by James Kraus
Why do Ferrari’s V8’s sound so delicious, almost as enticing and melodious as a V12? It comes from the use of a single-plane “flat” crankshaft in lieu of the typical cross-plane (two-plane) crankshaft. Workaday V8 engines utilize the cross-plane crank to optimize mechanical smoothness; an admittedly important consideration when transporting a hedge fund manager and his mistress to a performance of Die Walküre at the Théâtre de Genève in a BMW 750i, or a load of sensitive electronic test equipment behind a MAN TGX in route to the CERN Large Hadron Collider.
While a twin-plane crankshaft design is mechanically smoother, more power can be squeezed from a single-plane configuration. The power advantage stems from the firing order.
In a cross-plane configuration, the firing order results in sequential cylinder-firing occurring on both common and opposing banks. This results in exhaust pulses in each bank that are uneven, with large time gaps when two cylinders fire sequentially on the opposite bank. The unevenly spaced pulses make high rpm exhaust scavenging less effective and result (when lightly muffled) in the flatulent, staccato motorboat sound so beloved by muscle car, truck and Harley-Davidson enthusiasts.
In the flat crank design, used by all F1 teams and every Ferrari V8 road car ever constructed, sequentially-firing cylinders are always on opposing banks. This results in a even spacing of exhaust pulses within each bank of cylinders, allowing for very efficient scavenging of exhaust gas at high revs. Not surprisingly, it also emits a much smoother, more melodious soundtrack, rising from a smooth well-tempered rumble to an otherworldly fortissimo crescendo as it climbs toward the redline. Think of it as Luciano Pavarotti as compared to Ozzy Osbourne.
BMW took a novel approach to the exhaust-pulse timing issue when they designed their twin-turbo S63 V8 (currently used in the X5M and X6M.) They retained their traditional two-plane crankshaft, but they turned the cylinder heads inside-out so that the exhaust ports empty into the vee between the cylinder banks, and the intake ports face outwards.
By then placing the turbochargers into the vee and adopting clever manifolding, they made it possible to feed each turbo with sequentially-firing cylinders from either bank. In this way, each turbo impeller is accelerated by a smooth, uninterrupted flow of exhaust gas, drastically lowering spool-up time, and providing near instantaneous throttle response.
The S63 sounds similar to a V8 Ferrari except for the fact that the spinning turbine wheels muddy the exhaust note a bit, slightly masking the inherent sound signature.