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?
What Is A Crankshaft?
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.
So which is better?
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.
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.