Flat plane crankshafts are nothing new.
However, with the recent introductions of the Shelby GT350 and GT350R, flat plane crankshaft design has been garnering a lot more attention. And it’s been raising a few questions, too. Namely, what is a flat plane crankshaft? How does it differ from a crossplane crankshaft? And what are the benefits to a flat plane crankshaft?
In this post, we’ll look at flat plane crankshafts vs. crossplane crankshafts. We’ll start with the crossplane crank, since you may be a little more familiar with that design.
The crossplane crankshaft is used in most every production V8 sold in America today.
On a crossplane crank, the four crank journals are arranged at 90-degree intervals (see image at left). When you look at this type of crankshaft along its axis from either end, it resembles a +. Hence, the name crossplane.
Using the traditional V8 firing order of GM, AMC, Mopar, and most Ford OHV V8s (1-8-4-3-6-5-7-2), a cross plane crankshaft will produce unevenly spaced firings within each side (or cylinder bank) but ensures there’s a balance between the two banks. This is what gives American muscle cars that distinctive burbling sound, although there is some sacrifice in the amount of exhaust scavenging that takes place.
Another distinctive characteristic of the crossplane crankshaft is its counterweights. These counterweights are required to keep the engine properly balanced and prevent it from rocking up and down. This reduces vibration (a problem with many flat plane crankshafts) and fosters smoother operation; however, it also adds rotational mass, which makes this crankshaft less desirable for really high-revving engines.
That brings us to the flat plane crankshaft.
The flat plane crankshaft used in the new 5.2L engine is the first ever in a production Ford engine, but flat plane cranks have been around forever.
Flat plane crankshafts have two pairs of journals 180 degrees apart from each other. When viewed from either end, these crankshafts look flat (see illustration below right). No matter the firing order, flat plane engines will always alternate back and forth between the two cylinder banks. This produces more efficient exhaust scavenging without the need to have header primaries cross over from one bank to the other. It also gives the new Shelby GT350 a totally different sound from other American performance cars–as this video shows:
Another thing that differentiates a flat plane crankshaft from a crossplane crank is the lack of massive counterweights. Without the additional mass of the counterweights, the lighter flat plane cranks will spin more easily than their crossplane counterparts, making them more ideal for high-revving, high-rpm applications. The downside is flat-plane crankshafts tend to create more vibration within the engine without the help of counterweights.
Flat plane crankshafts are typically found on race cars and high-end exotic cars because of their high-rpm performance. In most cases, race car drivers don’t mind a little extra vibration in their engine, and exotic car companies will spend money on lighter-weight materials to reduce vibration in street-oriented cars. Since the flat plane 5.2L V8 will be used in the high-end GT350/GT350R, we’d expect many of those vibration problems to have been addressed by Ford as well.
Most flat plane crankshafts have a shorter stroke than crossplane designs. That means less crankcase space is required; however, shorter strokes often lead to lower torque output.
The Bottom Line
Crossplane Crankshaft Advantages: Smooth, vibration-free performance; distinctive American muscle car burble.
Crossplane Crankshaft Disadvantages: Heavier (harder to rev), requires larger crankcase.
Flat Plane Crankshaft Advantages: Lighter, more compact, more responsive (high-revving), better exhaust scavenging.
Flat Plane Crankshaft Disadvantages: Prone to vibration, lower torque levels.