Do you prefer a V8 that rumbles or one that screams? Do you favour refinement or engine response? A flat-plane or cross-plane crankshaft can have a major influence on a V8 engine’s characteristics. Graham Eagle investigates …
After years of producing high-performance V8 engines – some normally aspirated and some turbocharged but all admired for their rumbling exhaust notes – the announcement in 2020 by then AMG boss Tobias Moers that the forthcoming Mercedes-AMG GT Black Series would feature a flat-plane crankshaft created quite a stir. This was a first for AMG and the reaction was not dissimilar to that of a few years earlier when Ford launched the Mustang Shelby GT 350 featuring a 5,2-litre engine with a flat-plane crankshaft as the performance flagship model in the range. Brochures and marketing material at the time made references to Ferraris and racing engines, while a redline of 8 250 r/min clearly differentiated it from other established American performance machinery.
History shows that all early V8s featured flat-plane crankshafts, not for performance reasons but because they were easier to manufacture. They had also been conceived as a pair of in-line four-cylinder engines mated together and all in-line fours use flat-plane crankshafts. It was only in the early 1920s when Cadillac, in the quest for more smoothness and refinement, developed the cross-plane crankshaft V8, a configuration that has since become the norm for production vehicles.
Ferrari has always been a notable exception, with all their roadgoing and racing V8s utilising flat-plane crankshafts. McLaren has followed suit with its 3,8- and 4,0-litre V8s fitted to its range of supercars also going flat plane. Interestingly, the Ferrari F136 engine family, which won a total of eight International Engine of the Year Awards, was produced in flat-plane form for Ferrari (think F430, 458 Italia and California) but with a cross-plane crankshaft for use by Maserati (4,2- and 4,7-litre) and the Alfa Romeo 8C.
What does flat plane and cross plane mean exactly?
The terminology refers to the angle at which the crank journals are positioned relative to each other. On a flat-plane crankshaft, the journals are positioned at 180 degrees to each other and thus all journals lie on the same two-dimensional plane. On a cross-plane crankshaft, the journals are positioned at 90 degrees, meaning they lie on two different two-dimensional planes, perpendicular to each other. Explained another way, when viewed end-on, a flat-plane crankshaft looks like a minus symbol (-) while a cross-plane crankshaft forms a plus symbol (+).
Good (and bad) vibrations
As noted, the cross-plane crankshaft was developed for improved smoothness and refinement, so where does the flat-plane iteration fall short in this regard?
Primary balance is the vibration that occurs at the first harmonic of the engine; the frequency of crankshaft rotation. Just like an in-line four, a flat-plane V8 has excellent primary balance in that at any given point in the crankshaft’s rotation, there is an equal number of pistons (equal masses) moving in opposite directions to one another. When two pistons on one bank of cylinders are at the top of their stroke or top dead centre (TDC), the other two pistons are at the bottom of their stroke, bottom dead centre (BDC). Therefore, the mass of the two pistons and their conrods at TDC cancels out the mass of the two pistons and their conrods at BDC. This eliminates the need for heavy counterweights and the crankshaft remains light and compact.
The drawback of the flat-plane crank is it generates significant secondary vibrations; by definition, these occur at twice the frequency of crankshaft rotation. To understand why we need to examine the movement of the piston relative to the rotation of the crankshaft. As the crankshaft rotates clockwise from TDC to the 90-degree position, the angle of the connecting rod means the piston travels a distance greater than half the length of the stroke. As the crankshaft then rotates a further 90 degrees to BDC, the piston travel is, therefore, less than half of the length of the stroke. The reverse applies as the crankshaft completes a full rotation and the piston moves back to TDC.
The piston travels a greater distance during the top half of its movement between 270, TDC and 90 degrees than during the bottom half of its movement between 90, BDC and 270 degrees. This leads to higher rates of acceleration and deceleration during the top half of its movement than in the bottom half, in turn, resulting in higher inertia forces by the mass of a piston during the top half of crankshaft rotation than the bottom half. This disparity in the upward vs. downward inertia of the pistons creates an upward vibration that occurs twice per crankshaft revolution.
These vibrations could be cancelled out using two balancer shafts; however, these are not favoured on high-performance engines owing to the additional weight and friction. Vibrations are usually reduced to an acceptable level through the use of lightweight pistons and connecting rods, crankshaft dampers, dual-mass flywheels, active engine mounts and other scientific measures. Extreme examples are the 2,4-litre Formula One V8s used between 2006-2013. Revving to 18 000 r/min, some of these engines required as many as 13 dampers fitted to the crankshaft, camshafts and other parts of the valvetrain to control vibrations and avoid failures.
In contrast, each bank of a cross-plane crankshaft engine has four distinct piston phases, eliminating the secondary inertia forces and the resulting vibrations of flat-plane engines. However, with the middle and end crank throws being phased 180 degrees apart, a primary (engine speed) rocking moment is created which is countered by including heavy counterweights on each crank throw. As a result, cross-plane engines are extremely smooth – contrary to what their rumbling exhaust note suggests – but the heavy, counter-weighted crankshaft has high rotational inertia making it less responsive and free revving than a flat-plane engine.
Both engine types fire a cylinder every 90 degrees, equating to eight firing events per four-stroke cycle of 720 degrees, or two rotations. A key difference is that the flat-plane crankshaft fires a cylinder from alternating banks at equal intervals, which improves intake breathing and exhaust scavenging. With the exhaust pulses within the exhaust header of each bank evenly spaced at 180 degrees, the vacuum following each pulse assists the flow of the next pulse by creating a scavenging effect which is particularly important at higher engine speeds.
With the cross-plane crankshaft, the cylinder firing order is such that within each 720-degree cycle, two cylinders on each bank fire consecutively as opposed to alternating between banks. These consecutive exhaust pulses are only 90 degrees apart, interfering with each other and causing a pressure build-up which, in turn, compromises the scavenging effect. This is sometimes countered in road applications by balancer pipes and in racing applications by different header designs; the so-called “bundle of snakes” header design made famous on the Le Mans-winning Ford GT40s is one such example. In this design, two pipes from each bank cross over to combine with two pipes from the opposite bank into a common collector. They are also known as “180-degree” headers as the exhaust pulses in each collector are spaced 180 degrees apart – the same as the flat-plane crankshaft engine – and thus provide the same scavenging effect.
Which is better?
The fairest answer would be that neither is better; only that each has its own characteristics that make them suitable for a particular application.
Flat-plane crankshaft engines are better suited to high-performance or sporty applications. Their lighter crankshafts have less rotational inertia which makes them free revving and allows the use of more compact cylinder blocks, while their superior exhaust scavenging allows a more compact and efficient exhaust manifold design.
In the case of the Mercedes-AMG GT Black Series, the increased rev limit of 7 200 r/min – combined with revised camshafts, turbos, exhaust manifolds and larger intercoolers – has lifted outputs to 537 kW/800 N.m, an increase of 107 kW and 100 N.m over the already impressive GT R model fitted with a cross-plane crankshaft. In short, flat-plane crankshafts provide the ideal configuration for manufacturers like Ferrari and McLaren, and Mercedes-AMG’s most track-focused model. Vibrations are minimised by the relatively small engine capacities, use of lightweight materials, careful balancing and sophisticated engine mountings.
Cross-plane crankshaft engines, on the other hand, are ideally matched to high-end road cars. Their inherent balance and heavier crankshafts make them suitable for larger engine capacities and they generally deliver a wide spread of torque, ideal for road use. They can be tuned to deliver big power and torque but generally at lower peak engine speeds than flat-plane engines. Engine response might be slightly slower than flat-plane engines but with most of these engines coupled to responsive, multi-speed automatic transmissions, this is unlikely to be noticed by their drivers.