Engines Exposed: What is a Flat-Plane Crankshaft?


2016 Ford Shelby GT350 Flat-Plane Crank
Source: Ford

There are phrases in the automotive world that evoke a great deal of reverence and glee. Phrases like “turbo” or “hemi” conjure up images of speed, power, and social status. It seems like these phrases are misunderstood by most and complete mysteries to the rest. In an attempt to remedy that, this is the first in a series of articles in which we make sense of these words. Think of me as Ms. Frizzle, but a guy and without an iguana — with a sports car instead of a school bus.

Flat-plane engines

This term became one of the buzz words of 2015 thanks to Ford’s stellar Voodoo V8 (covered previously by us) found in the Mustang GT350. With 526 horsepower and a soundtrack like this, it’s easy to see why it has been attracting attention. Rather than the loping burble of a traditional V8, the sound of a flat-plane engine is much tighter. It’s a tenor to the cross-plane V8’s baritone. However, flat-plane production engines are not a new concept, as Ferrari and Lotus have been using them for years.

The engine in the 458 Italia, for instance, is a flat-plane crank engine. That’s the real term as it describes the construction of the crankshaft of the engine. Before we dive into the technical details, opportunities, and limitations of flat-plane crankshaft engines, watch the short animation below that displays a flat-plane crankshaft and a cross-plane crankshaft, which is what most V8s use, in action.

It’s the job of the crankshaft to transfer the linear force from the pistons into a rotary force that can be used by the wheels. A flat-plane crankshaft (FP), which has its connecting rod journals spaced 180 degrees apart, differs from a cross-plane crankshaft (CP), which has journals that are spaced 90 degrees apart.

Their names derive from their layouts: A flat-plane crank has all of the journals oriented in a (you guessed it) plane while a cross-plane looks like a cross or plus sign when viewed along its axis. Many of the early internal combustion engines were built using a flat-plane crankshaft because it was the simplest way to build them, but these engines tended to be small and unrefined.

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Source: Ford

As engines got more powerful and the public started to demand a more refined motoring experience, engineers figured out a way to make engines vibrate less while creating more power. Placing the connecting rod journals 180 degrees apart effectively creates two inline-four cylinder engines that are mated at the crank. While these are largely balanced mechanically on their own, they are prone to second-order vibrations (more about that later). The complicated engineering needed to quell these vibrations made a cross-plane solution more appealing to many engine designers.

There are two major benefits of FP engines. First, as you can see in the images above, they can rev to high heaven, which expands the usable range of the engine. This is due to the fact that they have less rotary and reciprocating mass within the engine thanks to the elimination of counterweights on the crankshaft. Designers take advantage of this fact by shortening the stroke of the engine and further increasing its RPM range. Second, it is easier to manage exhaust from an FP engine due to the alternating nature of its firing order.

However, all of this comes with one major disadvantage, and this is where you will need to put your science hat on for a minute. FP engines are not widely used due to Noise, Vibration, and Harshness (NVH) characteristics. Essentially, they vibrate quite a bit due to something called second-order vibrations.

Without getting too deep into the math, basically this means that the two pistons that are going to the top of the cylinders have more inertia than the two pistons that are going to the bottom of the cylinders due to their rates of acceleration and deceleration. A harmonic balancer (or harmonic damper) is added to the Voodoo V8 to help quell these vibrations. FP configurations are much more common in smaller engines because the vibrations are also smaller, due to the reduced reciprocating mass, and are therefore more manageable.

 The Mustang Shelby GT350 benefits from sharp styling and a flat-plane crank loaded V8 that generates tons of power without the assistance of a supercharger | Source: Ford
The Mustang Shelby GT350 benefits from a flat-plane crank loaded V8 that generates tons of power without the assistance of a supercharger | Ford

Due to all of these factors, it becomes clear why FP engines are favored for racing. They are inherently lightweight, have wide RPM ranges that can lead to high horsepower numbers, and their vibration characteristics matter far less when the focus is on function rather than comfort.

Through decades of experimentation and black magic, several car companies have figured out a recipe for a comfortable flat-plane engine that doesn’t skimp on the thrills. The Voodoo V8 is the latest power plant to take up the mantle. If the reviews are any indication, cross-plane engines suddenly have some serious competition.

Like classics? It’s always Throwback Thursday somewhere.