What's the difference, and which is better?
In the automotive world, some might argue that there isn't a better sounding engine than one with 8 cylinders in a V-shaped box. The humble V8 is one of the most common high performance engines, characterized by a deep rumble – the kind that sets your heart quivering in your chest and gets heads turning. But the iconic rumble of the V8 isn't the only signature note it makes – in fact it's just the one most associated with traditional American V8s.
Across the Atlantic, European V8s have an altogether different signature sound – a free-revving, loud scream of a note that makes the hair on the back of your neck stand up. Why the difference? It's all down to a single piece of the engine called the crankshaft, and the difference in layout of it – cross-plane in the American tradition, and flat-plane in the European setup. But what's the difference? How does a flat-plane plane make the sound any different to a cross-plane crank; and which is better?
Before we understand the difference between cross-plane and flat-plane cranks, we've got to get to grips with what a crankshaft is and what it does. It's a lobed shaft at the bottom of the engine to which the pistons are connected to, and it's one of the final output points of the engine itself. The crankshaft very simply converts reciprocating motion into rotational motion – that's how a piston moving up and down converts the energy into rotating the wheels. A cross-plane crankshaft is a crankshaft design with a 90° rotation between crank throws. In simpler terms, every time the crankshaft rotates by 90°, one of the cylinders fires.
In a traditional 4-stroke combustion cycle, the cross-plane crank with even firing pattern can only be used in engines with multiples of 8 cylinders – and they don't have to be in a V-configuration either. Inline-8, V8, V16, and even flat-8 and flat-16 engines can utilize cross-plane cranks. When it comes to V8 engines though, the cross-plane crank configuration is the most common across the world. Mass production V8s like the 5.0-liter V8 in the Ford Mustang GT, or the Hemi V8 in the Dodge Hellcats; even the famous Mercedes-AMG M156 6.2-liter V8 from the SLS AMG featured a cross-plane crank. Conversely, a flat-plane crank is a crankshaft design with a 180° rotation between crank throws. For every 180 degrees that the crank turns a piston fires.
In V8 engines, flat-plane cranks are less commonly used, but unlike the cross-plane crank, flat-plane cranks can be used, and are used almost always, in 4 cylinder engines. Historically, when flat-plane cranks have been used in V8s, it's been almost exclusively by European manufacturers. However there are exceptions to the rule. Engines using a flat-plane crank design include every Ferrari engine manufactured for a Ferrari and the McLaren 3.8-liter engine used from the 12C right through to the P1. Bucking the European trend of flat-plane usage, the latest Ford Mustang Shelby GT350 swapped out its cross-plane crank for a flat-plane one, giving it its unique sound and ability to seemingly rev through the atmospheric ceiling.
On the plus side for cross-plane cranks there's obviously the noise they make that could best be summed up as monumental – a tribute to the gods of horsepower and torque, and the sound that's defined 'American Muscle' for decades! But in addition to the glorious noise, cross-plane V8s are smooth running, with a cylinder firing every 90° there's a constant rotation going which means it runs smoothly. Importantly, because of the frequency of the cylinders firing being so regularly, cross-plane V8s develop big, big torque figures, available from low down.
It's why the Hemi V8 used in Chrysler and Dodge products feels like it can move a mountain from the moment you pull off, and it's also the reason you'll so easily turn tires into plumes of white smoke. A smooth engine with lots of torque, what's not to love! But there are downsides to cross-plane engines. The reason why they rev so smoothly is because they feature heavy counterweights. Without them, the rocking motion caused by the cross-plane crank would unsettle the engine in the car and cause advanced mechanical wear. But these counterweights are heavy, adding rotational mass to the crank.
Because of the additional rotational mass, cross-plane crank V8s don't enjoy revving, and have a fairly low rev-ceiling. The counterweights and configuration also require a larger crank case, making cross-pane V8 engines less compact than might be ideal. Flat-plane cranks are inherently well balanced – unlike cross-plane cranks. As a result, they can make do without the heavy counterweights. With the reduced rotational mass, the inertia point (amount of energy required to incite movement) is lower too. That makes engine responses quicker, and it allows the flat-plane V8 to rev far higher than a cross-plane crank V8 can. Take the Ford Mustang Shelby GT350 for example – revving to 8200 RPM compared to the 7000 RPM of the standard Mustang GT.
Ferrari engines are known to rev even higher – north of 9000 RPM. Due to the lack of counterweights, flat-plane cranks are also highly compact allowing better packaging within an engine bay. But there are downsides too – the 180° firing angle means there's ultimately more vibration and the engine doesn't rev as smoothly. It gets better the higher you rev, but at lower engine speeds vibrations are plentiful. But the biggest downside to the flat-plane crank is the lack of torque. With the wider angle between cylinders firing, there's less momentum and less torque – so ultimately you need to rev it harder and higher to generate torque. Why do you think Ferraris perform best at their screaming top end?
With each having their pros and cons, neither is better outright than the other. However, compact packaging, reduced inertia, and reduced rotational mass give the flat-plane crank V8 a clear advantage. Though it might have flaws, they're far easier to counter, easier to manage and offset in one way or another. To counter the high levels of vibrations, active engine mounts and engine damping have been engineered, and in the age of turbocharging, even flat-plane cranks V8's can develop massive amounts of torque from low down.
Cross-plane cranks may be more common in the mass-markets scenario, but from efficiency, packaging, and ultimately performance perspectives, flat plane crank V8s are the better configuration – technically speaking. But logic and sense when it comes to cars seldom prevail. Much like love, cars are an emotional subject. It's why we form emotional bonds to the hunks of metal and glass in our garages. Though flat-plane cranks may be technically better, the emotion of hearing a rumbling cross-plane V8 is a hard thing to ignore – and when sound is one of the most emotional aspects of motoring, we hope the cross-plane V8 never dies.