conference in the United States, but it took another eight years to bring it to production. Cadillac and Peerless (who had hired an ex-Cadillac mathematician for the job) applied for a patent on the cross-plane design simultaneously, and the two agreed to share the idea. Cadillac introduced their "Compensated Crankshaft" V8 in 1923, with the "Equipoised Eight" from Peerless appearing in November 1924.