What’s The Difference Between Flat-Plane and Cross-Plane Crankshafts?

Crankshafts, in the world of high-performance engines, are important. Actually, they’re pretty important in all engines, but when the power goes up and the revs get fruity they can play a pivotal role in defining a car’s character, performance and soundtrack.

The main types of crankshaft you’ll find beneath whirling pistons are flat-plane and cross-plane, and exactly which is best is a regular topic for debate among enthusiasts. Unsurprisingly, there’s no clear answer – both types have their own advantages and disadvantages, and are suited to different kinds of automotive uses. So, let’s take a look at them and see what’s what.

The basics

First things first, what’s a crankshaft? It’s essentially the backbone of the internal combustion process – a component within the engine that converts the linear motion of an engine’s pistons into rotational motion that, eventually, turns the car’s wheels. The difference between flat-plane and cross-plane is in the configuration of the crankpins, to which the piston connecting rods are attached, and how they balance the operation of the engine.

Flat-Plane Crankshaft

A flat-plane crankshaft has its crankpins arranged on a single (flat) plane, alternating them at 180-degree intervals.

You’ll generally find a flat-plane crankshaft in four-cylinder engines as well as V8s and other V-configuration layouts. They’re lighter and easier to make than cross-plane crankshafts, and therefore often find favour in performance cars.

In addition, the lighter weight means less rotational inertia, which allows quicker engine revs. You get better exhaust scavenging because of a flat-plane crankshaft’s firing order – it alternates between cylinder banks, which reduces overlap and increases efficiency. (Exhaust scavenging, by the way, is the process of removing spent gas from the engine’s combustion chamber.)

The byproduct of this is a high-pitched, raspy exhaust note – see examples like the Ferrari 458 Italia and the McLaren 720S. Flat-plane engines are often associated with high performance engines in race cars and high-end sports cars, and are all about precision and fast responses.

The downside of a flat-plane layout is increased vibration, due to the inherent imbalance of the design. This is particularly the case at higher revs, so while the crankshaft design itself is relatively simple, engineers have to work hard to balance and reinforce the rest of the engine (and car) to ensure reliability and comfort.

Cross-Plane Crankshaft

A cross-plane crankshaft has the crankpins arranged over two planes, spaced at 90-degree intervals. Viewed from the end, it looks like an X-shape.

The result is heavier and more complex, but is also inherently more balanced, which is great for smoother operation and improved torque at low-to-medium engine speeds. Engineers need to worry less about counterweights when designing the wider engine structure. However, exhaust scavenging will be less efficient, resulting in a deep, burbling exhaust note – think American muscle cars like the Ford Mustang and Chevrolet Corvette, and V8 grand tourers like the Bentley Continental GT V8.

An engine with a cross-plane crankshaft tends to be a more characterful one – not just because of the noise, but because they deliver performance across a wider range of speeds. You can happily smash down a drag strip with a cross-plane engine, but it’ll be equally happy on a leisurely cruise.

Downsides of a cross-plane setup? As well as lower efficiency, they can be harder to engineer in terms of responsiveness compared to a flat-plane design.

So, what’s better? As with… well, everything, it depends. Flat-plane and cross-plane crankshafts represent different paths with different results, and which one’s best for you depends on personal preference and what you’ll be using your engine for. If you want razor-sharp precision and lightning response from your engine, then flat-plane’s probably the one to go for. For broader use and, arguably, more character, then get yourself something with a cross-plane crankshaft.

Of course, whatever you choose, arguably the most important thing is that you bore your mates by repeatedly explaining the differences and justifying why your choice is the best.